strftime.c

#include <stdio.h>
#include <stdlib.h>
#include <langinfo.h>
#include <locale.h>
#include <time.h>
#include <limits.h>
#include "libc.h"

// FIXME: integer overflows

const char *__nl_langinfo_l(nl_item, locale_t);

static int is_leap(int y)
{
	/* Avoid overflow */
	if (y>INT_MAX-1900) y -= 2000;
	y += 1900;
	return !(y%4) && ((y%100) || !(y%400));
}

static int week_num(const struct tm *tm)
{
	int val = (tm->tm_yday + 7 - (tm->tm_wday+6)%7) / 7;
	/* If 1 Jan is just 1-3 days past Monday,
	 * the previous week is also in this year. */
	if ((tm->tm_wday - tm->tm_yday - 2 + 371) % 7 <= 2)
		val++;
	if (!val) {
		val = 52;
		/* If 31 December of prev year a Thursday,
		 * or Friday of a leap year, then the
		 * prev year has 53 weeks. */
		int dec31 = (tm->tm_wday - tm->tm_yday - 1 + 7) % 7;
		if (dec31 == 4 || (dec31 == 5 && is_leap(tm->tm_year%400-1)))
			val++;
	} else if (val == 53) {
		/* If 1 January is not a Thursday, and not
		 * a Wednesday of a leap year, then this
		 * year has only 52 weeks. */
		int jan1 = (tm->tm_wday - tm->tm_yday + 371) % 7;
		if (jan1 != 4 && (jan1 != 3 || !is_leap(tm->tm_year)))
			val = 1;
	}
	return val;
}

size_t __strftime_l(char *restrict, size_t, const char *restrict, const struct tm *restrict, locale_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;
	const char *fmt;

	switch (f) {
	case 'a':
		item = ABDAY_1 + tm->tm_wday;
		goto nl_strcat;
	case 'A':
		item = DAY_1 + tm->tm_wday;
		goto nl_strcat;
	case 'h':
	case 'b':
		item = ABMON_1 + tm->tm_mon;
		goto nl_strcat;
	case 'B':
		item = MON_1 + tm->tm_mon;
		goto nl_strcat;
	case 'c':
		item = D_T_FMT;
		goto nl_strftime;
	case 'C':
		val = (1900+tm->tm_year) / 100;
		fmt = "%02d";
		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;
		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;
		}
		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";
		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;
		return "\n";
	case 'p':
		item = tm->tm_hour >= 12 ? PM_STR : AM_STR;
		goto nl_strcat;
	case 'r':
		item = T_FMT_AMPM;
		goto nl_strftime;
	case 'R':
		fmt = "%H:%M";
		goto recu_strftime;
	case 'S':
		val = tm->tm_sec;
		fmt = "%02d";
		goto number;
	case 't':
		*l = 1;
		return "\t";
	case 'T':
		fmt = "%H:%M:%S";
		goto recu_strftime;
	case 'u':
		val = tm->tm_wday ? tm->tm_wday : 7;
		fmt = "%d";
		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";
		goto number;
	case 'x':
		item = D_FMT;
		goto nl_strftime;
	case 'X':
		item = T_FMT;
		goto nl_strftime;
	case 'y':
		val = tm->tm_year % 100;
		fmt = "%02d";
		goto number;
	case 'Y':
		val = tm->tm_year + 1900;
		fmt = "%04d";
		goto number;
	case 'z':
		val = -tm->__tm_gmtoff;
		*l = snprintf(*s, sizeof *s, "%+.2d%.2d", val/3600, abs(val%3600)/60);
		return *s;
	case 'Z':
		fmt = tm->__tm_zone;
		goto string;
	case '%':
		*l = 1;
		return "%";
	default:
		return 0;
	}
number:
	*l = snprintf(*s, sizeof *s, fmt, val);
	return *s;
nl_strcat:
	fmt = __nl_langinfo_l(item, loc);
string:
	*l = strlen(fmt);
	return fmt;
nl_strftime:
	fmt = __nl_langinfo_l(item, loc);
recu_strftime:
	*l = __strftime_l(*s, sizeof *s, fmt, tm, loc);
	if (!*l) return 0;
	return *s;
}

size_t __strftime_l(char *restrict s, size_t n, const char *restrict f, const struct tm *restrict tm, locale_t loc)
{
	size_t l, k;
	char buf[100];
	const char *t;
	for (l=0; l+1<n; f++) {
		if (!*f) {
			s[l] = 0;
			return l;
		}
		if (*f != '%') {
			s[l++] = *f;
			continue;
		}
		f++;
		if (*f == 'E' || *f == 'O') f++;
		t = __strftime_fmt_1(&buf, &k, *f, tm, loc);
		if (!t || k >= n-l)
			return 0;
		memcpy(s+l, t, k);
		l += k;
	}
	return 0;
}

size_t strftime(char *restrict s, size_t n, const char *restrict f, const struct tm *restrict tm)
{
	return __strftime_l(s, n, f, tm, 0);
}

weak_alias(__strftime_l, strftime_l);