puzzles: resync with upstream

This brings the upstream version to 9aa7b7c (with some of my changes as
well).

Change-Id: I5bf8a3e0b8672d82cb1bf34afc07adbe12a3ac53

60 files changed, 1889 insertions, 332 deletions

diff --git a/apps/plugins/puzzles/README.rockbox b/apps/plugins/puzzles/README.rockbox
index 3adce72412..26e6bbb289 100644
--- a/apps/plugins/puzzles/README.rockbox
+++ b/apps/plugins/puzzles/README.rockbox
@@ -59,3 +59,7 @@ April 2018: Finished up the rest of the games. All work now! Surely
 there's still bugs to fix, so stay tuned...
 
 December 2018: Resync to 3ece3d6. Happy holidays!
+
+May 2019: Resync to e2135d5.
+
+June 2020: Resync to 9aa7b7c.
diff --git a/apps/plugins/puzzles/help/blackbox.c b/apps/plugins/puzzles/help/blackbox.c
index 938810a19f..659d26cd7a 100644
--- a/apps/plugins/puzzles/help/blackbox.c
+++ b/apps/plugins/puzzles/help/blackbox.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/bridges.c b/apps/plugins/puzzles/help/bridges.c
index ed901c1309..b0cdcedd34 100644
--- a/apps/plugins/puzzles/help/bridges.c
+++ b/apps/plugins/puzzles/help/bridges.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/cube.c b/apps/plugins/puzzles/help/cube.c
index f8026a5946..7939437658 100644
--- a/apps/plugins/puzzles/help/cube.c
+++ b/apps/plugins/puzzles/help/cube.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/dominosa.c b/apps/plugins/puzzles/help/dominosa.c
index f1c61620ee..20a2341c64 100644
--- a/apps/plugins/puzzles/help/dominosa.c
+++ b/apps/plugins/puzzles/help/dominosa.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/fifteen.c b/apps/plugins/puzzles/help/fifteen.c
index eb3f84ab26..dc3e5373e8 100644
--- a/apps/plugins/puzzles/help/fifteen.c
+++ b/apps/plugins/puzzles/help/fifteen.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/filling.c b/apps/plugins/puzzles/help/filling.c
index 15ba8cbe43..6602890bd8 100644
--- a/apps/plugins/puzzles/help/filling.c
+++ b/apps/plugins/puzzles/help/filling.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/flip.c b/apps/plugins/puzzles/help/flip.c
index c451e8c672..9553df8b77 100644
--- a/apps/plugins/puzzles/help/flip.c
+++ b/apps/plugins/puzzles/help/flip.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/flood.c b/apps/plugins/puzzles/help/flood.c
index 1bee956902..a800ca4444 100644
--- a/apps/plugins/puzzles/help/flood.c
+++ b/apps/plugins/puzzles/help/flood.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/galaxies.c b/apps/plugins/puzzles/help/galaxies.c
index eed08b8ac4..d6bbf9360c 100644
--- a/apps/plugins/puzzles/help/galaxies.c
+++ b/apps/plugins/puzzles/help/galaxies.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/guess.c b/apps/plugins/puzzles/help/guess.c
index 1fc4f0695f..8260b0c6f6 100644
--- a/apps/plugins/puzzles/help/guess.c
+++ b/apps/plugins/puzzles/help/guess.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/inertia.c b/apps/plugins/puzzles/help/inertia.c
index 8d7902dc54..184893af4a 100644
--- a/apps/plugins/puzzles/help/inertia.c
+++ b/apps/plugins/puzzles/help/inertia.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/keen.c b/apps/plugins/puzzles/help/keen.c
index 8f0374dcdd..4539df4625 100644
--- a/apps/plugins/puzzles/help/keen.c
+++ b/apps/plugins/puzzles/help/keen.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/lightup.c b/apps/plugins/puzzles/help/lightup.c
index 303aa45beb..1dca0363c4 100644
--- a/apps/plugins/puzzles/help/lightup.c
+++ b/apps/plugins/puzzles/help/lightup.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/loopy.c b/apps/plugins/puzzles/help/loopy.c
index eb4adedfc8..85ad8e27c4 100644
--- a/apps/plugins/puzzles/help/loopy.c
+++ b/apps/plugins/puzzles/help/loopy.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/magnets.c b/apps/plugins/puzzles/help/magnets.c
index becfe20902..2f18033675 100644
--- a/apps/plugins/puzzles/help/magnets.c
+++ b/apps/plugins/puzzles/help/magnets.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/map.c b/apps/plugins/puzzles/help/map.c
index 731b5f4745..5608eb7619 100644
--- a/apps/plugins/puzzles/help/map.c
+++ b/apps/plugins/puzzles/help/map.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/mines.c b/apps/plugins/puzzles/help/mines.c
index 317d626292..c37e9298ab 100644
--- a/apps/plugins/puzzles/help/mines.c
+++ b/apps/plugins/puzzles/help/mines.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
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diff --git a/apps/plugins/puzzles/help/net.c b/apps/plugins/puzzles/help/net.c
index d83892c110..256012638e 100644
--- a/apps/plugins/puzzles/help/net.c
+++ b/apps/plugins/puzzles/help/net.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
 /* help text is compressed using LZ4; see compress.c for details */
 /* DO NOT EDIT! */
 
diff --git a/apps/plugins/puzzles/help/netslide.c b/apps/plugins/puzzles/help/netslide.c
index dc6a0d65e1..5d6e282e04 100644
--- a/apps/plugins/puzzles/help/netslide.c
+++ b/apps/plugins/puzzles/help/netslide.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/palisade.c b/apps/plugins/puzzles/help/palisade.c
index 3de6c2d06d..c3e1de1b51 100644
--- a/apps/plugins/puzzles/help/palisade.c
+++ b/apps/plugins/puzzles/help/palisade.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/pattern.c b/apps/plugins/puzzles/help/pattern.c
index 3c51afd66c..83bfaeb5fe 100644
--- a/apps/plugins/puzzles/help/pattern.c
+++ b/apps/plugins/puzzles/help/pattern.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/pearl.c b/apps/plugins/puzzles/help/pearl.c
index 1c7a7ee3ab..fe3f3534fc 100644
--- a/apps/plugins/puzzles/help/pearl.c
+++ b/apps/plugins/puzzles/help/pearl.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/pegs.c b/apps/plugins/puzzles/help/pegs.c
index d64cbe8d56..24513761c8 100644
--- a/apps/plugins/puzzles/help/pegs.c
+++ b/apps/plugins/puzzles/help/pegs.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/range.c b/apps/plugins/puzzles/help/range.c
index 62c308778e..34ae51a512 100644
--- a/apps/plugins/puzzles/help/range.c
+++ b/apps/plugins/puzzles/help/range.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/rect.c b/apps/plugins/puzzles/help/rect.c
index 768bd1a4ce..196dc077d9 100644
--- a/apps/plugins/puzzles/help/rect.c
+++ b/apps/plugins/puzzles/help/rect.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/samegame.c b/apps/plugins/puzzles/help/samegame.c
index 2f8e8a3003..02fc670b80 100644
--- a/apps/plugins/puzzles/help/samegame.c
+++ b/apps/plugins/puzzles/help/samegame.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/signpost.c b/apps/plugins/puzzles/help/signpost.c
index ceafcfd056..c0e4c44e9f 100644
--- a/apps/plugins/puzzles/help/signpost.c
+++ b/apps/plugins/puzzles/help/signpost.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/singles.c b/apps/plugins/puzzles/help/singles.c
index dffc14b35b..606b4f7b41 100644
--- a/apps/plugins/puzzles/help/singles.c
+++ b/apps/plugins/puzzles/help/singles.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/sixteen.c b/apps/plugins/puzzles/help/sixteen.c
index eb8feed049..d72226f8a1 100644
--- a/apps/plugins/puzzles/help/sixteen.c
+++ b/apps/plugins/puzzles/help/sixteen.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/slant.c b/apps/plugins/puzzles/help/slant.c
index 873863757e..b78c7c694c 100644
--- a/apps/plugins/puzzles/help/slant.c
+++ b/apps/plugins/puzzles/help/slant.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/solo.c b/apps/plugins/puzzles/help/solo.c
index a02bb94c04..449a352fe2 100644
--- a/apps/plugins/puzzles/help/solo.c
+++ b/apps/plugins/puzzles/help/solo.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/tents.c b/apps/plugins/puzzles/help/tents.c
index e53f92b0f6..d26afead48 100644
--- a/apps/plugins/puzzles/help/tents.c
+++ b/apps/plugins/puzzles/help/tents.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/towers.c b/apps/plugins/puzzles/help/towers.c
index 62c08e011b..b062c4f350 100644
--- a/apps/plugins/puzzles/help/towers.c
+++ b/apps/plugins/puzzles/help/towers.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/tracks.c b/apps/plugins/puzzles/help/tracks.c
index d3bdc709be..6872ddbbab 100644
--- a/apps/plugins/puzzles/help/tracks.c
+++ b/apps/plugins/puzzles/help/tracks.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/twiddle.c b/apps/plugins/puzzles/help/twiddle.c
index 72495874b8..69ce44e9b0 100644
--- a/apps/plugins/puzzles/help/twiddle.c
+++ b/apps/plugins/puzzles/help/twiddle.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/undead.c b/apps/plugins/puzzles/help/undead.c
index 3af048a3b8..11fb57cf48 100644
--- a/apps/plugins/puzzles/help/undead.c
+++ b/apps/plugins/puzzles/help/undead.c
@@ -1,4 +1,4 @@
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+/* auto-generated on Jun 25 2020 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/unequal.c b/apps/plugins/puzzles/help/unequal.c
index 8c5d2fa9b8..f7a612c01e 100644
--- a/apps/plugins/puzzles/help/unequal.c
+++ b/apps/plugins/puzzles/help/unequal.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/help/unruly.c b/apps/plugins/puzzles/help/unruly.c
index 55c4d5240f..88590aca32 100644
--- a/apps/plugins/puzzles/help/unruly.c
+++ b/apps/plugins/puzzles/help/unruly.c
@@ -1,4 +1,4 @@
-/* auto-generated on Dec 21 2018 by genhelp.sh */
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diff --git a/apps/plugins/puzzles/help/untangle.c b/apps/plugins/puzzles/help/untangle.c
index 24f9d83986..6992d97e96 100644
--- a/apps/plugins/puzzles/help/untangle.c
+++ b/apps/plugins/puzzles/help/untangle.c
@@ -1,4 +1,4 @@
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diff --git a/apps/plugins/puzzles/src/LICENCE b/apps/plugins/puzzles/src/LICENCE
index ce0418e6cc..85f67ba795 100644
--- a/apps/plugins/puzzles/src/LICENCE
+++ b/apps/plugins/puzzles/src/LICENCE
@@ -2,8 +2,8 @@ This software is copyright (c) 2004-2014 Simon Tatham.
 
 Portions copyright Richard Boulton, James Harvey, Mike Pinna, Jonas
 Kölker, Dariusz Olszewski, Michael Schierl, Lambros Lambrou, Bernd
-Schmidt, Steffen Bauer, Lennard Sprong, Rogier Goossens and Michael
-Quevillon.
+Schmidt, Steffen Bauer, Lennard Sprong, Rogier Goossens, Michael
+Quevillon and Asher Gordon.
 
 Permission is hereby granted, free of charge, to any person
 obtaining a copy of this software and associated documentation files
diff --git a/apps/plugins/puzzles/src/gamedesc.txt b/apps/plugins/puzzles/src/gamedesc.txt
deleted file mode 100644
index db0d9cbd01..0000000000
--- a/apps/plugins/puzzles/src/gamedesc.txt
+++ /dev/null
@@ -1,39 +0,0 @@
-blackbox:blackbox.exe:Black Box:Ball-finding puzzle:Find the hidden balls in the box by bouncing laser beams off them.
-bridges:bridges.exe:Bridges:Bridge-placing puzzle:Connect all the islands with a network of bridges.
-cube:cube.exe:Cube:Rolling cube puzzle:Pick up all the blue squares by rolling the cube over them.
-dominosa:dominosa.exe:Dominosa:Domino tiling puzzle:Tile the rectangle with a full set of dominoes.
-fifteen:fifteen.exe:Fifteen:Sliding block puzzle:Slide the tiles around to arrange them into order.
-filling:filling.exe:Filling:Polyomino puzzle:Mark every square with the area of its containing region.
-flip:flip.exe:Flip:Tile inversion puzzle:Flip groups of squares to light them all up at once.
-flood:flood.exe:Flood:Flood-filling puzzle:Turn the grid the same colour in as few flood fills as possible.
-galaxies:galaxies.exe:Galaxies:Symmetric polyomino puzzle:Divide the grid into rotationally symmetric regions each centred on a dot.
-guess:guess.exe:Guess:Combination-guessing puzzle:Guess the hidden combination of colours.
-inertia:inertia.exe:Inertia:Gem-collecting puzzle:Collect all the gems without running into any of the mines.
-keen:keen.exe:Keen:Arithmetic Latin square puzzle:Complete the latin square in accordance with the arithmetic clues.
-lightup:lightup.exe:Light Up:Light-bulb placing puzzle:Place bulbs to light up all the squares.
-loopy:loopy.exe:Loopy:Loop-drawing puzzle:Draw a single closed loop, given clues about number of adjacent edges.
-magnets:magnets.exe:Magnets:Magnet-placing puzzle:Place magnets to satisfy the clues and avoid like poles touching.
-map:map.exe:Map:Map-colouring puzzle:Colour the map so that adjacent regions are never the same colour.
-mines:mines.exe:Mines:Mine-finding puzzle:Find all the mines without treading on any of them.
-net:netgame.exe:Net:Network jigsaw puzzle:Rotate each tile to reassemble the network.
-netslide:netslide.exe:Netslide:Toroidal sliding network puzzle:Slide a row at a time to reassemble the network.
-palisade:palisade.exe:Palisade:Grid-division puzzle:Divide the grid into equal-sized areas in accordance with the clues.
-pattern:pattern.exe:Pattern:Pattern puzzle:Fill in the pattern in the grid, given only the lengths of runs of black squares.
-pearl:pearl.exe:Pearl:Loop-drawing puzzle:Draw a single closed loop, given clues about corner and straight squares.
-pegs:pegs.exe:Pegs:Peg solitaire puzzle:Jump pegs over each other to remove all but one.
-range:range.exe:Range:Visible-distance puzzle:Place black squares to limit the visible distance from each numbered cell.
-rect:rect.exe:Rectangles:Rectangles puzzle:Divide the grid into rectangles with areas equal to the numbers.
-samegame:samegame.exe:Same Game:Block-clearing puzzle:Clear the grid by removing touching groups of the same colour squares.
-signpost:signpost.exe:Signpost:Square-connecting puzzle:Connect the squares into a path following the arrows.
-singles:singles.exe:Singles:Number-removing puzzle:Black out the right set of duplicate numbers.
-sixteen:sixteen.exe:Sixteen:Toroidal sliding block puzzle:Slide a row at a time to arrange the tiles into order.
-slant:slant.exe:Slant:Maze-drawing puzzle:Draw a maze of slanting lines that matches the clues.
-solo:solo.exe:Solo:Number placement puzzle:Fill in the grid so that each row, column and square block contains one of every digit.
-tents:tents.exe:Tents:Tent-placing puzzle:Place a tent next to each tree.
-towers:towers.exe:Towers:Tower-placing Latin square puzzle:Complete the latin square of towers in accordance with the clues.
-tracks:tracks.exe:Tracks:Path-finding railway track puzzle:Fill in the railway track according to the clues.
-twiddle:twiddle.exe:Twiddle:Rotational sliding block puzzle:Rotate the tiles around themselves to arrange them into order.
-undead:undead.exe:Undead:Monster-placing puzzle:Place ghosts, vampires and zombies so that the right numbers of them can be seen in mirrors.
-unequal:unequal.exe:Unequal:Latin square puzzle:Complete the latin square in accordance with the > signs.
-unruly:unruly.exe:Unruly:Black and white grid puzzle:Fill in the black and white grid to avoid runs of three.
-untangle:untangle.exe:Untangle:Planar graph layout puzzle:Reposition the points so that the lines do not cross.
diff --git a/apps/plugins/puzzles/src/grid.c b/apps/plugins/puzzles/src/grid.c
index 89bde187be..5ea37439d4 100644
--- a/apps/plugins/puzzles/src/grid.c
+++ b/apps/plugins/puzzles/src/grid.c
@@ -2378,6 +2378,8 @@ static void grid_size_floret(int width, int height,
     *tilesize = FLORET_TILESIZE;
     *xextent = (6*px+3*qx)/2 * (width-1) + 4*qx + 2*px;
     *yextent = (5*qy-4*py) * (height-1) + 4*qy + 2*ry;
+    if (height == 1)
+        *yextent += (5*qy-4*py)/2;
 }
 
 static grid *grid_new_floret(int width, int height, const char *desc)
diff --git a/apps/plugins/puzzles/src/gtk.c b/apps/plugins/puzzles/src/gtk.c
index d41f8677b9..7588ee0dc6 100644
--- a/apps/plugins/puzzles/src/gtk.c
+++ b/apps/plugins/puzzles/src/gtk.c
@@ -44,6 +44,17 @@
 # endif
 #endif
 
