win32: limited version of timegm(3)

timegm(3) from musl checks that the calculated time_t value can
be broken out into a struct tm without overflow.  The limiting
factor is that tm_year is an int.

Our only use of timegm(3) is followed by a call to localtime(3).
The Microsoft version of this only works for time_t values between
0 and INT_MAX (32-bit) or 0 and 32535215999 (64-bit).

Enforce these limits in timegm(3), thus avoiding the use of musl's
__secs_to_tm() and saving 624 bytes.

1 files changed, 13 insertions, 84 deletions

diff --git a/win32/timegm.c b/win32/timegm.c
index abba2579f..8bfa041c4 100644
--- a/win32/timegm.c
+++ b/win32/timegm.c
@@ -111,95 +111,24 @@ static long long __tm_to_secs(const struct tm *tm)
         return t;
 }
 
-/* 2000-03-01 (mod 400 year, immediately after feb29 */
-#define LEAPOCH (946684800LL + 86400*(31+29))
-
-#define DAYS_PER_400Y (365*400 + 97)
-#define DAYS_PER_100Y (365*100 + 24)
-#define DAYS_PER_4Y   (365*4   + 1)
-
-static int __secs_to_tm(long long t, struct tm *tm)
-{
-        long long days, secs, years;
-        int remdays, remsecs, remyears;
-        int qc_cycles, c_cycles, q_cycles;
-        int months;
-        int wday, yday, leap;
-        static const char days_in_month[] = {31,30,31,30,31,31,30,31,30,31,31,29};
-
-        /* Reject time_t values whose year would overflow int */
-        if (t < INT_MIN * 31622400LL || t > INT_MAX * 31622400LL)
-                return -1;
-
-        secs = t - LEAPOCH;
-        days = secs / 86400;
-        remsecs = secs % 86400;
-        if (remsecs < 0) {
-                remsecs += 86400;
-                days--;
-        }
-
-        wday = (3+days)%7;
-        if (wday < 0) wday += 7;
-
-        qc_cycles = days / DAYS_PER_400Y;
-        remdays = days % DAYS_PER_400Y;
-        if (remdays < 0) {
-                remdays += DAYS_PER_400Y;
-                qc_cycles--;
-        }
-
-        c_cycles = remdays / DAYS_PER_100Y;
-        if (c_cycles == 4) c_cycles--;
-        remdays -= c_cycles * DAYS_PER_100Y;
-
-        q_cycles = remdays / DAYS_PER_4Y;
-        if (q_cycles == 25) q_cycles--;
-        remdays -= q_cycles * DAYS_PER_4Y;
-
-        remyears = remdays / 365;
-        if (remyears == 4) remyears--;
-        remdays -= remyears * 365;
-
-        leap = !remyears && (q_cycles || !c_cycles);
-        yday = remdays + 31 + 28 + leap;
-        if (yday >= 365+leap) yday -= 365+leap;
-
-        years = remyears + 4*q_cycles + 100*c_cycles + 400LL*qc_cycles;
-
-        for (months=0; days_in_month[months] <= remdays; months++)
-                remdays -= days_in_month[months];
-
-        if (months >= 10) {
-                months -= 12;
-                years++;
-        }
-
-        if (years+100 > INT_MAX || years+100 < INT_MIN)
-                return -1;
-
-        tm->tm_year = years + 100;
-        tm->tm_mon = months + 2;
-        tm->tm_mday = remdays + 1;
-        tm->tm_wday = wday;
-        tm->tm_yday = yday;
-
-        tm->tm_hour = remsecs / 3600;
-        tm->tm_min = remsecs / 60 % 60;
-        tm->tm_sec = remsecs % 60;
-
-        return 0;
-}
-
+/*
+ * Restricted version of timegm:
+ *
+ *   it doesn't normalise its argument
+ *   its return value is limited to the range Microsoft supports
+ */
 time_t timegm(struct tm *tm)
 {
-        struct tm new;
         long long t = __tm_to_secs(tm);
-        if (__secs_to_tm(t, &new) < 0) {
+        if (t < 0 ||
+#ifdef _USE_32BIT_TIME_T
+                t > INT_MAX                     /* 2038-01-19 03:14:07Z */
+#else
+                t > 32535215999         /* 3000-12-31 23:59:59Z */
+#endif
+        ) {
                 errno = EOVERFLOW;
                 return -1;
         }
-        *tm = new;
-        tm->tm_isdst = 0;
         return t;
 }