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authorMichael Sevakis <jethead71@rockbox.org>2017-01-25 19:34:13 -0500
committerMichael Sevakis <jethead71@rockbox.org>2017-02-03 14:40:55 -0500
commit248bff5eb8c56dc3c8b4ff2e47c05c7c09cd4849 (patch)
tree3956b2c20460816e25358985da3a3e45340ff8bb /firmware/libc
parent4d4b0c5a07c907efda20771fa6baca6f1c91802e (diff)
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Improve code for gmtime_r()
Unlike the current code, it does no looping to count days or do table lookups, which means running time doesn't increase with year or month. A good thing if it's call a lot, especially if the algorithm were asked to compute dates centuries or more from the epoch start. As a bonus, handles negative time values. Change-Id: I198a23daf621e40623e6b44dacf2387078b4db9c
Diffstat (limited to 'firmware/libc')
-rw-r--r--firmware/libc/gmtime.c154
1 files changed, 86 insertions, 68 deletions
diff --git a/firmware/libc/gmtime.c b/firmware/libc/gmtime.c
index c5deb593cd..b01050904d 100644
--- a/firmware/libc/gmtime.c
+++ b/firmware/libc/gmtime.c
@@ -7,6 +7,8 @@
* \/ \/ \/ \/ \/
* $Id$
*
+ * Copyright (C) 2017 by Michael Sevakis
+ *
* Copyright (C) 2012 by Bertrik Sikken
*
* Based on code from: rtc_as3514.c
@@ -21,96 +23,112 @@
* KIND, either express or implied.
*
****************************************************************************/
-#include <stdbool.h>
-#include <stdint.h>
#include "time.h"
-#define MINUTE_SECONDS 60
-#define HOUR_SECONDS 3600
-#define DAY_SECONDS 86400
-#define WEEK_SECONDS 604800
-#define YEAR_SECONDS 31536000
-#define LEAP_YEAR_SECONDS 31622400
+/* Constants that mark Thursday, 1 January 1970 */
+#define UNIX_EPOCH_DAY_NUM 134774
+#define UNIX_EPOCH_YEAR (1601 - 1900)
-/* Days in each month */
-static uint8_t days_in_month[] =
- {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
+/* Last day number it can do */
+#define MAX_DAY_NUM 551879
-static inline bool is_leapyear(int year)
+/* d is days since epoch start, Monday, 1 January 1601 (d = 0)
+ *
+ * That date is the beginning of a full 400 year cycle and so simplifies the
+ * calculations a bit, not requiring offsets before divisions to shift the
+ * leap year cycle.
+ *
+ * It can handle dates up through Sunday, 31 December 3111 (d = 551879).
+ *
+ * time_t can't get near the limits anyway for now but algorithm can be
+ * altered slightly to increase range if even needed.
+ */
+static void get_tmdate(unsigned int d, struct tm *tm)
{
- return (((year%4)==0) && (((year%100)!=0) || ((year%400)==0)));
-}
+ static const unsigned short mon_yday[13] =
+ {
+ /* year day of 1st of month (non-leap)
+ +31 +28 +31 +30 +31 +30 +31 +31 +30 +31 +30 +31 +31 */
+ 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365
+ };
-struct tm *gmtime(const time_t *timep)
-{
- static struct tm time;
- return gmtime_r(timep, &time);
-}
+ unsigned int x0, x1, x2, x3; /* scratch variables */
-struct tm *gmtime_r(const time_t *timep, struct tm *tm)
-{
- time_t seconds = *timep;
- int year, i, mday, hour, min;
+ /* calculate year from day number */
+ x0 = d / 1460;
+ x1 = x0 / 25;
+ x2 = x1 / 4;
- /* weekday */
- tm->tm_wday = (seconds / DAY_SECONDS + 4) % 7;
+ unsigned int y = (d - x0 + x1 - x2) / 365;
- /* Year */
- year = 1970;
- while (seconds >= LEAP_YEAR_SECONDS)
- {
- if (is_leapyear(year)){
- seconds -= LEAP_YEAR_SECONDS;
- } else {
- seconds -= YEAR_SECONDS;
- }
+ tm->tm_year = y + UNIX_EPOCH_YEAR; /* year - 1900 */
- year++;
- }
+ /* calculate year day from year number and day number */
+ x0 = y / 4;
+ x1 = x0 / 25;
+ x2 = x1 / 4;
+
+ unsigned int yday = d - x0 + x1 - x2 - y * 365;
- if (is_leapyear(year)) {
- days_in_month[1] = 29;
- } else {
- days_in_month[1] = 28;
- if(seconds>YEAR_SECONDS){
- year++;
- seconds -= YEAR_SECONDS;
+ tm->tm_yday = x3 = yday; /* 0..364/365 */
+
+ /* check if leap year; adjust February->March transition if so rather
+ than keeping a leap year version of mon_yday[] */
+ if (y - x0 * 4 == 3 && (x0 - x1 * 25 != 24 || x1 - x2 * 4 == 3)) {
+ /* retard month lookup to make year day 59 into 29 Feb, both to make
+ year day 60 into 01 March, lagging one day for remainder of year */
+ if (x3 >= mon_yday[2] && --x3 >= mon_yday[2]) {
+ yday--;
}
}
- tm->tm_year = year - 1900;
- /* Month */
- for (i = 0; i < 12; i++)
- {
- if (seconds < days_in_month[i]*DAY_SECONDS){
- tm->tm_mon = i;
- break;
- }
+ /* stab approximately at current month based on year day; advance if
+ it fell short (never initially more than 1 short). */
+ x0 = x3 / 32;
+ if (mon_yday[x0 + 1] <= x3) {
+ x0++;
+ }
+
+ tm->tm_mon = x0; /* 0..11 */
+ tm->tm_mday = yday - mon_yday[x0] + 1; /* 1..31 */
+ tm->tm_wday = (d + 1) % 7; /* 0..6 */
+}
+
+struct tm * gmtime_r(const time_t *timep, struct tm *tm)
+{
+ time_t t = *timep;
+
+ int d = t / 86400; /* day number (-24856..24855) */
+ int s = t - (time_t)d * 86400; /* second # of day (0..86399) */
- seconds -= days_in_month[i]*DAY_SECONDS;
+ if (s < 0) {
+ /* round towards 0 -> floored division */
+ d--;
+ s += 86400;
}
- /* Month Day */
- mday = seconds/DAY_SECONDS;
- seconds -= mday*DAY_SECONDS;
- tm->tm_mday = mday + 1; /* 1 ... 31 */
+ unsigned int x;
- /* Hour */
- hour = seconds/HOUR_SECONDS;
- seconds -= hour*HOUR_SECONDS;
- tm->tm_hour = hour;
+ x = s / 3600;
+ tm->tm_hour = x; /* 0..23 */
- /* Minute */
- min = seconds/MINUTE_SECONDS;
- seconds -= min*MINUTE_SECONDS;
- tm->tm_min = min;
+ s -= x * 3600;
+ x = s / 60;
+ tm->tm_min = x; /* 0..59 */
- /* Second */
- tm->tm_sec = seconds;
+ s -= x * 60;
+ tm->tm_sec = s; /* 0..59 */
- tm->tm_yday = 0; /* Not implemented for now */
- tm->tm_isdst = -1; /* Not implemented for now */
+ /* not implemented right now */
+ tm->tm_isdst = -1;
+
+ get_tmdate(d + UNIX_EPOCH_DAY_NUM, tm);
return tm;
}
+struct tm * gmtime(const time_t *timep)
+{
+ static struct tm time;
+ return gmtime_r(timep, &time);
+}