+#if defined USE_CAIRO && GTK_CHECK_VERSION(2,10,0)
+/* We can only use printing if we are using Cairo for drawing and we
+   have a GTK version >= 2.10 (when GtkPrintOperation was added). */
+# define USE_PRINTING
+# if GTK_CHECK_VERSION(2,18,0)
+/* We can embed the page setup. Before 2.18, we needed to have a
+   separate page setup. */
+#  define USE_EMBED_PAGE_SETUP
+# endif
+#endif
+
 #if GTK_CHECK_VERSION(3,0,0)
 /* The old names are still more concise! */
 #define gtk_hbox_new(x,y) gtk_box_new(GTK_ORIENTATION_HORIZONTAL,y)
@@ -132,6 +143,18 @@ struct font {
 };
 
 /*
+ * An internal API for functions which need to be different for
+ * printing and drawing.
+ */
+struct internal_drawing_api {
+    void (*set_colour)(frontend *fe, int colour);
+#ifdef USE_CAIRO
+    void (*fill)(frontend *fe);
+    void (*fill_preserve)(frontend *fe);
+#endif
+};
+
+/*
  * This structure holds all the data relevant to a single window.
  * In principle this would allow us to open multiple independent
  * puzzle windows, although I can't currently see any real point in
@@ -180,7 +203,7 @@ struct frontend {
     GtkWidget *cfgbox;
     void *paste_data;
     int paste_data_len;
-    int pw, ph;                        /* pixmap size (w, h are area size) */
+    int pw, ph, ps;  /* pixmap size (w, h are area size, s is GDK scale) */
     int ox, oy;                        /* offset of pixmap in drawing area */
 #ifdef OLD_FILESEL
     char *filesel_name;
@@ -225,6 +248,23 @@ struct frontend {
      */
     bool awaiting_resize_ack;
 #endif
+#ifdef USE_CAIRO
+    int printcount, printw, printh;
+    float printscale;
+    bool printsolns, printcolour;
+    int hatch;
+    float hatchthick, hatchspace;
+    drawing *print_dr;
+    document *doc;
+#endif
+#ifdef USE_PRINTING
+    GtkPrintOperation *printop;
+    GtkPrintContext *printcontext;
+    GtkSpinButton *printcount_spin_button, *printw_spin_button,
+        *printh_spin_button, *printscale_spin_button;
+    GtkCheckButton *soln_check_button, *colour_check_button;
+#endif
+    const struct internal_drawing_api *dr_api;
 };
 
 struct blitter {
@@ -247,15 +287,19 @@ void get_random_seed(void **randseed, int *randseedsize)
 void frontend_default_colour(frontend *fe, float *output)
 {
 #if !GTK_CHECK_VERSION(3,0,0)
-    /*
-     * Use the widget style's default background colour as the
-     * background for the puzzle drawing area.
-     */
-    GdkColor col = gtk_widget_get_style(fe->window)->bg[GTK_STATE_NORMAL];
-    output[0] = col.red / 65535.0;
-    output[1] = col.green / 65535.0;
-    output[2] = col.blue / 65535.0;
-#else
+    if (!fe->headless) {
+        /*
+         * If we have a widget and it has a style that specifies a
+         * default background colour, use that as the background for
+         * the puzzle drawing area.
+         */
+        GdkColor col = gtk_widget_get_style(fe->window)->bg[GTK_STATE_NORMAL];
+        output[0] = col.red / 65535.0;
+        output[1] = col.green / 65535.0;
+        output[2] = col.blue / 65535.0;
+    }
+#endif
+
     /*
      * GTK 3 has decided that there's no such thing as a 'default
      * background colour' any more, because widget styles might set
@@ -263,9 +307,11 @@ void frontend_default_colour(frontend *fe, float *output)
      * image. We don't want to get into overlaying our entire puzzle
      * on an arbitrary background image, so we'll just make up a
      * reasonable shade of grey.
+     *
+     * This is also what we do on GTK 2 in headless mode, where we
+     * don't have a widget style to query.
      */
     output[0] = output[1] = output[2] = 0.9F;
-#endif
 }
 
 void gtk_status_bar(void *handle, const char *text)
@@ -290,6 +336,7 @@ void gtk_status_bar(void *handle, const char *text)
 static void setup_drawing(frontend *fe)
 {
     fe->cr = cairo_create(fe->image);
+    cairo_scale(fe->cr, fe->ps, fe->ps);
     cairo_set_antialias(fe->cr, CAIRO_ANTIALIAS_GRAY);
     cairo_set_line_width(fe->cr, 1.0);
     cairo_set_line_cap(fe->cr, CAIRO_LINE_CAP_SQUARE);
@@ -321,12 +368,23 @@ static void snaffle_colours(frontend *fe)
     fe->colours = midend_colours(fe->me, &fe->ncolours);
 }
 
-static void set_colour(frontend *fe, int colour)
+static void draw_set_colour(frontend *fe, int colour)
 {
     cairo_set_source_rgb(fe->cr,
-			 fe->colours[3*colour + 0],
-			 fe->colours[3*colour + 1],
-			 fe->colours[3*colour + 2]);
+                         fe->colours[3*colour + 0],
+                         fe->colours[3*colour + 1],
+                         fe->colours[3*colour + 2]);
+}
+
+static void print_set_colour(frontend *fe, int colour)
+{
+    float r, g, b;
+
+    print_get_colour(fe->print_dr, colour, fe->printcolour,
+                     &(fe->hatch), &r, &g, &b);
+
+    if (fe->hatch < 0)
+        cairo_set_source_rgb(fe->cr, r, g, b);
 }
 
 static void set_window_background(frontend *fe, int colour)
@@ -395,6 +453,82 @@ static void save_screenshot_png(frontend *fe, const char *screenshot_file)
     cairo_surface_write_to_png(fe->image, screenshot_file);
 }
 
+static void do_hatch(frontend *fe)
+{
+    double i, x, y, width, height, maxdim;
+
+    /* Get the dimensions of the region to be hatched. */
+    cairo_path_extents(fe->cr, &x, &y, &width, &height);
+
+    maxdim = max(width, height);
+
+    cairo_save(fe->cr);
+
+    /* Set the line color and width. */
+    cairo_set_source_rgb(fe->cr, 0, 0, 0);
+    cairo_set_line_width(fe->cr, fe->hatchthick);
+    /* Clip to the region. */
+    cairo_clip(fe->cr);
+    /* Hatch the bounding area of the fill region. */
+    if (fe->hatch == HATCH_VERT || fe->hatch == HATCH_PLUS) {
+        for (i = 0.0; i <= width; i += fe->hatchspace) {
+            cairo_move_to(fe->cr, i, 0);
+            cairo_rel_line_to(fe->cr, 0, height);
+        }
+    }
+    if (fe->hatch == HATCH_HORIZ || fe->hatch == HATCH_PLUS) {
+        for (i = 0.0; i <= height; i += fe->hatchspace) {
+            cairo_move_to(fe->cr, 0, i);
+            cairo_rel_line_to(fe->cr, width, 0);
+        }
+    }
+    if (fe->hatch == HATCH_SLASH || fe->hatch == HATCH_X) {
+        for (i = -height; i <= width; i += fe->hatchspace * ROOT2) {
+            cairo_move_to(fe->cr, i, 0);
+            cairo_rel_line_to(fe->cr, maxdim, maxdim);
+        }
+    }
+    if (fe->hatch == HATCH_BACKSLASH || fe->hatch == HATCH_X) {
+        for (i = 0.0; i <= width + height; i += fe->hatchspace * ROOT2) {
+            cairo_move_to(fe->cr, i, 0);
+            cairo_rel_line_to(fe->cr, -maxdim, maxdim);
+        }
+    }
+    cairo_stroke(fe->cr);
+
+    cairo_restore(fe->cr);
+}
+
+static void do_draw_fill(frontend *fe)
+{
+    cairo_fill(fe->cr);
+}
+
+static void do_draw_fill_preserve(frontend *fe)
+{
+    cairo_fill_preserve(fe->cr);
+}
+
+static void do_print_fill(frontend *fe)
+{
+    if (fe->hatch < 0)
+        cairo_fill(fe->cr);
+    else
+        do_hatch(fe);
+}
+
+static void do_print_fill_preserve(frontend *fe)
+{
+    if (fe->hatch < 0) {
+        cairo_fill_preserve(fe->cr);
+    } else {
+        cairo_path_t *oldpath;
+        oldpath = cairo_copy_path(fe->cr);
+        do_hatch(fe);
+        cairo_append_path(fe->cr, oldpath);
+    }
+}
+
 static void do_clip(frontend *fe, int x, int y, int w, int h)
 {
     cairo_new_path(fe->cr);
@@ -413,7 +547,7 @@ static void do_draw_rect(frontend *fe, int x, int y, int w, int h)
     cairo_new_path(fe->cr);
     cairo_set_antialias(fe->cr, CAIRO_ANTIALIAS_NONE);
     cairo_rectangle(fe->cr, x, y, w, h);
-    cairo_fill(fe->cr);
+    fe->dr_api->fill(fe);
     cairo_restore(fe->cr);
 }
 
@@ -447,11 +581,11 @@ static void do_draw_poly(frontend *fe, int *coords, int npoints,
 	cairo_line_to(fe->cr, coords[i*2] + 0.5, coords[i*2 + 1] + 0.5);
     cairo_close_path(fe->cr);
     if (fillcolour >= 0) {
-        set_colour(fe, fillcolour);
-	cairo_fill_preserve(fe->cr);
+        fe->dr_api->set_colour(fe, fillcolour);
+	fe->dr_api->fill_preserve(fe);
     }
     assert(outlinecolour >= 0);
-    set_colour(fe, outlinecolour);
+    fe->dr_api->set_colour(fe, outlinecolour);
     cairo_stroke(fe->cr);
 }
 
@@ -462,11 +596,11 @@ static void do_draw_circle(frontend *fe, int cx, int cy, int radius,
     cairo_arc(fe->cr, cx + 0.5, cy + 0.5, radius, 0, 2*PI);
     cairo_close_path(fe->cr);		/* Just in case... */
     if (fillcolour >= 0) {
-	set_colour(fe, fillcolour);
-	cairo_fill_preserve(fe->cr);
+	fe->dr_api->set_colour(fe, fillcolour);
+	fe->dr_api->fill_preserve(fe);
     }
     assert(outlinecolour >= 0);
-    set_colour(fe, outlinecolour);
+    fe->dr_api->set_colour(fe, outlinecolour);
     cairo_stroke(fe->cr);
 }
 
@@ -512,23 +646,24 @@ static void wipe_and_maybe_destroy_cairo(frontend *fe, cairo_t *cr,
 static void setup_backing_store(frontend *fe)
 {
 #ifndef USE_CAIRO_WITHOUT_PIXMAP
-    if (fe->headless) {
-        fprintf(stderr, "headless mode does not work with GDK pixmaps\n");
-        exit(1);
+    if (!fe->headless) {
+        fe->pixmap = gdk_pixmap_new(gtk_widget_get_window(fe->area),
+                                    fe->pw*fe->ps, fe->ph*fe->ps, -1);
+    } else {
+        fe->pixmap = NULL;
     }
-
-    fe->pixmap = gdk_pixmap_new(gtk_widget_get_window(fe->area),
-                                fe->pw, fe->ph, -1);
 #endif
+
     fe->image = cairo_image_surface_create(CAIRO_FORMAT_RGB24,
-					   fe->pw, fe->ph);
+					   fe->pw*fe->ps, fe->ph*fe->ps);
 
     wipe_and_maybe_destroy_cairo(fe, cairo_create(fe->image), true);
 #ifndef USE_CAIRO_WITHOUT_PIXMAP
-    wipe_and_maybe_destroy_cairo(fe, gdk_cairo_create(fe->pixmap), true);
+    if (!fe->headless)
+        wipe_and_maybe_destroy_cairo(fe, gdk_cairo_create(fe->pixmap), true);
 #endif
-#if GTK_CHECK_VERSION(3,22,0)
     if (!fe->headless) {
+#if GTK_CHECK_VERSION(3,22,0)
         GdkWindow *gdkwin;
         cairo_region_t *region;
         GdkDrawingContext *drawctx;
@@ -541,11 +676,11 @@ static void setup_backing_store(frontend *fe)
         wipe_and_maybe_destroy_cairo(fe, cr, false);
         gdk_window_end_draw_frame(gdkwin, drawctx);
         cairo_region_destroy(region);
-    }
 #else
-    wipe_and_maybe_destroy_cairo(
-        fe, gdk_cairo_create(gtk_widget_get_window(fe->area)), true);
+        wipe_and_maybe_destroy_cairo(
+            fe, gdk_cairo_create(gtk_widget_get_window(fe->area)), true);
 #endif
+    }
 }
 
 static bool backing_store_ok(frontend *fe)
@@ -616,7 +751,7 @@ static void set_window_background(frontend *fe, int colour)
     gdk_window_set_background(fe->window->window, &fe->colours[colour]);
 }
 
-static void set_colour(frontend *fe, int colour)
+static void draw_set_colour(frontend *fe, int colour)
 {
     gdk_gc_set_foreground(fe->gc, &fe->colours[colour]);
 }
@@ -709,11 +844,11 @@ static void do_draw_poly(frontend *fe, int *coords, int npoints,
     }
 
     if (fillcolour >= 0) {
-	set_colour(fe, fillcolour);
+	fe->dr_api->set_colour(fe, fillcolour);
 	gdk_draw_polygon(fe->pixmap, fe->gc, true, points, npoints);
     }
     assert(outlinecolour >= 0);
-    set_colour(fe, outlinecolour);
+    fe->dr_api->set_colour(fe, outlinecolour);
 
     /*
      * In principle we ought to be able to use gdk_draw_polygon for
@@ -733,14 +868,14 @@ static void do_draw_circle(frontend *fe, int cx, int cy, int radius,
 			   int fillcolour, int outlinecolour)
 {
     if (fillcolour >= 0) {
-	set_colour(fe, fillcolour);
+	fe->dr_api->set_colour(fe, fillcolour);
 	gdk_draw_arc(fe->pixmap, fe->gc, true,
 		     cx - radius, cy - radius,
 		     2 * radius, 2 * radius, 0, 360 * 64);
     }
 
     assert(outlinecolour >= 0);
-    set_colour(fe, outlinecolour);
+    fe->dr_api->set_colour(fe, outlinecolour);
     gdk_draw_arc(fe->pixmap, fe->gc, false,
 		 cx - radius, cy - radius,
 		 2 * radius, 2 * radius, 0, 360 * 64);
@@ -1045,21 +1180,21 @@ void gtk_draw_text(void *handle, int x, int y, int fonttype, int fontsize,
     /*
      * Do the job.
      */
-    set_colour(fe, colour);
+    fe->dr_api->set_colour(fe, colour);
     align_and_draw_text(fe, i, align, x, y, text);
 }
 
 void gtk_draw_rect(void *handle, int x, int y, int w, int h, int colour)
 {
     frontend *fe = (frontend *)handle;
-    set_colour(fe, colour);
+    fe->dr_api->set_colour(fe, colour);
     do_draw_rect(fe, x, y, w, h);
 }
 
 void gtk_draw_line(void *handle, int x1, int y1, int x2, int y2, int colour)
 {
     frontend *fe = (frontend *)handle;
-    set_colour(fe, colour);
+    fe->dr_api->set_colour(fe, colour);
     do_draw_line(fe, x1, y1, x2, y2);
 }
 
@@ -1067,7 +1202,7 @@ void gtk_draw_thick_line(void *handle, float thickness,
 			 float x1, float y1, float x2, float y2, int colour)
 {
     frontend *fe = (frontend *)handle;
-    set_colour(fe, colour);
+    fe->dr_api->set_colour(fe, colour);
     do_draw_thick_line(fe, thickness, x1, y1, x2, y2);
 }
 
@@ -1161,6 +1296,105 @@ char *gtk_text_fallback(void *handle, const char *const *strings, int nstrings)
 }
 #endif
 
