simplify strftime and fix integer overflows

use a long long value so that even with offsets, values cannot
overflow. instead of using different format strings for different
numeric formats, simply use a per-format width and %0*lld for all of
them.

this width specifier is not for use with strftime field widths; that
will be a separate step in the caller.

src/time/strftime.c

@@ -6,8 +6,6 @@
 #include <limits.h>
 #include "libc.h"
 
-// FIXME: integer overflows
-
 const char *__nl_langinfo_l(nl_item, locale_t);
 
 static int is_leap(int y)
@@ -49,8 +47,9 @@ size_t __strftime_l(char *restrict, size_t, const char *restrict, const struct t
 const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *tm, locale_t loc)
 {
 	nl_item item;
-	int val;
+	long long val;
 	const char *fmt;
+	int width = 2;
 
 	switch (f) {
 	case 'a':
@@ -70,55 +69,45 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		item = D_T_FMT;
 		goto nl_strftime;
 	case 'C':
-		val = (1900+tm->tm_year) / 100;
-		fmt = "%02d";
+		val = (1900LL+tm->tm_year) / 100;
 		goto number;
 	case 'd':
 		val = tm->tm_mday;
-		fmt = "%02d";
 		goto number;
 	case 'D':
 		fmt = "%m/%d/%y";
 		goto recu_strftime;
 	case 'e':
 		val = tm->tm_mday;
-		fmt = "%2d";
 		goto number;
 	case 'F':
 		fmt = "%Y-%m-%d";
 		goto recu_strftime;
 	case 'g':
 	case 'G':
-		fmt = "%04d";
-		val = tm->tm_year + 1900;
+		val = tm->tm_year + 1900LL;
 		if (tm->tm_yday < 3 && week_num(tm) != 1) val--;
 		else if (tm->tm_yday > 360 && week_num(tm) == 1) val++;
-		if (f=='g') {
-			fmt = "%02d";
-			val %= 100;
-		}
+		if (f=='g') val %= 100;
+		else width = 4;
 		goto number;
 	case 'H':
 		val = tm->tm_hour;
-		fmt = "%02d";
 		goto number;
 	case 'I':
 		val = tm->tm_hour;
 		if (!val) val = 12;
 		else if (val > 12) val -= 12;
-		fmt = "%02d";
 		goto number;
 	case 'j':
 		val = tm->tm_yday+1;
-		fmt = "%03d";
+		width = 3;
 		goto number;
 	case 'm':
 		val = tm->tm_mon+1;
-		fmt = "%02d";
 		goto number;
 	case 'M':
 		val = tm->tm_min;
-		fmt = "%02d";
 		goto number;
 	case 'n':
 		*l = 1;
@@ -134,7 +123,6 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		goto recu_strftime;
 	case 'S':
 		val = tm->tm_sec;
-		fmt = "%02d";
 		goto number;
 	case 't':
 		*l = 1;
@@ -144,23 +132,20 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		goto recu_strftime;
 	case 'u':
 		val = tm->tm_wday ? tm->tm_wday : 7;
-		fmt = "%d";
+		width = 1;
 		goto number;
 	case 'U':
 		val = (tm->tm_yday + 7 - tm->tm_wday) / 7;
-		fmt = "%02d";
 		goto number;
 	case 'W':
 		val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
-		fmt = "%02d";
 		goto number;
 	case 'V':
 		val = week_num(tm);
-		fmt = "%02d";
 		goto number;
 	case 'w':
 		val = tm->tm_wday;
-		fmt = "%d";
+		width = 1;
 		goto number;
 	case 'x':
 		item = D_FMT;
@@ -170,11 +155,10 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		goto nl_strftime;
 	case 'y':
 		val = tm->tm_year % 100;
-		fmt = "%02d";
 		goto number;
 	case 'Y':
-		val = tm->tm_year + 1900;
-		fmt = "%04d";
+		val = tm->tm_year + 1900LL;
+		width = 4;
 		goto number;
 	case 'z':
 		val = -tm->__tm_gmtoff;
@@ -190,7 +174,7 @@ const char *__strftime_fmt_1(char (*s)[100], size_t *l, int f, const struct tm *
 		return 0;
 	}
 number:
-	*l = snprintf(*s, sizeof *s, fmt, val);
+	*l = snprintf(*s, sizeof *s, "%0*lld", width, val);
 	return *s;
 nl_strcat:
 	fmt = __nl_langinfo_l(item, loc);