gmtime_r: rewrite, now truly thread-safe

compat/gmtime_r.c

 /*****************************************************************************
  * gmtime_r.c: POSIX gmtime_r() replacement
  *****************************************************************************
- * Copyright © 1998-2008 VLC authors and VideoLAN
+ * Copyright © 2011-2012 Rémi Denis-Courmont
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU Lesser General Public License as published by
@@ -23,13 +23,73 @@
 #endif
 
 #include <time.h>
+#include <stdio.h>
+#include <stdlib.h>
 
-struct tm *gmtime_r (const time_t *timep, struct tm *result)
+struct tm *gmtime_r (const time_t *timep, struct tm *tm)
 {
-    struct tm *s = gmtime (timep);
-    if (s == NULL)
-        return NULL;
+    static const unsigned short normal[12] =
+        { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+    static const unsigned short leap[12] =
+        { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+    lldiv_t d;
+    const unsigned short *months;
 
-    *result = *s;
-    return result;
+    /* Rebase from 1970 Unix epoch to fictitious year 0 (i.e. would-be
+     * year 1 BC had the Gregorian calendar already existed then) */
+    d.quot = 60LL * 60 * 24 * (4 * 146097 + 135140) + *timep;
+
+    d = lldiv (d.quot, 60); /* 60 seconds per minute */
+    if (*timep < 0)
+    {
+        d.rem += 60;
+        d.quot--;
+    }
+    tm->tm_sec = d.rem;
+
+    d = lldiv (d.quot, 60); /* 60 minutes per hour */
+    tm->tm_min = d.rem;
+
+    d = lldiv (d.quot, 24); /* 24 hours per day */
+    tm->tm_hour = d.rem;
+
+    tm->tm_wday = (d.quot + 6) % 7; /* 7 days per week */
+
+    d = lldiv (d.quot, 146097); /* 146097 days per 4 centuries */
+    tm->tm_year = 400 * d.quot;
+
+    if (d.rem >= 36525)
+    {    /* Century with 24 leap years */
+        d.rem -= 36525;
+        tm->tm_year += 100;
+
+        d = lldiv (d.rem, 36524); /* 36524 days per normal century */
+        tm->tm_year += d.quot * 100;
+        if (d.rem >= 59) /* not in January or February of year 00 */
+            d.rem++; /* insert missing February 29th 00*/
+    }
+    tm->tm_year -= 1900; /* Rebase to 1900 for struct tm */
+
+    d = lldiv (d.rem, 1461); /* 1461 days per 4 years */
+    tm->tm_year += 4 * d.quot;
+
+    if (d.rem >= 366)
+    {    /* Non-leap year */
+        d.rem -= 366;
+        tm->tm_year++;
+
+        d = lldiv (d.rem, 365); /* 365 days per normal year */
+        tm->tm_year += d.quot;
+        months = normal;
+    }
+    else
+        months = leap;
+    tm->tm_yday = d.rem;
+
+    for (tm->tm_mon = 0; d.rem >= months[tm->tm_mon]; tm->tm_mon++)
+        d.rem -= months[tm->tm_mon]; /* 28 to 31 days in a month */
+    tm->tm_mday = d.rem + 1;
+
+    tm->tm_isdst = 0; /* UTC time */
+    return tm;
 }