+#ifdef USE_PRINTING
+void gtk_begin_doc(void *handle, int pages)
+{
+    frontend *fe = (frontend *)handle;
+    gtk_print_operation_set_n_pages(fe->printop, pages);
+}
+
+void gtk_begin_page(void *handle, int number)
+{
+}
+
+void gtk_begin_puzzle(void *handle, float xm, float xc,
+                      float ym, float yc, int pw, int ph, float wmm)
+{
+    frontend *fe = (frontend *)handle;
+    double ppw, pph, pox, poy, dpmmx, dpmmy;
+    double scale;
+
+    ppw = gtk_print_context_get_width(fe->printcontext);
+    pph = gtk_print_context_get_height(fe->printcontext);
+    dpmmx = gtk_print_context_get_dpi_x(fe->printcontext) / 25.4;
+    dpmmy = gtk_print_context_get_dpi_y(fe->printcontext) / 25.4;
+
+    /*
+     * Compute the puzzle's position in pixels on the logical page.
+     */
+    pox = xm * ppw + xc * dpmmx;
+    poy = ym * pph + yc * dpmmy;
+
+    /*
+     * And determine the scale.
+     *
+     * I need a scale such that the maximum puzzle-coordinate
+     * extent of the rectangle (pw * scale) is equal to the pixel
+     * equivalent of the puzzle's millimetre width (wmm * dpmmx).
+     */
+    scale = wmm * dpmmx / pw;
+
+    /*
+     * Now instruct Cairo to transform points based on our calculated
+     * values (order here *is* important).
+     */
+    cairo_save(fe->cr);
+    cairo_translate(fe->cr, pox, poy);
+    cairo_scale(fe->cr, scale, scale);
+
+    fe->hatchthick = 0.2 * pw / wmm;
+    fe->hatchspace = 1.0 * pw / wmm;
+}
+
+void gtk_end_puzzle(void *handle)
+{
+    frontend *fe = (frontend *)handle;
+    cairo_restore(fe->cr);
+}
+
+void gtk_end_page(void *handle, int number)
+{
+}
+
+void gtk_end_doc(void *handle)
+{
+}
+
+void gtk_line_width(void *handle, float width)
+{
+    frontend *fe = (frontend *)handle;
+    cairo_set_line_width(fe->cr, width);
+}
+
+void gtk_line_dotted(void *handle, bool dotted)
+{
+    frontend *fe = (frontend *)handle;
+
+    if (dotted) {
+        const double dash = 35.0;
+        cairo_set_dash(fe->cr, &dash, 1, 0);
+    } else {
+        cairo_set_dash(fe->cr, NULL, 0, 0);
+    }
+}
+#endif /* USE_PRINTING */
+
+const struct internal_drawing_api internal_drawing = {
+    draw_set_colour,
+#ifdef USE_CAIRO
+    do_draw_fill,
+    do_draw_fill_preserve,
+#endif
+};
+
+#ifdef USE_CAIRO
+const struct internal_drawing_api internal_printing = {
+    print_set_colour,
+    do_print_fill,
+    do_print_fill_preserve,
+};
+#endif
+
 const struct drawing_api gtk_drawing = {
     gtk_draw_text,
     gtk_draw_rect,
@@ -1177,8 +1411,19 @@ const struct drawing_api gtk_drawing = {
     gtk_blitter_free,
     gtk_blitter_save,
     gtk_blitter_load,
+#ifdef USE_PRINTING
+    gtk_begin_doc,
+    gtk_begin_page,
+    gtk_begin_puzzle,
+    gtk_end_puzzle,
+    gtk_end_page,
+    gtk_end_doc,
+    gtk_line_width,
+    gtk_line_dotted,
+#else
     NULL, NULL, NULL, NULL, NULL, NULL, /* {begin,end}_{doc,page,puzzle} */
     NULL, NULL,			       /* line_width, line_dotted */
+#endif
 #ifdef USE_PANGO
     gtk_text_fallback,
 #else
@@ -1340,12 +1585,22 @@ static gint draw_area(GtkWidget *widget, cairo_t *cr, gpointer data)
     frontend *fe = (frontend *)data;
     GdkRectangle dirtyrect;
 
+    cairo_surface_t *target_surface = cairo_get_target(cr);
+    cairo_matrix_t m;
+    cairo_get_matrix(cr, &m);
+    double orig_sx, orig_sy;
+    cairo_surface_get_device_scale(target_surface, &orig_sx, &orig_sy);
+    cairo_surface_set_device_scale(target_surface, 1.0, 1.0);
+    cairo_translate(cr, m.x0 * (orig_sx - 1.0), m.y0 * (orig_sy - 1.0));
+
     gdk_cairo_get_clip_rectangle(cr, &dirtyrect);
     cairo_set_source_surface(cr, fe->image, fe->ox, fe->oy);
     cairo_rectangle(cr, dirtyrect.x, dirtyrect.y,
                     dirtyrect.width, dirtyrect.height);
     cairo_fill(cr);
 
+    cairo_surface_set_device_scale(target_surface, orig_sx, orig_sy);
+
     return true;
 }
 #else
@@ -1390,16 +1645,22 @@ static gint map_window(GtkWidget *widget, GdkEvent *event,
 static void resize_puzzle_to_area(frontend *fe, int x, int y)
 {
     int oldw = fe->w, oldpw = fe->pw, oldh = fe->h, oldph = fe->ph;
+    int oldps = fe->ps;
 
     fe->w = x;
     fe->h = y;
     midend_size(fe->me, &x, &y, true);
     fe->pw = x;
     fe->ph = y;
+#if GTK_CHECK_VERSION(3,10,0)
+    fe->ps = gtk_widget_get_scale_factor(fe->area);
+#else
+    fe->ps = 1;
+#endif
     fe->ox = (fe->w - fe->pw) / 2;
     fe->oy = (fe->h - fe->ph) / 2;
 
-    if (oldw != fe->w || oldpw != fe->pw ||
+    if (oldw != fe->w || oldpw != fe->pw || oldps != fe->ps ||
         oldh != fe->h || oldph != fe->ph || !backing_store_ok(fe)) {
         if (backing_store_ok(fe))
             teardown_backing_store(fe);
@@ -1413,6 +1674,7 @@ static gint configure_area(GtkWidget *widget,
                            GdkEventConfigure *event, gpointer data)
 {
     frontend *fe = (frontend *)data;
+
     resize_puzzle_to_area(fe, event->width, event->height);
 #if GTK_CHECK_VERSION(3,0,0)
     fe->awaiting_resize_ack = false;
@@ -2245,6 +2507,317 @@ static char *file_selector(frontend *fe, const char *title, bool save)
 
 #endif
 
+#ifdef USE_PRINTING
+GObject *create_print_widget(GtkPrintOperation *print, gpointer data)
+{
+    GtkLabel *count_label, *width_label, *height_label,
+        *scale_llabel, *scale_rlabel;
+    GtkBox *scale_hbox;
+    GtkWidget *grid;
+    frontend *fe = (frontend *)data;
+
+    fe->printcount_spin_button =
+        GTK_SPIN_BUTTON(gtk_spin_button_new_with_range(1, 999, 1));
+    gtk_spin_button_set_numeric(fe->printcount_spin_button, true);
+    gtk_spin_button_set_snap_to_ticks(fe->printcount_spin_button, true);
+    fe->printw_spin_button =
+        GTK_SPIN_BUTTON(gtk_spin_button_new_with_range(1, 99, 1));
+    gtk_spin_button_set_numeric(fe->printw_spin_button, true);
+    gtk_spin_button_set_snap_to_ticks(fe->printw_spin_button, true);
+    fe->printh_spin_button =
+        GTK_SPIN_BUTTON(gtk_spin_button_new_with_range(1, 99, 1));
+    gtk_spin_button_set_numeric(fe->printh_spin_button, true);
+    gtk_spin_button_set_snap_to_ticks(fe->printh_spin_button, true);
+    fe->printscale_spin_button =
+        GTK_SPIN_BUTTON(gtk_spin_button_new_with_range(1, 1000, 1));
+    gtk_spin_button_set_digits(fe->printscale_spin_button, 1);
+    gtk_spin_button_set_numeric(fe->printscale_spin_button, true);
+    if (thegame.can_solve) {
+        fe->soln_check_button =
+            GTK_CHECK_BUTTON(
+                gtk_check_button_new_with_label("Print solutions"));
+    }
+    if (thegame.can_print_in_colour) {
+        fe->colour_check_button =
+            GTK_CHECK_BUTTON(
+                gtk_check_button_new_with_label("Print in color"));
+    }
+
+    /* Set defaults to what was selected last time. */
+    gtk_spin_button_set_value(fe->printcount_spin_button,
+                              (gdouble)fe->printcount);
+    gtk_spin_button_set_value(fe->printw_spin_button,
+                              (gdouble)fe->printw);
+    gtk_spin_button_set_value(fe->printh_spin_button,
+                              (gdouble)fe->printh);
+    gtk_spin_button_set_value(fe->printscale_spin_button,
+                              (gdouble)fe->printscale);
+    if (thegame.can_solve) {
+        gtk_toggle_button_set_active(
+            GTK_TOGGLE_BUTTON(fe->soln_check_button), fe->printsolns);
+    }
+    if (thegame.can_print_in_colour) {
+        gtk_toggle_button_set_active(
+            GTK_TOGGLE_BUTTON(fe->colour_check_button), fe->printcolour);
+    }
+
+    count_label = GTK_LABEL(gtk_label_new("Puzzles to print:"));
+    width_label = GTK_LABEL(gtk_label_new("Puzzles across:"));
+    height_label = GTK_LABEL(gtk_label_new("Puzzles down:"));
+    scale_llabel = GTK_LABEL(gtk_label_new("Puzzle scale:"));
+    scale_rlabel = GTK_LABEL(gtk_label_new("%"));
+#if GTK_CHECK_VERSION(3,0,0)
+    gtk_widget_set_halign(GTK_WIDGET(count_label), GTK_ALIGN_START);
+    gtk_widget_set_halign(GTK_WIDGET(width_label), GTK_ALIGN_START);
+    gtk_widget_set_halign(GTK_WIDGET(height_label), GTK_ALIGN_START);
+    gtk_widget_set_halign(GTK_WIDGET(scale_llabel), GTK_ALIGN_START);
+#else
+    gtk_misc_set_alignment(GTK_MISC(count_label), 0, 0);
+    gtk_misc_set_alignment(GTK_MISC(width_label), 0, 0);
+    gtk_misc_set_alignment(GTK_MISC(height_label), 0, 0);
+    gtk_misc_set_alignment(GTK_MISC(scale_llabel), 0, 0);
+#endif
+
+    scale_hbox = GTK_BOX(gtk_hbox_new(false, 6));
+    gtk_box_pack_start(scale_hbox, GTK_WIDGET(fe->printscale_spin_button),
+                       false, false, 0);
+    gtk_box_pack_start(scale_hbox, GTK_WIDGET(scale_rlabel),
+                       false, false, 0);
+
+#if GTK_CHECK_VERSION(3,0,0)
+    grid = gtk_grid_new();
+    gtk_grid_set_column_spacing(GTK_GRID(grid), 18);
+    gtk_grid_set_row_spacing(GTK_GRID(grid), 18);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(count_label), 0, 0, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(width_label), 0, 1, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(height_label), 0, 2, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(scale_llabel), 0, 3, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(fe->printcount_spin_button),
+		    1, 0, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(fe->printw_spin_button),
+		    1, 1, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(fe->printh_spin_button),
+		    1, 2, 1, 1);
+    gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(scale_hbox), 1, 3, 1, 1);
+    if (thegame.can_solve) {
+        gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(fe->soln_check_button),
+                        0, 4, 1, 1);
+    }
+    if (thegame.can_print_in_colour) {
+        gtk_grid_attach(GTK_GRID(grid), GTK_WIDGET(fe->colour_check_button),
+                        thegame.can_solve, 4, 1, 1);
+    }
+#else
+    grid = gtk_table_new((thegame.can_solve || thegame.can_print_in_colour) ?
+                         5 : 4, 2, false);
+    gtk_table_set_col_spacings(GTK_TABLE(grid), 18);
+    gtk_table_set_row_spacings(GTK_TABLE(grid), 18);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(count_label), 0, 1, 0, 1,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(width_label), 0, 1, 1, 2,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(height_label), 0, 1, 2, 3,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(scale_llabel), 0, 1, 3, 4,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(fe->printcount_spin_button),
+                     1, 2, 0, 1,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(fe->printw_spin_button),
+                     1, 2, 1, 2,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(fe->printh_spin_button),
+                     1, 2, 2, 3,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(scale_hbox), 1, 2, 3, 4,
+                     GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    if (thegame.can_solve) {
+        gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(fe->soln_check_button),
+                         0, 1, 4, 5,
+                         GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    }
+    if (thegame.can_print_in_colour) {
+        gtk_table_attach(GTK_TABLE(grid), GTK_WIDGET(fe->colour_check_button),
+                         thegame.can_solve, thegame.can_solve + 1, 4, 5,
+                         GTK_SHRINK | GTK_FILL, GTK_SHRINK | GTK_FILL, 0, 0);
+    }
+#endif
+    gtk_container_set_border_width(GTK_CONTAINER(grid), 12);
+
+    gtk_widget_show_all(grid);
+
+    return G_OBJECT(grid);
+}
+
+void apply_print_widget(GtkPrintOperation *print,
+                        GtkWidget *widget, gpointer data)
+{
+    frontend *fe = (frontend *)data;
+
+    /* We ignore `widget' because it is easier and faster to store the
+       widgets we need in `fe' then to get the children of `widget'. */
+    fe->printcount =
+        gtk_spin_button_get_value_as_int(fe->printcount_spin_button);
+    fe->printw = gtk_spin_button_get_value_as_int(fe->printw_spin_button);
+    fe->printh = gtk_spin_button_get_value_as_int(fe->printh_spin_button);
+    fe->printscale = gtk_spin_button_get_value(fe->printscale_spin_button);
+    if (thegame.can_solve) {
+        fe->printsolns =
+            gtk_toggle_button_get_active(
+                GTK_TOGGLE_BUTTON(fe->soln_check_button));
+    }
+    if (thegame.can_print_in_colour) {
+        fe->printcolour =
+            gtk_toggle_button_get_active(
+                GTK_TOGGLE_BUTTON(fe->colour_check_button));
+    }
+}
+
+void print_begin(GtkPrintOperation *printop,
+                 GtkPrintContext *context, gpointer data)
+{
+    frontend *fe = (frontend *)data;
+    midend *nme = NULL;  /* non-interactive midend for bulk puzzle generation */
+    int i;
+
+    fe->printcontext = context;
+    fe->cr = gtk_print_context_get_cairo_context(context);
+
+    /*
+     * Create our document structure and fill it up with puzzles.
+     */
+    fe->doc = document_new(fe->printw, fe->printh, fe->printscale / 100.0F);
+
+    for (i = 0; i < fe->printcount; i++) {
+        const char *err;
+
+        if (i == 0) {
+            err = midend_print_puzzle(fe->me, fe->doc, fe->printsolns);
+        } else {
+	    if (!nme) {
+		game_params *params;
+
+		nme = midend_new(NULL, &thegame, NULL, NULL);
+
+		/*
+		 * Set the non-interactive mid-end to have the same
+		 * parameters as the standard one.
+		 */
+		params = midend_get_params(fe->me);
+		midend_set_params(nme, params);
+		thegame.free_params(params);
+	    }
+
+            midend_new_game(nme);
+            err = midend_print_puzzle(nme, fe->doc, fe->printsolns);
+        }
+
+        if (err) {
+            error_box(fe->window, err);
+            return;
+        }
+    }
+
+    if (nme)
+        midend_free(nme);
+
+    /* Begin the document. */
+    document_begin(fe->doc, fe->print_dr);
+}
+
+void draw_page(GtkPrintOperation *printop,
+               GtkPrintContext *context,
+               gint page_nr, gpointer data)
+{
+    frontend *fe = (frontend *)data;
+    document_print_page(fe->doc, fe->print_dr, page_nr);
+}
+
+void print_end(GtkPrintOperation *printop,
+               GtkPrintContext *context, gpointer data)
+{
+    frontend *fe = (frontend *)data;
+
+    /* End and free the document. */
+    document_end(fe->doc, fe->print_dr);
+    document_free(fe->doc);
+    fe->doc = NULL;
+}
+
+static void print_dialog(frontend *fe)
+{
+    GError *error;
+    static GtkPrintSettings *settings = NULL;
+    static GtkPageSetup *page_setup = NULL;
+#ifndef USE_EMBED_PAGE_SETUP
+    GtkPageSetup *new_page_setup;
+#endif
+
+    fe->printop = gtk_print_operation_new();
+    gtk_print_operation_set_use_full_page(fe->printop, true);
+    gtk_print_operation_set_custom_tab_label(fe->printop, "Puzzle Settings");
+    g_signal_connect(fe->printop, "create-custom-widget",
+                     G_CALLBACK(create_print_widget), fe);
+    g_signal_connect(fe->printop, "custom-widget-apply",
+                     G_CALLBACK(apply_print_widget), fe);
+    g_signal_connect(fe->printop, "begin-print", G_CALLBACK(print_begin), fe);
+    g_signal_connect(fe->printop, "draw-page", G_CALLBACK(draw_page), fe);
+    g_signal_connect(fe->printop, "end-print", G_CALLBACK(print_end), fe);
+#ifdef USE_EMBED_PAGE_SETUP
+    gtk_print_operation_set_embed_page_setup(fe->printop, true);
+#else
+    if (page_setup == NULL) {
+        page_setup =
+            g_object_ref(
+                gtk_print_operation_get_default_page_setup(fe->printop));
+    }
+    if (settings == NULL) {
+        settings =
+            g_object_ref(gtk_print_operation_get_print_settings(fe->printop));
+    }
+    new_page_setup = gtk_print_run_page_setup_dialog(GTK_WINDOW(fe->window),
+                                                     page_setup, settings);
+    g_object_unref(page_setup);
+    page_setup = new_page_setup;
+    gtk_print_operation_set_default_page_setup(fe->printop, page_setup);
+#endif
+
+    if (settings != NULL)
+        gtk_print_operation_set_print_settings(fe->printop, settings);
+    if (page_setup != NULL)
+        gtk_print_operation_set_default_page_setup(fe->printop, page_setup);
+
+    switch (gtk_print_operation_run(fe->printop,
+                                    GTK_PRINT_OPERATION_ACTION_PRINT_DIALOG,
+                                    GTK_WINDOW(fe->window), &error)) {
+    case GTK_PRINT_OPERATION_RESULT_ERROR:
+        error_box(fe->window, error->message);
+        g_error_free(error);
+        break;
+    case GTK_PRINT_OPERATION_RESULT_APPLY:
+        if (settings != NULL)
+            g_object_unref(settings);
+        settings =
+            g_object_ref(gtk_print_operation_get_print_settings(fe->printop));
+#ifdef USE_EMBED_PAGE_SETUP
+        if (page_setup != NULL)
+            g_object_unref(page_setup);
+        page_setup =
+            g_object_ref(
+                gtk_print_operation_get_default_page_setup(fe->printop));
+#endif
+        break;
+    default:
+        /* Don't error out on -Werror=switch. */
+        break;
+    }
+
+    g_object_unref(fe->printop);
+    fe->printop = NULL;
+    fe->printcontext = NULL;
+}
+#endif /* USE_PRINTING */
+
 struct savefile_write_ctx {
     FILE *fp;
     int error;
@@ -2346,6 +2919,15 @@ static void menu_load_event(GtkMenuItem *menuitem, gpointer data)
     }
 }
 
