From 48ddce5e9a063d89689ffe4be1680767186e13ee Mon Sep 17 00:00:00 2001
From: Ron Yorston <rmy@pobox.com>
Date: Sat, 18 Sep 2021 09:33:21 +0100
Subject: 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.
---
 win32/timegm.c | 97 ++++++++--------------------------------------------------
 1 file 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;
 }
-- 
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