+#ifdef USE_PRINTING
+static void menu_print_event(GtkMenuItem *menuitem, gpointer data)
+{
+    frontend *fe = (frontend *)data;
+
+    print_dialog(fe);
+}
+#endif
+
 static void menu_solve_event(GtkMenuItem *menuitem, gpointer data)
 {
     frontend *fe = (frontend *)data;
@@ -2388,18 +2970,31 @@ static void menu_about_event(GtkMenuItem *menuitem, gpointer data)
     frontend *fe = (frontend *)data;
 
 #if GTK_CHECK_VERSION(3,0,0)
+# define ABOUT_PARAMS                                               \
+    "program-name", thegame.name,                                   \
+    "version", ver,                                                 \
+    "comments", "Part of Simon Tatham's Portable Puzzle Collection"
+
     extern char *const *const xpm_icons[];
     extern const int n_xpm_icons;
-    GdkPixbuf *icon = gdk_pixbuf_new_from_xpm_data
-        ((const gchar **)xpm_icons[n_xpm_icons-1]);
-    gtk_show_about_dialog
-        (GTK_WINDOW(fe->window),
-         "program-name", thegame.name,
-         "version", ver,
-         "comments", "Part of Simon Tatham's Portable Puzzle Collection",
-         "logo", icon,
-         (const gchar *)NULL);
-    g_object_unref(G_OBJECT(icon));
+
+    if (n_xpm_icons) {
+        GdkPixbuf *icon = gdk_pixbuf_new_from_xpm_data
+            ((const gchar **)xpm_icons[n_xpm_icons-1]);
+
+        gtk_show_about_dialog
+            (GTK_WINDOW(fe->window),
+             ABOUT_PARAMS,
+             "logo", icon,
+             (const gchar *)NULL);
+        g_object_unref(G_OBJECT(icon));
+    }
+    else {
+        gtk_show_about_dialog
+            (GTK_WINDOW(fe->window),
+             ABOUT_PARAMS,
+             (const gchar *)NULL);
+    }
 #else
     char titlebuf[256];
     char textbuf[1024];
@@ -2489,6 +3084,9 @@ static frontend *new_window(
     char *arg, int argtype, char **error, bool headless)
 {
     frontend *fe;
+#ifdef USE_PRINTING
+    frontend *print_fe = NULL;
+#endif
     GtkBox *vbox, *hbox;
     GtkWidget *menu, *menuitem;
     GList *iconlist;
@@ -2501,13 +3099,14 @@ static frontend *new_window(
     fe = snew(frontend);
     memset(fe, 0, sizeof(frontend));
 
-#if !GTK_CHECK_VERSION(3,0,0)
+#ifndef USE_CAIRO
     if (headless) {
-        fprintf(stderr, "headless mode not supported below GTK 3\n");
+        fprintf(stderr, "headless mode not supported for non-Cairo drawing\n");
         exit(1);
     }
 #else
     fe->headless = headless;
+    fe->ps = 1; /* in headless mode, configure_area won't have set this */
 #endif
 
     fe->timer_active = false;
@@ -2515,6 +3114,32 @@ static frontend *new_window(
 
     fe->me = midend_new(fe, &thegame, &gtk_drawing, fe);
 
+    fe->dr_api = &internal_drawing;
+
+#ifdef USE_PRINTING
+    if (thegame.can_print) {
+        print_fe = snew(frontend);
+        memset(print_fe, 0, sizeof(frontend));
+
+        /* Defaults */
+        print_fe->printcount = print_fe->printw = print_fe->printh = 1;
+        print_fe->printscale = 100;
+        print_fe->printsolns = false;
+        print_fe->printcolour = thegame.can_print_in_colour;
+
+        /*
+         * We need to use the same midend as the main frontend because
+         * we need midend_print_puzzle() to be able to print the
+         * current puzzle.
+         */
+        print_fe->me = fe->me;
+
+        print_fe->print_dr = drawing_new(&gtk_drawing, print_fe->me, print_fe);
+
+        print_fe->dr_api = &internal_printing;
+    }
+#endif
+
     if (arg) {
 	const char *err;
 	FILE *fp;
@@ -2568,6 +3193,12 @@ static frontend *new_window(
 	    *error = dupstr(errbuf);
 	    midend_free(fe->me);
 	    sfree(fe);
+#ifdef USE_PRINTING
+            if (thegame.can_print) {
+                drawing_free(print_fe->print_dr);
+                sfree(print_fe);
+            }
+#endif
 	    return NULL;
 	}
 
@@ -2711,6 +3342,16 @@ static frontend *new_window(
     g_signal_connect(G_OBJECT(menuitem), "activate",
                      G_CALLBACK(menu_save_event), fe);
     gtk_widget_show(menuitem);
+#ifdef USE_PRINTING
+    if (thegame.can_print) {
+        add_menu_separator(GTK_CONTAINER(menu));
+        menuitem = gtk_menu_item_new_with_label("Print...");
+        gtk_container_add(GTK_CONTAINER(menu), menuitem);
+        g_signal_connect(G_OBJECT(menuitem), "activate",
+                         G_CALLBACK(menu_print_event), print_fe);
+        gtk_widget_show(menuitem);
+    }
+#endif
 #ifndef STYLUS_BASED
     add_menu_separator(GTK_CONTAINER(menu));
     add_menu_ui_item(fe, GTK_CONTAINER(menu), "Undo", UI_UNDO, 'u', 0);
diff --git a/apps/plugins/puzzles/src/keen.c b/apps/plugins/puzzles/src/keen.c
index baa1d81802..70e3e5432c 100644
--- a/apps/plugins/puzzles/src/keen.c
+++ b/apps/plugins/puzzles/src/keen.c
@@ -591,6 +591,92 @@ static int solver_hard(struct latin_solver *solver, void *vctx)
 #define SOLVER(upper,title,func,lower) func,
 static usersolver_t const keen_solvers[] = { DIFFLIST(SOLVER) };
 
+static int transpose(int index, int w)
+{
+    return (index % w) * w + (index / w);
+}
+
+static bool keen_valid(struct latin_solver *solver, void *vctx)
+{
+    struct solver_ctx *ctx = (struct solver_ctx *)vctx;
+    int w = ctx->w;
+    int box, i;
+
+    /*
+     * Iterate over each clue box and check it's satisfied.
+     */
+    for (box = 0; box < ctx->nboxes; box++) {
+	int *sq = ctx->boxlist + ctx->boxes[box];
+	int n = ctx->boxes[box+1] - ctx->boxes[box];
+	long value = ctx->clues[box] & ~CMASK;
+	long op = ctx->clues[box] & CMASK;
+        bool fail = false;
+
+        switch (op) {
+          case C_ADD: {
+            long sum = 0;
+            for (i = 0; i < n; i++)
+                sum += solver->grid[transpose(sq[i], w)];
+            fail = (sum != value);
+            break;
+          }
+
+          case C_MUL: {
+            long remaining = value;
+            for (i = 0; i < n; i++) {
+                if (remaining % solver->grid[transpose(sq[i], w)]) {
+                    fail = true;
+                    break;
+                }
+                remaining /= solver->grid[transpose(sq[i], w)];
+            }
+            if (remaining != 1)
+                fail = true;
+            break;
+          }
+
+          case C_SUB:
+            assert(n == 2);
+            if (value != labs(solver->grid[transpose(sq[0], w)] -
+                              solver->grid[transpose(sq[1], w)]))
+                fail = true;
+            break;
+
+          case C_DIV: {
+            int num, den;
+            assert(n == 2);
+            num = max(solver->grid[transpose(sq[0], w)],
+                      solver->grid[transpose(sq[1], w)]);
+            den = min(solver->grid[transpose(sq[0], w)],
+                      solver->grid[transpose(sq[1], w)]);
+            if (den * value != num)
+                fail = true;
+            break;
+          }
+        }
+
+        if (fail) {
+#ifdef STANDALONE_SOLVER
+	    if (solver_show_working) {
+		printf("%*sclue at (%d,%d) is violated\n",
+                       solver_recurse_depth*4, "",
+                       sq[0]/w+1, sq[0]%w+1);
+		printf("%*s  (%s clue with target %ld containing [",
+                       solver_recurse_depth*4, "",
+                       (op == C_ADD ? "addition" : op == C_SUB ? "subtraction":
+                        op == C_MUL ? "multiplication" : "division"), value);
+                for (i = 0; i < n; i++)
+                    printf(" %d", (int)solver->grid[transpose(sq[i], w)]);
+                printf(" ]\n");
+            }
+#endif
+            return false;
+        }
+    }
+
+    return true;
+}
+
 static int solver(int w, int *dsf, long *clues, digit *soln, int maxdiff)
 {
     int a = w*w;
@@ -638,7 +724,7 @@ static int solver(int w, int *dsf, long *clues, digit *soln, int maxdiff)
     ret = latin_solver(soln, w, maxdiff,
 		       DIFF_EASY, DIFF_HARD, DIFF_EXTREME,
 		       DIFF_EXTREME, DIFF_UNREASONABLE,
-		       keen_solvers, &ctx, NULL, NULL);
+		       keen_solvers, keen_valid, &ctx, NULL, NULL);
 
     sfree(ctx.dscratch);
     sfree(ctx.iscratch);
diff --git a/apps/plugins/puzzles/src/latin.c b/apps/plugins/puzzles/src/latin.c
index 9d06ccd938..39930166e7 100644
--- a/apps/plugins/puzzles/src/latin.c
+++ b/apps/plugins/puzzles/src/latin.c
@@ -19,8 +19,8 @@
 static int latin_solver_top(struct latin_solver *solver, int maxdiff,
 			    int diff_simple, int diff_set_0, int diff_set_1,
 			    int diff_forcing, int diff_recursive,
-			    usersolver_t const *usersolvers, void *ctx,
-			    ctxnew_t ctxnew, ctxfree_t ctxfree);
+			    usersolver_t const *usersolvers, validator_t valid,
+                            void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree);
 
 #ifdef STANDALONE_SOLVER
 int solver_show_working, solver_recurse_depth;
@@ -711,7 +711,7 @@ int latin_solver_diff_set(struct latin_solver *solver,
 static int latin_solver_recurse
     (struct latin_solver *solver, int diff_simple, int diff_set_0,
      int diff_set_1, int diff_forcing, int diff_recursive,
-     usersolver_t const *usersolvers, void *ctx,
+     usersolver_t const *usersolvers, validator_t valid, void *ctx,
      ctxnew_t ctxnew, ctxfree_t ctxfree)
 {
     int best, bestcount;
@@ -817,7 +817,8 @@ static int latin_solver_recurse
             ret = latin_solver_top(&subsolver, diff_recursive,
 				   diff_simple, diff_set_0, diff_set_1,
 				   diff_forcing, diff_recursive,
-				   usersolvers, newctx, ctxnew, ctxfree);
+				   usersolvers, valid, newctx,
+                                   ctxnew, ctxfree);
 	    latin_solver_free(&subsolver);
 	    if (ctxnew)
 		ctxfree(newctx);
@@ -879,8 +880,8 @@ static int latin_solver_recurse
 static int latin_solver_top(struct latin_solver *solver, int maxdiff,
 			    int diff_simple, int diff_set_0, int diff_set_1,
 			    int diff_forcing, int diff_recursive,
-			    usersolver_t const *usersolvers, void *ctx,
-			    ctxnew_t ctxnew, ctxfree_t ctxfree)
+			    usersolver_t const *usersolvers, validator_t valid,
+                            void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree)
 {
     struct latin_solver_scratch *scratch = latin_solver_new_scratch(solver);
     int ret, diff = diff_simple;
@@ -941,7 +942,8 @@ static int latin_solver_top(struct latin_solver *solver, int maxdiff,
         int nsol = latin_solver_recurse(solver,
 					diff_simple, diff_set_0, diff_set_1,
 					diff_forcing, diff_recursive,
-					usersolvers, ctx, ctxnew, ctxfree);
+					usersolvers, valid, ctx,
+                                        ctxnew, ctxfree);
         if (nsol < 0) diff = diff_impossible;
         else if (nsol == 1) diff = diff_recursive;
         else if (nsol > 1) diff = diff_ambiguous;
@@ -990,6 +992,17 @@ static int latin_solver_top(struct latin_solver *solver, int maxdiff,
     }
 #endif
 
+    if (diff != diff_impossible && diff != diff_unfinished &&
+        diff != diff_ambiguous && valid && !valid(solver, ctx)) {
+#ifdef STANDALONE_SOLVER
+        if (solver_show_working) {
+            printf("%*ssolution failed final validation!\n",
+                   solver_recurse_depth*4, "");
+        }
+#endif
+        diff = diff_impossible;
+    }
+
     latin_solver_free_scratch(scratch);
 
     return diff;
@@ -998,8 +1011,8 @@ static int latin_solver_top(struct latin_solver *solver, int maxdiff,
 int latin_solver_main(struct latin_solver *solver, int maxdiff,
 		      int diff_simple, int diff_set_0, int diff_set_1,
 		      int diff_forcing, int diff_recursive,
-		      usersolver_t const *usersolvers, void *ctx,
-		      ctxnew_t ctxnew, ctxfree_t ctxfree)
+		      usersolver_t const *usersolvers, validator_t valid,
+                      void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree)
 {
     int diff;
 #ifdef STANDALONE_SOLVER
@@ -1027,7 +1040,7 @@ int latin_solver_main(struct latin_solver *solver, int maxdiff,
     diff = latin_solver_top(solver, maxdiff,
 			    diff_simple, diff_set_0, diff_set_1,
 			    diff_forcing, diff_recursive,
-			    usersolvers, ctx, ctxnew, ctxfree);
+			    usersolvers, valid, ctx, ctxnew, ctxfree);
 
 #ifdef STANDALONE_SOLVER
     sfree(names);
@@ -1040,8 +1053,8 @@ int latin_solver_main(struct latin_solver *solver, int maxdiff,
 int latin_solver(digit *grid, int o, int maxdiff,
 		 int diff_simple, int diff_set_0, int diff_set_1,
 		 int diff_forcing, int diff_recursive,
-		 usersolver_t const *usersolvers, void *ctx,
-		 ctxnew_t ctxnew, ctxfree_t ctxfree)
+		 usersolver_t const *usersolvers, validator_t valid,
+                 void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree)
 {
     struct latin_solver solver;
     int diff;
@@ -1050,7 +1063,7 @@ int latin_solver(digit *grid, int o, int maxdiff,
     diff = latin_solver_main(&solver, maxdiff,
 			     diff_simple, diff_set_0, diff_set_1,
 			     diff_forcing, diff_recursive,
-			     usersolvers, ctx, ctxnew, ctxfree);
+			     usersolvers, valid, ctx, ctxnew, ctxfree);
     latin_solver_free(&solver);
     return diff;
 }
diff --git a/apps/plugins/puzzles/src/latin.h b/apps/plugins/puzzles/src/latin.h
index ff6f07c922..bb172ec3c7 100644
--- a/apps/plugins/puzzles/src/latin.h
+++ b/apps/plugins/puzzles/src/latin.h
@@ -85,6 +85,7 @@ int latin_solver_diff_set(struct latin_solver *solver,
                           bool extreme);
 
 typedef int (*usersolver_t)(struct latin_solver *solver, void *ctx);
+typedef bool (*validator_t)(struct latin_solver *solver, void *ctx);
 typedef void *(*ctxnew_t)(void *ctx);
 typedef void (*ctxfree_t)(void *ctx);
 
@@ -96,15 +97,15 @@ enum { diff_impossible = 10, diff_ambiguous, diff_unfinished };
 int latin_solver(digit *grid, int o, int maxdiff,
 		 int diff_simple, int diff_set_0, int diff_set_1,
 		 int diff_forcing, int diff_recursive,
-		 usersolver_t const *usersolvers, void *ctx,
-		 ctxnew_t ctxnew, ctxfree_t ctxfree);
+		 usersolver_t const *usersolvers, validator_t valid,
+                 void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree);
 
 /* Version you can call if you want to alloc and free latin_solver yourself */
 int latin_solver_main(struct latin_solver *solver, int maxdiff,
 		      int diff_simple, int diff_set_0, int diff_set_1,
 		      int diff_forcing, int diff_recursive,
-		      usersolver_t const *usersolvers, void *ctx,
-		      ctxnew_t ctxnew, ctxfree_t ctxfree);
+		      usersolver_t const *usersolvers, validator_t valid,
+                      void *ctx, ctxnew_t ctxnew, ctxfree_t ctxfree);
 
 void latin_solver_debug(unsigned char *cube, int o);
 
diff --git a/apps/plugins/puzzles/src/mines.c b/apps/plugins/puzzles/src/mines.c
index c9d9852573..ae717d3f37 100644
--- a/apps/plugins/puzzles/src/mines.c
+++ b/apps/plugins/puzzles/src/mines.c
@@ -258,6 +258,8 @@ static const char *validate_params(const game_params *params, bool full)
      */
     if (full && params->unique && (params->w <= 2 || params->h <= 2))
 	return "Width and height must both be greater than two";
+    if (params->n < 0)
+	return "Mine count may not be negative";
     if (params->n > params->w * params->h - 9)
 	return "Too many mines for grid size";
     if (params->n < 1)
diff --git a/apps/plugins/puzzles/src/pattern.c b/apps/plugins/puzzles/src/pattern.c
index 42f3fc55ce..a43982f452 100644
--- a/apps/plugins/puzzles/src/pattern.c
+++ b/apps/plugins/puzzles/src/pattern.c
@@ -20,6 +20,7 @@ enum {
     COL_GRID,
     COL_CURSOR,
     COL_ERROR,
+    COL_CURSOR_GUIDE,
     NCOLOURS
 };
 
@@ -1660,11 +1661,12 @@ static float *game_colours(frontend *fe, int *ncolours)
     frontend_default_colour(fe, &ret[COL_BACKGROUND * 3]);
 
     for (i = 0; i < 3; i++) {
-        ret[COL_GRID    * 3 + i] = 0.3F;
-        ret[COL_UNKNOWN * 3 + i] = 0.5F;
-        ret[COL_TEXT    * 3 + i] = 0.0F;
-        ret[COL_FULL    * 3 + i] = 0.0F;
-        ret[COL_EMPTY   * 3 + i] = 1.0F;
+        ret[COL_GRID         * 3 + i] = 0.3F;
+        ret[COL_UNKNOWN      * 3 + i] = 0.5F;
+        ret[COL_TEXT         * 3 + i] = 0.0F;
+        ret[COL_FULL         * 3 + i] = 0.0F;
+        ret[COL_EMPTY        * 3 + i] = 1.0F;
+        ret[COL_CURSOR_GUIDE * 3 + i] = 0.5F;
     }
     ret[COL_CURSOR * 3 + 0] = 1.0F;
     ret[COL_CURSOR * 3 + 1] = 0.25F;
@@ -1891,6 +1893,9 @@ static void game_redraw(drawing *dr, game_drawstate *ds,
      */
     for (i = 0; i < state->common->w + state->common->h; i++) {
         int colour = check_errors(state, i) ? COL_ERROR : COL_TEXT;
+        if (colour == COL_TEXT && ((cx >= 0 && i == cx) || (cy >= 0 && i == cy + ds->w))) {
+            colour = COL_CURSOR_GUIDE;
+        }
         if (ds->numcolours[i] != colour) {
             draw_numbers(dr, ds, state, i, true, colour);
             ds->numcolours[i] = colour;
diff --git a/apps/plugins/puzzles/src/printing.c b/apps/plugins/puzzles/src/printing.c
index 98fdd841d3..d1a3badaaa 100644
--- a/apps/plugins/puzzles/src/printing.c
+++ b/apps/plugins/puzzles/src/printing.c
@@ -3,6 +3,8 @@
  * setup and layout.
  */
 
+#include <assert.h>
+
 #include "puzzles.h"
 
 struct puzzle {
@@ -88,7 +90,7 @@ void document_add_puzzle(document *doc, const game *game, game_params *par,
 	doc->got_solns = true;
 }
 
-static void get_puzzle_size(document *doc, struct puzzle *pz,
+static void get_puzzle_size(const document *doc, struct puzzle *pz,
 			    float *w, float *h, float *scale)
 {
     float ww, hh, ourscale;
@@ -115,133 +117,177 @@ static void get_puzzle_size(document *doc, struct puzzle *pz,
 }
 
 /*
- * Having accumulated a load of puzzles, actually do the printing.
+ * Calculate the the number of pages for a document.
  */
-void document_print(document *doc, drawing *dr)
+int document_npages(const document *doc)
 {
     int ppp;			       /* puzzles per page */
     int pages, passes;
+
+    ppp = doc->pw * doc->ph;
+    pages = (doc->npuzzles + ppp - 1) / ppp;
+    passes = (doc->got_solns ? 2 : 1);
+
+    return pages * passes;
+}
+
+/*
+ * Begin a document.
+ */
+void document_begin(const document *doc, drawing *dr)
+{
+    print_begin_doc(dr, document_npages(doc));
+}
+
+/*
+ * End a document.
+ */
+void document_end(const document *doc, drawing *dr)
+{
+    print_end_doc(dr);
+}
+
+/*
+ * Print a single page of a document.
+ */
+void document_print_page(const document *doc, drawing *dr, int page_nr)
+{
+    int ppp;			       /* puzzles per page */
+    int pages;
     int page, pass;
     int pageno;
+    int i, n, offset;
+    float colsum, rowsum;
 
     ppp = doc->pw * doc->ph;
     pages = (doc->npuzzles + ppp - 1) / ppp;
-    passes = (doc->got_solns ? 2 : 1);
 
-    print_begin_doc(dr, pages * passes);
-
-    pageno = 1;
-    for (pass = 0; pass < passes; pass++) {
-	for (page = 0; page < pages; page++) {
-	    int i, n, offset;
-	    float colsum, rowsum;
-
-	    print_begin_page(dr, pageno);
-
-	    offset = page * ppp;
-	    n = min(ppp, doc->npuzzles - offset);
-
-	    for (i = 0; i < doc->pw; i++)
-		doc->colwid[i] = 0;
-	    for (i = 0; i < doc->ph; i++)
-		doc->rowht[i] = 0;
-
-	    /*
-	     * Lay the page out by computing all the puzzle sizes.
-	     */
-	    for (i = 0; i < n; i++) {
-		struct puzzle *pz = doc->puzzles + offset + i;
-		int x = i % doc->pw, y = i / doc->pw;
-		float w, h, scale;
-
-		get_puzzle_size(doc, pz, &w, &h, &scale);
-
-		/* Update the maximum width/height of this column. */
-		doc->colwid[x] = max(doc->colwid[x], w);
-		doc->rowht[y] = max(doc->rowht[y], h);
-	    }
-
-	    /*
-	     * Add up the maximum column/row widths to get the
-	     * total amount of space used up by puzzles on the
-	     * page. We will use this to compute gutter widths.
-	     */
-	    colsum = 0.0;
-	    for (i = 0; i < doc->pw; i++)
-		colsum += doc->colwid[i];
-	    rowsum = 0.0;
-	    for (i = 0; i < doc->ph; i++)
-		rowsum += doc->rowht[i];
-
-	    /*
-	     * Now do the printing.
-	     */
-	    for (i = 0; i < n; i++) {
-		struct puzzle *pz = doc->puzzles + offset + i;
-		int x = i % doc->pw, y = i / doc->pw, j;
-		float w, h, scale, xm, xc, ym, yc;
-		int pixw, pixh, tilesize;
-
-		if (pass == 1 && !pz->st2)
-		    continue;	       /* nothing to do */
-
-		/*
-		 * The total amount of gutter space is the page
-		 * width minus colsum. This is divided into pw+1
-		 * gutters, so the amount of horizontal gutter
-		 * space appearing to the left of this puzzle
-		 * column is
-		 * 
-		 *   (width-colsum) * (x+1)/(pw+1)
-		 * = width * (x+1)/(pw+1) - (colsum * (x+1)/(pw+1))
-		 */
-		xm = (float)(x+1) / (doc->pw + 1);
-		xc = -xm * colsum;
-		/* And similarly for y. */
-		ym = (float)(y+1) / (doc->ph + 1);
-		yc = -ym * rowsum;
-
-		/*
-		 * However, the amount of space to the left of this
-		 * puzzle isn't just gutter space: we must also
-		 * count the widths of all the previous columns.
-		 */
-		for (j = 0; j < x; j++)
-		    xc += doc->colwid[j];
-		/* And similarly for rows. */
-		for (j = 0; j < y; j++)
-		    yc += doc->rowht[j];
-
-		/*
-		 * Now we adjust for this _specific_ puzzle, which
-		 * means centring it within the cell we've just
-		 * computed.
-		 */
-		get_puzzle_size(doc, pz, &w, &h, &scale);
-		xc += (doc->colwid[x] - w) / 2;
-		yc += (doc->rowht[y] - h) / 2;
-
-		/*
-		 * And now we know where and how big we want to
-		 * print the puzzle, just go ahead and do so. For
-		 * the moment I'll pick a standard pixel tile size
-		 * of 512.
-		 * 
-		 * (FIXME: would it be better to pick this value
-		 * with reference to the printer resolution? Or
-		 * permit each game to choose its own?)
-		 */
-		tilesize = 512;
-		pz->game->compute_size(pz->par, tilesize, &pixw, &pixh);
-		print_begin_puzzle(dr, xm, xc, ym, yc, pixw, pixh, w, scale);
-		pz->game->print(dr, pass == 0 ? pz->st : pz->st2, tilesize);
-		print_end_puzzle(dr);
-	    }
-
-	    print_end_page(dr, pageno);
-	    pageno++;
-	}
+    /* Get the current page, pass, and pageno based on page_nr. */
+    if (page_nr < pages) {
+        page = page_nr;
+        pass = 0;
+    }
+    else {
+        assert(doc->got_solns);
+        page = page_nr - pages;
+        pass = 1;
+    }
+    pageno = page_nr + 1;
+
+    offset = page * ppp;
+    n = min(ppp, doc->npuzzles - offset);
+
+    print_begin_page(dr, pageno);
+
+    for (i = 0; i < doc->pw; i++)
+        doc->colwid[i] = 0;
+    for (i = 0; i < doc->ph; i++)
+        doc->rowht[i] = 0;
+
+    /*
+     * Lay the page out by computing all the puzzle sizes.
+     */
+    for (i = 0; i < n; i++) {
+        struct puzzle *pz = doc->puzzles + offset + i;
+        int x = i % doc->pw, y = i / doc->pw;
+        float w, h, scale;
+
+        get_puzzle_size(doc, pz, &w, &h, &scale);
+
+        /* Update the maximum width/height of this column. */
+        doc->colwid[x] = max(doc->colwid[x], w);
+        doc->rowht[y] = max(doc->rowht[y], h);
+    }
+
+    /*
+     * Add up the maximum column/row widths to get the
+     * total amount of space used up by puzzles on the
+     * page. We will use this to compute gutter widths.
+     */
+    colsum = 0.0;
+    for (i = 0; i < doc->pw; i++)
+        colsum += doc->colwid[i];
+    rowsum = 0.0;
+    for (i = 0; i < doc->ph; i++)
+        rowsum += doc->rowht[i];
+
+    /*
+     * Now do the printing.
+     */
+    for (i = 0; i < n; i++) {
+        struct puzzle *pz = doc->puzzles + offset + i;
+        int x = i % doc->pw, y = i / doc->pw, j;
+        float w, h, scale, xm, xc, ym, yc;
+        int pixw, pixh, tilesize;
+
+        if (pass == 1 && !pz->st2)
+            continue;	       /* nothing to do */
+
+        /*
+         * The total amount of gutter space is the page
+         * width minus colsum. This is divided into pw+1
+         * gutters, so the amount of horizontal gutter
+         * space appearing to the left of this puzzle
+         * column is
+         *
+         *   (width-colsum) * (x+1)/(pw+1)
+         * = width * (x+1)/(pw+1) - (colsum * (x+1)/(pw+1))
+         */
+        xm = (float)(x+1) / (doc->pw + 1);
+        xc = -xm * colsum;
+        /* And similarly for y. */
+        ym = (float)(y+1) / (doc->ph + 1);
+        yc = -ym * rowsum;
+
+        /*
+         * However, the amount of space to the left of this
+         * puzzle isn't just gutter space: we must also
+         * count the widths of all the previous columns.
+         */
+        for (j = 0; j < x; j++)
+            xc += doc->colwid[j];
+        /* And similarly for rows. */
+        for (j = 0; j < y; j++)
+            yc += doc->rowht[j];
+
+        /*
+         * Now we adjust for this _specific_ puzzle, which
+         * means centring it within the cell we've just
+         * computed.
+         */
+        get_puzzle_size(doc, pz, &w, &h, &scale);
+        xc += (doc->colwid[x] - w) / 2;
+        yc += (doc->rowht[y] - h) / 2;
+
+        /*
+         * And now we know where and how big we want to
+         * print the puzzle, just go ahead and do so. For
+         * the moment I'll pick a standard pixel tile size
+         * of 512.
+         *
+         * (FIXME: would it be better to pick this value
+         * with reference to the printer resolution? Or
+         * permit each game to choose its own?)
+         */
+        tilesize = 512;
+        pz->game->compute_size(pz->par, tilesize, &pixw, &pixh);
+        print_begin_puzzle(dr, xm, xc, ym, yc, pixw, pixh, w, scale);
+        pz->game->print(dr, pass == 0 ? pz->st : pz->st2, tilesize);
+        print_end_puzzle(dr);
     }
 
+    print_end_page(dr, pageno);
+}
+
+/*
+ * Having accumulated a load of puzzles, actually do the printing.
+ */
+void document_print(const document *doc, drawing *dr)
+{
+    int page, pages;
+    pages = document_npages(doc);
+    print_begin_doc(dr, pages);
+    for (page = 0; page < pages; page++)
+        document_print_page(doc, dr, page);
     print_end_doc(dr);
 }
diff --git a/apps/plugins/puzzles/src/ps.c b/apps/plugins/puzzles/src/ps.c
index 94a708648a..ab8a1589f4 100644
--- a/apps/plugins/puzzles/src/ps.c
+++ b/apps/plugins/puzzles/src/ps.c
@@ -9,8 +9,6 @@
 
 #include "puzzles.h"
 
-#define ROOT2 1.414213562
-
 struct psdata {
     FILE *fp;
     bool colour;
diff --git a/apps/plugins/puzzles/src/puzzles.but b/apps/plugins/puzzles/src/puzzles.but
index e22e63d544..ee519b8aa1 100644
--- a/apps/plugins/puzzles/src/puzzles.but
+++ b/apps/plugins/puzzles/src/puzzles.but
@@ -3360,7 +3360,8 @@ This software is \i{copyright} 2004-2014 Simon Tatham.
 
 Portions copyright Richard Boulton, James Harvey, Mike Pinna, Jonas
 K\u00F6{oe}lker, Dariusz Olszewski, Michael Schierl, Lambros Lambrou,
-Bernd Schmidt, Steffen Bauer, Lennard Sprong and Rogier Goossens.
+Bernd Schmidt, Steffen Bauer, Lennard Sprong, Rogier Goossens, Michael
+Quevillon and Asher Gordon.
 
 Permission is hereby granted, free of charge, to any person
 obtaining a copy of this software and associated documentation files
diff --git a/apps/plugins/puzzles/src/puzzles.h b/apps/plugins/puzzles/src/puzzles.h
index 1732abe3e9..45ae321cc6 100644
--- a/apps/plugins/puzzles/src/puzzles.h
+++ b/apps/plugins/puzzles/src/puzzles.h
@@ -11,6 +11,7 @@
 #include <stdbool.h>
 
 #define PI 3.141592653589793238462643383279502884197169399
+#define ROOT2 1.414213562373095048801688724209698078569672
 
 #define lenof(array) ( sizeof(array) / sizeof(*(array)) )
 
@@ -530,7 +531,11 @@ document *document_new(int pw, int ph, float userscale);
 void document_free(document *doc);
 void document_add_puzzle(document *doc, const game *game, game_params *par,
 			 game_state *st, game_state *st2);
-void document_print(document *doc, drawing *dr);
+int document_npages(const document *doc);
+void document_begin(const document *doc, drawing *dr);
+void document_end(const document *doc, drawing *dr);
+void document_print_page(const document *doc, drawing *dr, int page_nr);
+void document_print(const document *doc, drawing *dr);
 
 /*
  * ps.c
diff --git a/apps/plugins/puzzles/src/towers.c b/apps/plugins/puzzles/src/towers.c
index a72cae680d..aee088fb54 100644
--- a/apps/plugins/puzzles/src/towers.c
+++ b/apps/plugins/puzzles/src/towers.c
@@ -574,6 +574,38 @@ static int solver_hard(struct latin_solver *solver, void *vctx)
 #define SOLVER(upper,title,func,lower) func,
 static usersolver_t const towers_solvers[] = { DIFFLIST(SOLVER) };
 
+static bool towers_valid(struct latin_solver *solver, void *vctx)
+{
+    struct solver_ctx *ctx = (struct solver_ctx *)vctx;
+    int w = ctx->w;
+    int c, i, n, best, clue, start, step;
+    for (c = 0; c < 4*w; c++) {
+	clue = ctx->clues[c];
+	if (!clue)
+	    continue;
+
+        STARTSTEP(start, step, c, w);
+        n = best = 0;
+        for (i = 0; i < w; i++) {
+            if (solver->grid[start+i*step] > best) {
+                best = solver->grid[start+i*step];
+                n++;
+            }
+        }
+
+        if (n != clue) {
+#ifdef STANDALONE_SOLVER
+            if (solver_show_working)
+		printf("%*sclue %s %d is violated\n",
+			solver_recurse_depth*4, "",
+			cluepos[c/w], c%w+1);
+#endif
+            return false;
+        }
+    }
+    return true;
+}
+
 static int solver(int w, int *clues, digit *soln, int maxdiff)
 {
     int ret;
@@ -589,7 +621,7 @@ static int solver(int w, int *clues, digit *soln, int maxdiff)
     ret = latin_solver(soln, w, maxdiff,
 		       DIFF_EASY, DIFF_HARD, DIFF_EXTREME,
 		       DIFF_EXTREME, DIFF_UNREASONABLE,
-		       towers_solvers, &ctx, NULL, NULL);
+		       towers_solvers, towers_valid, &ctx, NULL, NULL);
 
     sfree(ctx.iscratch);
     sfree(ctx.dscratch);
diff --git a/apps/plugins/puzzles/src/tracks.R b/apps/plugins/puzzles/src/tracks.R
index f88dfb03eb..8b0ac97e0f 100644
--- a/apps/plugins/puzzles/src/tracks.R
+++ b/apps/plugins/puzzles/src/tracks.R
@@ -8,6 +8,9 @@ tracks  : [G] WINDOWS COMMON tracks TRACKS_EXTRA tracks.res|noicon.res
 
 ALL += tracks[COMBINED] TRACKS_EXTRA
 
+trackssolver : [U] tracks[STANDALONE_SOLVER] TRACKS_EXTRA STANDALONE
+trackssolver : [C] tracks[STANDALONE_SOLVER] TRACKS_EXTRA STANDALONE
+
 !begin am gtk
 GAMES += tracks
 !end
diff --git a/apps/plugins/puzzles/src/tracks.c b/apps/plugins/puzzles/src/tracks.c
index 8f29faa0fd..924836afa9 100644
--- a/apps/plugins/puzzles/src/tracks.c
+++ b/apps/plugins/puzzles/src/tracks.c
@@ -12,6 +12,7 @@
  * #113   8x8:gCx5xAf,1,S4,2,5,4,6,2,3,4,2,5,2,S4,4,5,1
  * #114   8x8:p5fAzkAb,1,6,3,3,3,S6,2,3,5,4,S3,3,5,1,5,1
  * #115   8x8:zi9d5tAb,1,3,4,5,3,S4,2,4,2,6,2,3,6,S3,3,1
+ * #942   8x8:n5iCfAzAe,2,2,S5,5,3,5,4,5,4,5,2,S5,3,4,5,3
  */
 
 #include <stdio.h>
@@ -29,9 +30,11 @@
  * Difficulty levels. I do some macro ickery here to ensure that my
  * enum and the various forms of my name list always match up.
  */
-#define DIFFLIST(A) \
-    A(EASY,Easy,e) \
-    A(TRICKY,Tricky,t)
+#define DIFFLIST(A)                             \
+    A(EASY,Easy,e)                              \
+    A(TRICKY,Tricky,t)                          \
+    A(HARD,Hard,h)                              \
+    /* end of list */
 
 #define ENUM(upper,title,lower) DIFF_ ## upper,
 #define TITLE(upper,title,lower) #title,
@@ -65,10 +68,12 @@ static const struct game_params tracks_presets[] = {
     {10, 8, DIFF_TRICKY, 1 },
     {10, 10, DIFF_EASY, 1},
     {10, 10, DIFF_TRICKY, 1},
+    {10, 10, DIFF_HARD, 1},
     {15, 10, DIFF_EASY, 1},
     {15, 10, DIFF_TRICKY, 1},
     {15, 15, DIFF_EASY, 1},
     {15, 15, DIFF_TRICKY, 1},
+    {15, 15, DIFF_HARD, 1},
 };
 
 static bool game_fetch_preset(int i, char **name, game_params **params)
@@ -452,7 +457,7 @@ start:
     state->numbers->col_s = px;
 }
 
-static int tracks_solve(game_state *state, int diff);
+static int tracks_solve(game_state *state, int diff, int *max_diff_out);
 static void debug_state(game_state *state, const char *what);
 
 /* Clue-setting algorithm:
@@ -533,6 +538,26 @@ static game_state *copy_and_strip(const game_state *state, game_state *ret, int
     return ret;
 }
 
+#ifdef STANDALONE_SOLVER
+#include <stdarg.h>
+static FILE *solver_diagnostics_fp = NULL;
+static void solver_diagnostic(const char *fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    vfprintf(solver_diagnostics_fp, fmt, ap);
+    va_end(ap);
+    fputc('\n', solver_diagnostics_fp);
+}
+#define solverdebug(printf_params) do {         \
+        if (solver_diagnostics_fp) {            \
+            solver_diagnostic printf_params;    \
+        }                                       \
+    } while (0)
+#else
+#define solverdebug(printf_params) ((void)0)
+#endif
+
 static int solve_progress(const game_state *state) {
     int i, w = state->p.w, h = state->p.h, progress = 0;
 
@@ -575,6 +600,7 @@ static int add_clues(game_state *state, random_state *rs, int diff)
     int *positions = snewn(w*h, int), npositions = 0;
     int *nedges_previous_solve = snewn(w*h, int);
     game_state *scratch = dup_game(state);
+    int diff_used;
 
     debug_state(state, "gen: Initial board");
 
@@ -591,17 +617,13 @@ static int add_clues(game_state *state, random_state *rs, int diff)
 
     /* First, check whether the puzzle is already either too easy, or just right */
     scratch = copy_and_strip(state, scratch, -1);
-    if (diff > 0) {
-        sr = tracks_solve(scratch, diff-1);
-        if (sr < 0)
-            assert(!"Generator should not have created impossible puzzle");
-        if (sr > 0) {
-            ret = -1; /* already too easy, even without adding clues. */
-            debug(("gen:  ...already too easy, need new board."));
-            goto done;
-        }
+    sr = tracks_solve(scratch, diff, &diff_used);
+    if (diff_used < diff) {
+        ret = -1; /* already too easy, even without adding clues. */
+        debug(("gen:  ...already too easy, need new board."));
+        goto done;
     }
-    sr = tracks_solve(scratch, diff);
+
     if (sr < 0)
         assert(!"Generator should not have created impossible puzzle");
     if (sr > 0) {
@@ -629,12 +651,10 @@ static int add_clues(game_state *state, random_state *rs, int diff)
         if (check_phantom_moves(scratch))
             continue; /* adding a clue here would add phantom track */
 
-        if (diff > 0) {
-            if (tracks_solve(scratch, diff-1) > 0) {
+        if (tracks_solve(scratch, diff, &diff_used) > 0) {
+            if (diff_used < diff) {
                 continue; /* adding a clue here makes it too easy */
             }
-        }
-        if (tracks_solve(scratch, diff) > 0) {
             /* we're now soluble (and we weren't before): add this clue, and then
                start stripping clues */
             debug(("gen:  ...adding clue at (%d,%d), now soluble", i%w, i/w));
@@ -676,7 +696,7 @@ strip_clues:
         if (check_phantom_moves(scratch))
             continue; /* removing a clue here would add phantom track */
 
-        if (tracks_solve(scratch, diff) > 0) {
+        if (tracks_solve(scratch, diff, NULL) > 0) {
             debug(("gen:  ... removing clue at (%d,%d), still soluble without it", i%w, i/w));
             state->sflags[i] &= ~S_CLUE; /* still soluble without this clue. */
         }
@@ -686,6 +706,7 @@ strip_clues:
 
 done:
     sfree(positions);
+    sfree(nedges_previous_solve);
     free_game(scratch);
     return ret;
 }
@@ -780,7 +801,7 @@ newpath:
     }
     *p++ = '\0';
 
-    ret = tracks_solve(state, DIFFCOUNT);
+    ret = tracks_solve(state, DIFFCOUNT, NULL);
     assert(ret >= 0);
     free_game(state);
 
@@ -882,6 +903,10 @@ static game_state *new_game(midend *me, const game_params *params, const char *d
     return state;
 }
 
+struct solver_scratch {
+    int *dsf;
+};
+
 static int solve_set_sflag(game_state *state, int x, int y,
                            unsigned int f, const char *why)
 {
@@ -889,10 +914,10 @@ static int solve_set_sflag(game_state *state, int x, int y,
 
     if (state->sflags[i] & f)
         return 0;
-    debug(("solve: square (%d,%d) -> %s: %s",
+    solverdebug(("square (%d,%d) -> %s: %s",
            x, y, (f == S_TRACK ? "TRACK" : "NOTRACK"), why));
     if (state->sflags[i] & (f == S_TRACK ? S_NOTRACK : S_TRACK)) {
-        debug(("solve: opposite flag already set there, marking IMPOSSIBLE"));
+        solverdebug(("opposite flag already set there, marking IMPOSSIBLE"));
         state->impossible = true;
     }
     state->sflags[i] |= f;
@@ -906,11 +931,11 @@ static int solve_set_eflag(game_state *state, int x, int y, int d,
 
     if (sf & f)
         return 0;
-    debug(("solve: edge (%d,%d)/%c -> %s: %s", x, y,
+    solverdebug(("edge (%d,%d)/%c -> %s: %s", x, y,
            (d == U) ? 'U' : (d == D) ? 'D' : (d == L) ? 'L' : 'R',
            (f == S_TRACK ? "TRACK" : "NOTRACK"), why));
     if (sf & (f == E_TRACK ? E_NOTRACK : E_TRACK)) {
-        debug(("solve: opposite flag already set there, marking IMPOSSIBLE"));
+        solverdebug(("opposite flag already set there, marking IMPOSSIBLE"));
         state->impossible = true;
     }
     S_E_SET(state, x, y, d, f);
@@ -1063,7 +1088,7 @@ static int solve_check_single_sub(game_state *state, int si, int id, int n,
     if (ctrack != (target-1)) return 0;
     if (nperp > 0 || n1edge != 1) return 0;
 
-    debug(("check_single from (%d,%d): 1 match from (%d,%d)",
+    solverdebug(("check_single from (%d,%d): 1 match from (%d,%d)",
            si%w, si/w, i1edge%w, i1edge/w));
 
     /* We have a match: anything that's more than 1 away from this square
@@ -1120,12 +1145,12 @@ static int solve_check_loose_sub(game_state *state, int si, int id, int n,
     }
 
     if (nloose > (target - e2count)) {
-        debug(("check %s from (%d,%d): more loose (%d) than empty (%d), IMPOSSIBLE",
+        solverdebug(("check %s from (%d,%d): more loose (%d) than empty (%d), IMPOSSIBLE",
                what, si%w, si/w, nloose, target-e2count));
         state->impossible = true;
     }
     if (nloose > 0 && nloose == (target - e2count)) {
-        debug(("check %s from (%d,%d): nloose = empty (%d), forcing loners out.",
+        solverdebug(("check %s from (%d,%d): nloose = empty (%d), forcing loners out.",
                what, si%w, si/w, nloose));
         for (j = 0, i = si; j < n; j++, i += id) {
             if (!(state->sflags[i] & S_MARK))
@@ -1146,7 +1171,7 @@ static int solve_check_loose_sub(game_state *state, int si, int id, int n,
         }
     }
     if (nloose == 1 && (target - e2count) == 2 && nperp == 0) {
-        debug(("check %s from (%d,%d): 1 loose end, 2 empty squares, forcing parallel",
+        solverdebug(("check %s from (%d,%d): 1 loose end, 2 empty squares, forcing parallel",
                what, si%w, si/w));
         for (j = 0, i = si; j < n; j++, i += id) {
             if (!(state->sflags[i] & S_MARK))
@@ -1176,6 +1201,110 @@ static int solve_check_loose_ends(game_state *state)
     return did;
 }
 
+static void solve_check_neighbours_count(
+    game_state *state, int start, int step, int n, int clueindex,
+    bool *onefill, bool *oneempty)
+{
+    int to_fill = state->numbers->numbers[clueindex];
+    int to_empty = n - to_fill;
+    int i;
+    for (i = 0; i < n; i++) {
+        int p = start + i*step;
+        if (state->sflags[p] & S_TRACK)
+            to_fill--;
+        if (state->sflags[p] & S_NOTRACK)
+            to_empty--;
+    }
+    *onefill = (to_fill == 1);
+    *oneempty = (to_empty == 1);
+}
+
+static int solve_check_neighbours_try(game_state *state, int x, int y,
+                                      int X, int Y, bool onefill,
+                                      bool oneempty, unsigned dir,
+                                      const char *what)
+{
+    int w = state->p.w, p = y*w+x, P = Y*w+X;
+
+    /*
+     * We're given a neighbouring pair of squares p,P, with 'dir'
+     * being the direction from the former to the latter. We aim to
+     * spot situations in which, if p is a track square, then P must
+     * also be one (because p doesn't have enough free exits to avoid
+     * using the one that goes towards P).
+     *
+     * Then, if the target number of track squares on their shared
+     * row/column says that there's only one track square left to
+     * place, it can't be p, because P would have to be one too,
+     * violating the clue. So in that situation we can mark p as
+     * unfilled. Conversely, if there's only one _non_-track square
+     * left to place, it can't be P, so we can mark P as filled.
+     */
+
+    if ((state->sflags[p] | state->sflags[P]) & (S_TRACK | S_NOTRACK))
+        return 0; /* no need: we already know something about these squares */
+
+    int possible_exits_except_dir = nbits[
+        ALLDIR & ~dir & ~S_E_DIRS(state, x, y, E_NOTRACK)];
+    if (possible_exits_except_dir >= 2)
+        return 0; /* square p need not connect to P, even if it is filled */
+
+    /* OK, now we know that if p is filled, P must be filled too. */
+
+    int did = 0;
+    if (onefill) {
+        /* But at most one of them can be filled, so it can't be p. */
+        state->sflags[p] |= S_NOTRACK;
+        solverdebug(("square (%d,%d) -> NOTRACK: otherwise, that and (%d,%d) "
+                     "would make too many TRACK in %s", x, y, X, Y, what));
+        did++;
+    }
+    if (oneempty) {
+        /* Alternatively, at least one of them _must_ be filled, so P
+         * must be. */
+        state->sflags[P] |= S_TRACK;
+        solverdebug(("square (%d,%d) -> TRACK: otherwise, that and (%d,%d) "
+                     "would make too many NOTRACK in %s", X, Y, x, y, what));
+        did++;
+    }
+    return did;
+}
+
+static int solve_check_neighbours(game_state *state, bool both_ways)
+{
+    int w = state->p.w, h = state->p.h, x, y, did = 0;
+    bool onefill, oneempty;
+
+    for (x = 0; x < w; x++) {
+        solve_check_neighbours_count(state, x, w, h, x, &onefill, &oneempty);
+        if (!both_ways)
+            oneempty = false; /* disable the harder version of the deduction */
+        if (!onefill && !oneempty)
+            continue;
+        for (y = 0; y+1 < h; y++) {
+            did += solve_check_neighbours_try(state, x, y, x, y+1,
+                                              onefill, oneempty, D, "column");
+            did += solve_check_neighbours_try(state, x, y+1, x, y,
+                                              onefill, oneempty, U, "column");
+        }
+    }
+    for (y = 0; y < h; y++) {
+        solve_check_neighbours_count(state, y*w, 1, w, w+y,
+                                     &onefill, &oneempty);
+        if (!both_ways)
+            oneempty = false; /* disable the harder version of the deduction */
+        if (!onefill && !oneempty)
+            continue;
+        for (x = 0; x+1 < w; x++) {
+            did += solve_check_neighbours_try(state, x, y, x+1, y,
+                                              onefill, oneempty, R, "row");
+            did += solve_check_neighbours_try(state, x+1, y, x, y,
+                                              onefill, oneempty, L, "row");
+        }
+    }
+    return did;
+}
+
 static int solve_check_loop_sub(game_state *state, int x, int y, int dir,
                                 int *dsf, int startc, int endc)
 {
@@ -1195,7 +1324,7 @@ static int solve_check_loop_sub(game_state *state, int x, int y, int dir,
             return solve_set_eflag(state, x, y, dir, E_NOTRACK, "would close loop");
         }
         if ((ic == startc && jc == endc) || (ic == endc && jc == startc)) {
-            debug(("Adding link at (%d,%d) would join start to end", x, y));
+            solverdebug(("Adding link at (%d,%d) would join start to end", x, y));
             /* We mustn't join the start to the end if:
                - there are other bits of track that aren't attached to either end
                - the clues are not fully satisfied yet
@@ -1287,10 +1416,145 @@ static void solve_discount_edge(game_state *state, int x, int y, int d)
     solve_set_eflag(state, x, y, d, E_NOTRACK, "outer edge");
 }
 
-static int tracks_solve(game_state *state, int diff)
+static int solve_bridge_sub(game_state *state, int x, int y, int d,
+                            struct solver_scratch *sc)
+{
+    /*
+     * Imagine a graph on the squares of the grid, with an edge
+     * connecting neighbouring squares only if it's not yet known
+     * whether there's a track between them.
+     *
+     * This function is called if the edge between x,y and X,Y is a
+     * bridge in that graph: that is, it's not part of any loop in the
+     * graph, or equivalently, removing it would increase the number
+     * of connected components in the graph.
+     *
+     * In that situation, we can fill in the edge by a parity
+     * argument. Construct a closed loop of edges in the grid, all of
+     * whose states are known except this one. The track starts and
+     * ends outside this loop, so it must cross the boundary of the
+     * loop an even number of times. So if we count up how many times
+     * the track is known to cross the edges of our loop, then we can
+     * fill in the last edge in whichever way makes that number even.
+     *
+     * In fact, there's not even any need to go to the effort of
+     * constructing a _single_ closed loop. The simplest thing is to
+     * delete the bridge edge from the graph, find a connected
+     * component of the reduced graph whose boundary includes that
+     * edge, and take every edge separating that component from
+     * another. This may not lead to _exactly one_ cycle - the
+     * component could be non-simply connected and have a hole in the
+     * middle - but that doesn't matter, because the same parity
+     * constraint applies just as well with more than one disjoint
+     * loop.
+     */
+    int w = state->p.w, h = state->p.h, wh = w*h;
+    int X = x + DX(d), Y = y + DY(d);
+    int xi, yi, di;
+
+    assert(d == D || d == R);
+
+    if (!sc->dsf)
+        sc->dsf = snew_dsf(wh);
+    dsf_init(sc->dsf, wh);
+
+    for (xi = 0; xi < w; xi++) {
+        for (yi = 0; yi < h; yi++) {
+            /* We expect to have been called with X,Y either to the
+             * right of x,y or below it, not the other way round. If
+             * that were not true, the tests in this loop to exclude
+             * the bridge edge would have to be twice as annoying. */
+
+            if (yi+1 < h && !S_E_FLAGS(state, xi, yi, D) &&
+                !(xi == x && yi == y && xi == X && yi+1 == Y))
+                dsf_merge(sc->dsf, yi*w+xi, (yi+1)*w+xi);
+
+            if (xi+1 < w && !S_E_FLAGS(state, xi, yi, R) &&
+                !(xi == x && yi == y && xi+1 == X && yi == Y))
+                dsf_merge(sc->dsf, yi*w+xi, yi*w+(xi+1));
+        }
+    }
+
+    int component = dsf_canonify(sc->dsf, y*w+x);
+    int parity = 0;
+    for (xi = 0; xi < w; xi++) {
+        for (yi = 0; yi < h; yi++) {
+            if (dsf_canonify(sc->dsf, yi*w+xi) != component)
+                continue;
+            for (di = 1; di < 16; di *= 2) {
+                int Xi = xi + DX(di), Yi = yi + DY(di);
+                if ((Xi < 0 || Xi >= w || Yi < 0 || Yi >= h ||
+                     dsf_canonify(sc->dsf, Yi*w+Xi) != component) &&
+                    (S_E_DIRS(state, xi, yi, E_TRACK) & di))
+                    parity ^= 1;
+            }
+        }
+    }
+
+    solve_set_eflag(state, x, y, d, parity ? E_TRACK : E_NOTRACK, "parity");
+    return 1;
+}
+
+struct solve_bridge_neighbour_ctx {
+    game_state *state;
+    int x, y, dirs;
+};
+static int solve_bridge_neighbour(int vertex, void *vctx)
+{
+    struct solve_bridge_neighbour_ctx *ctx =
+        (struct solve_bridge_neighbour_ctx *)vctx;
+    int w = ctx->state->p.w;
+
+    if (vertex >= 0) {
+        ctx->x = vertex % w;
+        ctx->y = vertex / w;
+        ctx->dirs = ALLDIR
+            & ~S_E_DIRS(ctx->state, ctx->x, ctx->y, E_TRACK)
+            & ~S_E_DIRS(ctx->state, ctx->x, ctx->y, E_NOTRACK);
+    }
+    unsigned dir = ctx->dirs & -ctx->dirs; /* isolate lowest set bit */
+    if (!dir)
+        return -1;
+    ctx->dirs &= ~dir;
+    int xr = ctx->x + DX(dir), yr = ctx->y + DY(dir);
+    assert(0 <= xr && xr < w);
+    assert(0 <= yr && yr < ctx->state->p.h);
+    return yr * w + xr;
+}
+
+static int solve_check_bridge_parity(game_state *state,
+                                     struct solver_scratch *sc)
+{
+    int w = state->p.w, h = state->p.h, wh = w*h;
+    struct findloopstate *fls;
+    struct solve_bridge_neighbour_ctx ctx[1];
+    int x, y, did = 0;
+
+    ctx->state = state;
+    fls = findloop_new_state(wh);
+    findloop_run(fls, wh, solve_bridge_neighbour, ctx);
+
+    for (x = 0; x < w; x++) {
+        for (y = 0; y < h; y++) {
+            if (y+1 < h && !findloop_is_loop_edge(fls, y*w+x, (y+1)*w+x))
+                did += solve_bridge_sub(state, x, y, D, sc);
+            if (x+1 < w && !findloop_is_loop_edge(fls, y*w+x, y*w+(x+1)))
+                did += solve_bridge_sub(state, x, y, R, sc);
+        }
+    }
+
+    findloop_free_state(fls);
+
+    return did;
+}
+
+static int tracks_solve(game_state *state, int diff, int *max_diff_out)
 {
     int x, y, w = state->p.w, h = state->p.h;
-    bool didsth;
+    struct solver_scratch sc[1];
+    int max_diff = DIFF_EASY;
+
+    sc->dsf = NULL;
 
     debug(("solve..."));
     state->impossible = false;
@@ -1305,21 +1569,37 @@ static int tracks_solve(game_state *state, int diff)
         solve_discount_edge(state, w-1, y, R);
     }
 
-    while (1) {
-        didsth = false;
+    while (!state->impossible) {
 
-        didsth |= solve_update_flags(state);
-        didsth |= solve_count_clues(state);
-        didsth |= solve_check_loop(state);
+/* Can't use do ... while (0) because we need a 'continue' in this macro */
+#define TRY(curr_diff, funcall)                         \
+        if (diff >= (curr_diff) && (funcall)) {         \
+            if (max_diff < curr_diff)                   \
+                max_diff = curr_diff;                   \
+            continue;                                   \
+        } else ((void)0)
 
-        if (diff >= DIFF_TRICKY) {
-            didsth |= solve_check_single(state);
-            didsth |= solve_check_loose_ends(state);
-        }
+        TRY(DIFF_EASY, solve_update_flags(state));
+        TRY(DIFF_EASY, solve_count_clues(state));
+        TRY(DIFF_EASY, solve_check_loop(state));
 
-        if (!didsth || state->impossible) break;
+        TRY(DIFF_TRICKY, solve_check_single(state));
+        TRY(DIFF_TRICKY, solve_check_loose_ends(state));
+        TRY(DIFF_TRICKY, solve_check_neighbours(state, false));
+
+        TRY(DIFF_HARD, solve_check_neighbours(state, true));
+        TRY(DIFF_HARD, solve_check_bridge_parity(state, sc));
+
+#undef TRY
+
+        break;
     }
 
+    sfree(sc->dsf);
+
+    if (max_diff_out)
+        *max_diff_out = max_diff;
+
     return state->impossible ? -1 : check_completion(state, false) ? 1 : 0;
 }
 
@@ -1379,11 +1659,11 @@ static char *solve_game(const game_state *state, const game_state *currstate,
     char *move;
 
     solved = dup_game(currstate);
-    ret = tracks_solve(solved, DIFFCOUNT);
+    ret = tracks_solve(solved, DIFFCOUNT, NULL);
     if (ret < 1) {
         free_game(solved);
         solved = dup_game(state);
-        ret = tracks_solve(solved, DIFFCOUNT);
+        ret = tracks_solve(solved, DIFFCOUNT, NULL);
     }
 
     if (ret < 1) {
@@ -2094,7 +2374,7 @@ static game_state *execute_move(const game_state *state, const char *move)
                 goto badmove;
             move += n;
         } else if (c == 'H') {
-            tracks_solve(ret, DIFFCOUNT);
+            tracks_solve(ret, DIFFCOUNT, NULL);
             move++;
         } else {
             goto badmove;
@@ -2675,4 +2955,87 @@ const struct game thegame = {
     0,				       /* flags */
 };
 
+#ifdef STANDALONE_SOLVER
+
+int main(int argc, char **argv)
+{
+    game_params *p;
+    game_state *s;
+    char *id = NULL, *desc;
+    int maxdiff = DIFFCOUNT, diff_used;
+    const char *err;
+    bool diagnostics = false, grade = false;
+    int retd;
+
+    while (--argc > 0) {
+        char *p = *++argv;
+        if (!strcmp(p, "-v")) {
+            diagnostics = true;
+        } else if (!strcmp(p, "-g")) {
+            grade = true;
+        } else if (!strncmp(p, "-d", 2) && p[2] && !p[3]) {
+            int i;
+            bool bad = true;
+            for (i = 0; i < lenof(tracks_diffchars); i++)
+                if (tracks_diffchars[i] == p[2]) {
+                    bad = false;
+                    maxdiff = i;
+                    break;
+                }
+            if (bad) {
+                fprintf(stderr, "%s: unrecognised difficulty `%c'\n",
+                        argv[0], p[2]);
+                return 1;
+            }
+        } else if (*p == '-') {
+            fprintf(stderr, "%s: unrecognised option `%s'\n", argv[0], p);
+            return 1;
+        } else {
+            id = p;
+        }
+    }
+
+    if (!id) {
+        fprintf(stderr, "usage: %s [-v | -g] <game_id>\n", argv[0]);
+        return 1;
+    }
+
+    desc = strchr(id, ':');
+    if (!desc) {
+        fprintf(stderr, "%s: game id expects a colon in it\n", argv[0]);
+        return 1;
+    }
+    *desc++ = '\0';
+
+    p = default_params();
+    decode_params(p, id);
+    err = validate_desc(p, desc);
+    if (err) {
+        fprintf(stderr, "%s: %s\n", argv[0], err);
+        return 1;
+    }
+    s = new_game(NULL, p, desc);
+
+    solver_diagnostics_fp = (diagnostics ? stdout : NULL);
+    retd = tracks_solve(s, maxdiff, &diff_used);
+    if (retd < 0) {
+        printf("Puzzle is inconsistent\n");
+    } else if (grade) {
+        printf("Difficulty rating: %s\n",
+               (retd == 0 ? "Ambiguous" : tracks_diffnames[diff_used]));
+    } else {
+        char *text = game_text_format(s);
+        fputs(text, stdout);
+        sfree(text);
+        if (retd == 0)
+            printf("Could not deduce a unique solution\n");
+    }
+    free_game(s);
+    free_params(p);
+
+    return 0;
+}
+
+#endif
+
 /* vim: set shiftwidth=4 tabstop=8: */
diff --git a/apps/plugins/puzzles/src/twiddle.c b/apps/plugins/puzzles/src/twiddle.c
index 1d91559e37..5f2ea02e6f 100644
--- a/apps/plugins/puzzles/src/twiddle.c
+++ b/apps/plugins/puzzles/src/twiddle.c
@@ -552,6 +552,12 @@ static char *game_text_format(const game_state *state)
     int i, x, y, col, maxlen;
     bool o = state->orientable;
 
+    /* Pedantic check: ensure buf is large enough to format an int in
+     * decimal, using the bound log10(2) < 1/3. (Obviously in practice
+     * int is not going to be larger than even 32 bits any time soon,
+     * but.) */
+    assert(sizeof(buf) >= 1 + sizeof(int) * CHAR_BIT/3);
+
     /*
      * First work out how many characters we need to display each
      * number. We're pretty flexible on grid contents here, so we
@@ -563,6 +569,11 @@ static char *game_text_format(const game_state *state)
 	if (col < x) col = x;
     }
 
+    /* Reassure sprintf-checking compilers like gcc that the field
+     * width we've just computed is not now excessive */
+    if (col >= sizeof(buf))
+        col = sizeof(buf)-1;
+
     /*
      * Now we know the exact total size of the grid we're going to
      * produce: it's got h rows, each containing w lots of col+o,
diff --git a/apps/plugins/puzzles/src/unequal.c b/apps/plugins/puzzles/src/unequal.c
index 951e9ac80f..556cf01c45 100644
--- a/apps/plugins/puzzles/src/unequal.c
+++ b/apps/plugins/puzzles/src/unequal.c
@@ -816,6 +816,74 @@ static int solver_set(struct latin_solver *solver, void *vctx)
 #define SOLVER(upper,title,func,lower) func,
 static usersolver_t const unequal_solvers[] = { DIFFLIST(SOLVER) };
 
+static bool unequal_valid(struct latin_solver *solver, void *vctx)
+{
+    struct solver_ctx *ctx = (struct solver_ctx *)vctx;
+    if (ctx->state->mode == MODE_ADJACENT) {
+        int o = solver->o;
+        int x, y, nx, ny, v, nv, i;
+
+        for (x = 0; x+1 < o; x++) {
+            for (y = 0; y+1 < o; y++) {
+                v = grid(x, y);
+                for (i = 0; i < 4; i++) {
+                    bool is_adj, should_be_adj;
+
+                    should_be_adj =
+                        (GRID(ctx->state, flags, x, y) & adjthan[i].f);
+
+                    nx = x + adjthan[i].dx, ny = y + adjthan[i].dy;
+                    if (nx < 0 || ny < 0 || nx >= o || ny >= o)
+                        continue;
+
+                    nv = grid(nx, ny);
+                    is_adj = (labs(v - nv) == 1);
+
+                    if (is_adj && !should_be_adj) {
+#ifdef STANDALONE_SOLVER
+                        if (solver_show_working)
+                            printf("%*s(%d,%d):%d and (%d,%d):%d have "
+                                   "adjacent values, but should not\n",
+                                   solver_recurse_depth*4, "",
+                                   x+1, y+1, v, nx+1, ny+1, nv);
+#endif
+                        return false;
+                    }
+
+                    if (!is_adj && should_be_adj) {
+#ifdef STANDALONE_SOLVER
+                        if (solver_show_working)
+                            printf("%*s(%d,%d):%d and (%d,%d):%d do not have "
+                                   "adjacent values, but should\n",
+                                   solver_recurse_depth*4, "",
+                                   x+1, y+1, v, nx+1, ny+1, nv);
+#endif
+                        return false;
+                    }
+                }
+            }
+        }
+    } else {
+        int i;
+        for (i = 0; i < ctx->nlinks; i++) {
+            struct solver_link *link = &ctx->links[i];
+            int gv = grid(link->gx, link->gy);
+            int lv = grid(link->lx, link->ly);
+            if (gv <= lv) {
+#ifdef STANDALONE_SOLVER
+                if (solver_show_working)
+                    printf("%*s(%d,%d):%d should be greater than (%d,%d):%d, "
+                           "but is not\n", solver_recurse_depth*4, "",
+                           link->gx+1, link->gy+1, gv,
+                           link->lx+1, link->ly+1, lv);
+#endif
+                return false;
+            }
+        }
+    }
+    return true;
+}
+
 static int solver_state(game_state *state, int maxdiff)
 {
     struct solver_ctx *ctx = new_ctx(state);
@@ -827,7 +895,8 @@ static int solver_state(game_state *state, int maxdiff)
     diff = latin_solver_main(&solver, maxdiff,
 			     DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
 			     DIFF_EXTREME, DIFF_RECURSIVE,
-			     unequal_solvers, ctx, clone_ctx, free_ctx);
+			     unequal_solvers, unequal_valid, ctx,
+                             clone_ctx, free_ctx);
 
     memcpy(state->hints, solver.cube, state->order*state->order*state->order);
 
@@ -2155,7 +2224,8 @@ static int solve(game_params *p, char *desc, int debug)
     diff = latin_solver_main(&solver, DIFF_RECURSIVE,
 			     DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
 			     DIFF_EXTREME, DIFF_RECURSIVE,
-			     unequal_solvers, ctx, clone_ctx, free_ctx);
+			     unequal_solvers, unequal_valid, ctx,
+                             clone_ctx, free_ctx);
 
     free_ctx(ctx);
 
diff --git a/apps/plugins/puzzles/src/unfinished/group.c b/apps/plugins/puzzles/src/unfinished/group.c
index ef7ffba349..006a9e0ee6 100644
--- a/apps/plugins/puzzles/src/unfinished/group.c
+++ b/apps/plugins/puzzles/src/unfinished/group.c
@@ -43,7 +43,7 @@
 #define DIFFLIST(A) \
     A(TRIVIAL,Trivial,NULL,t) \
     A(NORMAL,Normal,solver_normal,n) \
-    A(HARD,Hard,NULL,h) \
+    A(HARD,Hard,solver_hard,h) \
     A(EXTREME,Extreme,NULL,x) \
     A(UNREASONABLE,Unreasonable,NULL,u)
 #define ENUM(upper,title,func,lower) DIFF_ ## upper,
@@ -280,6 +280,23 @@ static const char *validate_params(const game_params *params, bool full)
  * Solver.
  */
 
+static int find_identity(struct latin_solver *solver)
+{
+    int w = solver->o;
+    digit *grid = solver->grid;
+    int i, j;
+
+    for (i = 0; i < w; i++)
+	for (j = 0; j < w; j++) {
+            if (grid[i*w+j] == i+1)
+                return j+1;
+            if (grid[i*w+j] == j+1)
+                return i+1;
+        }
+
+    return 0;
+}
+
 static int solver_normal(struct latin_solver *solver, void *vctx)
 {
     int w = solver->o;
@@ -295,9 +312,9 @@ static int solver_normal(struct latin_solver *solver, void *vctx)
      * So we pick any a,b,c we like; then if we know ab, bc, and
      * (ab)c we can fill in a(bc).
      */
-    for (i = 1; i < w; i++)
-	for (j = 1; j < w; j++)
-	    for (k = 1; k < w; k++) {
+    for (i = 0; i < w; i++)
+	for (j = 0; j < w; j++)
+	    for (k = 0; k < w; k++) {
 		if (!grid[i*w+j] || !grid[j*w+k])
 		    continue;
 		if (grid[(grid[i*w+j]-1)*w+k] &&
@@ -358,12 +375,206 @@ static int solver_normal(struct latin_solver *solver, void *vctx)
 		}
 	    }
 
+    /*
+     * Fill in the row and column for the group identity, if it's not
+     * already known and if we've just found out what it is.
+     */
+    i = find_identity(solver);
+    if (i) {
+        bool done_something = false;
+        for (j = 1; j <= w; j++) {
+            if (!grid[(i-1)*w+(j-1)] || !grid[(j-1)*w+(i-1)]) {
+                done_something = true;
+            }
+        }
+        if (done_something) {
+#ifdef STANDALONE_SOLVER
+            if (solver_show_working) {
+                printf("%*s%s is the group identity\n",
+                       solver_recurse_depth*4, "", names[i-1]);
+            }
+#endif
+            for (j = 1; j <= w; j++) {
+                if (!grid[(j-1)*w+(i-1)]) {
+                    if (!cube(i-1, j-1, j)) {
+#ifdef STANDALONE_SOLVER
+                        if (solver_show_working) {
+                            printf("%*s  but %s cannot go at (%d,%d) - "
+                                   "contradiction!\n",
+                                   solver_recurse_depth*4, "",
+                                   names[j-1], i, j);
+                        }
+#endif
+                        return -1;
+                    }
+#ifdef STANDALONE_SOLVER
+                    if (solver_show_working) {
+                        printf("%*s  placing %s at (%d,%d)\n",
+                               solver_recurse_depth*4, "",
+                               names[j-1], i, j);
+                    }
+#endif
+                    latin_solver_place(solver, i-1, j-1, j);
+                }
+                if (!grid[(i-1)*w+(j-1)]) {
+                    if (!cube(j-1, i-1, j)) {
+#ifdef STANDALONE_SOLVER
+                        if (solver_show_working) {
+                            printf("%*s  but %s cannot go at (%d,%d) - "
+                                   "contradiction!\n",
+                                   solver_recurse_depth*4, "",
+                                   names[j-1], j, i);
+                        }
+#endif
+                        return -1;
+                    }
+#ifdef STANDALONE_SOLVER
+                    if (solver_show_working) {
+                        printf("%*s  placing %s at (%d,%d)\n",
+                               solver_recurse_depth*4, "",
+                               names[j-1], j, i);
+                    }
+#endif
+                    latin_solver_place(solver, j-1, i-1, j);
+                }
+            }
+            return 1;
+        }
+    }
+
     return 0;
 }
 
+static int solver_hard(struct latin_solver *solver, void *vctx)
+{
+    bool done_something = false;
+    int w = solver->o;
+#ifdef STANDALONE_SOLVER
+    char **names = solver->names;
+#endif
+    int i, j;
+
+    /*
+     * In identity-hidden mode, systematically rule out possibilities
+     * for the group identity.
+     *
+     * In solver_normal, we used the fact that any filled square in
+     * the grid whose contents _does_ match one of the elements it's
+     * the product of - that is, ab=a or ab=b - tells you immediately
+     * that the other element is the identity.
+     *
+     * Here, we use the flip side of that: any filled square in the
+     * grid whose contents does _not_ match either its row or column -
+     * that is, if ab is neither a nor b - tells you immediately that
+     * _neither_ of those elements is the identity. And if that's
+     * true, then we can also immediately rule out the possibility
+     * that it acts as the identity on any element at all.
+     */
+    for (i = 0; i < w; i++) {
+        bool i_can_be_id = true;
+#ifdef STANDALONE_SOLVER
+        char title[80];
+#endif
+
+	for (j = 0; j < w; j++) {
+            if (grid(i,j) && grid(i,j) != j+1) {
+#ifdef STANDALONE_SOLVER
+                if (solver_show_working)
+                    sprintf(title, "%s cannot be the identity: "
+                            "%s%s = %s =/= %s", names[i], names[i], names[j],
+                            names[grid(i,j)-1], names[j]);
+#endif
+                i_can_be_id = false;
+                break;
+            }
+            if (grid(j,i) && grid(j,i) != j+1) {
+#ifdef STANDALONE_SOLVER
+                if (solver_show_working)
+                    sprintf(title, "%s cannot be the identity: "
+                            "%s%s = %s =/= %s", names[i], names[j], names[i],
+                            names[grid(j,i)-1], names[j]);
+#endif
+                i_can_be_id = false;
+                break;
+            }
+        }
+
+        if (!i_can_be_id) {
+            /* Now rule out ij=j or ji=j for all j. */
+            for (j = 0; j < w; j++) {
+                if (cube(i, j, j+1)) {
+#ifdef STANDALONE_SOLVER
+                    if (solver_show_working) {
+                        if (title[0]) {
+                            printf("%*s%s\n", solver_recurse_depth*4, "",
+                                   title);
+                            title[0] = '\0';
+                        }
+                        printf("%*s  ruling out %s at (%d,%d)\n",
+                               solver_recurse_depth*4, "", names[j], i, j);
+                    }
+#endif
+                    cube(i, j, j+1) = false;
+                }
+                if (cube(j, i, j+1)) {
+#ifdef STANDALONE_SOLVER
+                    if (solver_show_working) {
+                        if (title[0]) {
+                            printf("%*s%s\n", solver_recurse_depth*4, "",
+                                   title);
+                            title[0] = '\0';
+                        }
+                        printf("%*s  ruling out %s at (%d,%d)\n",
+                               solver_recurse_depth*4, "", names[j], j, i);
+                    }
+#endif
+                    cube(j, i, j+1) = false;
+                }
+            }
+        }
+    }
+
+    return done_something;
+}
+
 #define SOLVER(upper,title,func,lower) func,
 static usersolver_t const group_solvers[] = { DIFFLIST(SOLVER) };
 
+static bool group_valid(struct latin_solver *solver, void *ctx)
+{
+    int w = solver->o;
+#ifdef STANDALONE_SOLVER
+    char **names = solver->names;
+#endif
+    int i, j, k;
+
+    for (i = 0; i < w; i++)
+	for (j = 0; j < w; j++)
+	    for (k = 0; k < w; k++) {
+                int ij = grid(i, j) - 1;
+                int jk = grid(j, k) - 1;
+                int ij_k = grid(ij, k) - 1;
+                int i_jk = grid(i, jk) - 1;
+                if (ij_k != i_jk) {
+#ifdef STANDALONE_SOLVER
+                    if (solver_show_working) {
+                        printf("%*sfailure of associativity: "
+                               "(%s%s)%s = %s%s = %s but "
+                               "%s(%s%s) = %s%s = %s\n",
+                               solver_recurse_depth*4, "",
+                               names[i], names[j], names[k],
+                               names[ij], names[k], names[ij_k],
+                               names[i], names[j], names[k],
+                               names[i], names[jk], names[i_jk]);
+                    }
+#endif
+                    return false;
+                }
+            }
+
+    return true;
+}
+
 static int solver(const game_params *params, digit *grid, int maxdiff)
 {
     int w = params->w;
@@ -387,7 +598,7 @@ static int solver(const game_params *params, digit *grid, int maxdiff)
     ret = latin_solver_main(&solver, maxdiff,
 			    DIFF_TRIVIAL, DIFF_HARD, DIFF_EXTREME,
 			    DIFF_EXTREME, DIFF_UNREASONABLE,
-			    group_solvers, NULL, NULL, NULL);
+			    group_solvers, group_valid, NULL, NULL, NULL);
 
     latin_solver_free(&solver);
 
@@ -2183,6 +2394,11 @@ int main(int argc, char **argv)
     }
 
     if (diff == DIFFCOUNT) {
+        if (really_show_working) {
+	    solver_show_working = true;
+	    memcpy(grid, s->grid, p->w * p->w);
+	    ret = solver(&s->par, grid, DIFFCOUNT - 1);
+        }
 	if (grade)
 	    printf("Difficulty rating: ambiguous\n");
 	else
diff --git a/apps/plugins/puzzles/src/unfinished/path.c b/apps/plugins/puzzles/src/unfinished/path.c
index 829fbc6c75..fe5a47fd2a 100644
--- a/apps/plugins/puzzles/src/unfinished/path.c
+++ b/apps/plugins/puzzles/src/unfinished/path.c
@@ -69,6 +69,103 @@
  */
 
 /*
+ * 2020-05-11: some thoughts on a solver.
+ *
+ * Consider this example puzzle, from Wikipedia:
+ *
+ *     ---4---
+ *     -3--25-
+ *     ---31--
+ *     ---5---
+ *     -------
+ *     --1----
+ *     2---4--
+ *
+ * The kind of deduction that a human wants to make here is: which way
+ * does the path between the 4s go? In particular, does it go round
+ * the left of the W-shaped cluster of endpoints, or round the right
+ * of it? It's clear at a glance that it must go to the right, because
+ * _any_ path between the 4s that goes to the left of that cluster, no
+ * matter what detailed direction it takes, will disconnect the
+ * remaining grid squares into two components, with the two 2s not in
+ * the same component. So we immediately know that the path between
+ * the 4s _must_ go round the right-hand side of the grid.
+ *
+ * How do you model that global and topological reasoning in a
+ * computer?
+ *
+ * The most plausible idea I've seen so far is to use fundamental
+ * groups. The fundamental group of loops based at a given point in a
+ * space is a free group, under loop concatenation and up to homotopy,
+ * generated by the loops that go in each direction around each hole
+ * in the space. In this case, the 'holes' are clues, or connected
+ * groups of clues.
+ *
+ * So you might be able to enumerate all the homotopy classes of paths
+ * between (say) the two 4s as follows. Start with any old path
+ * between them (say, find the first one that breadth-first search
+ * will give you). Choose one of the 4s to regard as the base point
+ * (arbitrarily). Then breadth-first search among the space of _paths_
+ * by the following procedure. Given a candidate path, append to it
+ * each of the possible loops that starts from the base point,
+ * circumnavigates one clue cluster, and returns to the base point.
+ * The result will typically be a path that retraces its steps and
+ * self-intersects. Now adjust it homotopically so that it doesn't. If
+ * that can't be done, then we haven't generated a fresh candidate
+ * path; if it can, then we've got a new path that is not homotopic to
+ * any path we already had, so add it to our list and queue it up to
+ * become the starting point of this search later.
+ *
+ * The idea is that this should exhaustively enumerate, up to
+ * homotopy, the different ways in which the two 4s can connect to
+ * each other within the constraint that you have to actually fit the
+ * path non-self-intersectingly into this grid. Then you can keep a
+ * list of those homotopy classes in mind, and start ruling them out
+ * by techniques like the connectivity approach described above.
+ * Hopefully you end up narrowing down to few enough homotopy classes
+ * that you can deduce something concrete about actual squares of the
+ * grid - for example, here, that if the path between 4s has to go
+ * round the right, then we know some specific squares it must go
+ * through, so we can fill those in. And then, having filled in a
+ * piece of the middle of a path, you can now regard connecting the
+ * ultimate endpoints to that mid-section as two separate subproblems,
+ * so you've reduced to a simpler instance of the same puzzle.
+ *
+ * But I don't know whether all of this actually works. I more or less
+ * believe the process for enumerating elements of the free group; but
+ * I'm not as confident that when you find a group element that won't
+ * fit in the grid, you'll never have to consider its descendants in
+ * the BFS either. And I'm assuming that 'unwind the self-intersection
+ * homotopically' is a thing that can actually be turned into a
+ * sensible algorithm.
+ *
+ * --------
+ *
+ * Another thing that might be needed is to characterise _which_
+ * homotopy class a given path is in.
+ *
+ * For this I think it's sufficient to choose a collection of paths
+ * along the _edges_ of the square grid, each of which connects two of
+ * the holes in the grid (including the grid exterior, which counts as
+ * a huge hole), such that they form a spanning tree between the
+ * holes. Then assign each of those paths an orientation, so that
+ * crossing it in one direction counts as 'positive' and the other
+ * 'negative'. Now analyse a candidate path from one square to another
+ * by following it and noting down which of those paths it crosses in
+ * which direction, then simplifying the result like a free group word
+ * (i.e. adjacent + and - crossings of the same path cancel out).
+ *
+ * --------
+ *
+ * If we choose those paths to be of minimal length, then we can get
+ * an upper bound on the number of homotopy classes by observing that
+ * you can't traverse any of those barriers more times than will fit
+ * non-self-intersectingly in the grid. That might be an alternative
+ * method of bounding the search through the fundamental group to only
+ * finitely many possibilities.
+ */
+
+/*
  * Standard notation for directions.
  */
 #define L 0