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

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 + 7U - (tm->tm_wday + 6U) % 7) / 7;
  /* If 1 Jan is just 1-3 days past Monday,
   * the previous week is also in this year. */
  if ((tm->tm_wday + 371U - tm->tm_yday - 2) % 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 + 7U - tm->tm_yday - 1) % 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 + 371U - tm->tm_yday) % 7;
    if (jan1 != 4 && (jan1 != 3 || !is_leap(tm->tm_year)))
      val = 1;
  }
  return val;
}

const char* __tm_to_tzname(const struct tm*);
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;
  long long val;
  const char* fmt = "-";
  int width = 2;

  switch (f) {
    case 'a':
      if (tm->tm_wday > 6U)
        goto string;
      item = ABDAY_1 + tm->tm_wday;
      goto nl_strcat;
    case 'A':
      if (tm->tm_wday > 6U)
        goto string;
      item = DAY_1 + tm->tm_wday;
      goto nl_strcat;
    case 'h':
    case 'b':
      if (tm->tm_mon > 11U)
        goto string;
      item = ABMON_1 + tm->tm_mon;
      goto nl_strcat;
    case 'B':
      if (tm->tm_mon > 11U)
        goto string;
      item = MON_1 + tm->tm_mon;
      goto nl_strcat;
    case 'c':
      item = D_T_FMT;
      goto nl_strftime;
    case 'C':
      val = (1900LL + tm->tm_year) / 100;
      goto number;
    case 'd':
      val = tm->tm_mday;
      goto number;
    case 'D':
      fmt = "%m/%d/%y";
      goto recu_strftime;
    case 'e':
      *l = snprintf(*s, sizeof *s, "%2d", tm->tm_mday);
      return *s;
    case 'F':
      fmt = "%Y-%m-%d";
      goto recu_strftime;
    case 'g':
    case 'G':
      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')
        val %= 100;
      else
        width = 4;
      goto number;
    case 'H':
      val = tm->tm_hour;
      goto number;
    case 'I':
      val = tm->tm_hour;
      if (!val)
        val = 12;
      else if (val > 12)
        val -= 12;
      goto number;
    case 'j':
      val = tm->tm_yday + 1;
      width = 3;
      goto number;
    case 'm':
      val = tm->tm_mon + 1;
      goto number;
    case 'M':
      val = tm->tm_min;
      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_to_secs(tm) - tm->__tm_gmtoff;
      width = 1;
      goto number;
    case 'S':
      val = tm->tm_sec;
      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;
      width = 1;
      goto number;
    case 'U':
      val = (tm->tm_yday + 7U - tm->tm_wday) / 7;
      goto number;
    case 'W':
      val = (tm->tm_yday + 7U - (tm->tm_wday + 6U) % 7) / 7;
      goto number;
    case 'V':
      val = week_num(tm);
      goto number;
    case 'w':
      val = tm->tm_wday;
      width = 1;
      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;
      goto number;
    case 'Y':
      val = tm->tm_year + 1900LL;
      if (val >= 10000) {
        *l = snprintf(*s, sizeof *s, "+%lld", val);
        return *s;
      }
      width = 4;
      goto number;
    case 'z':
      if (tm->tm_isdst < 0) {
        *l = 0;
        return "";
      }
      *l = snprintf(*s, sizeof *s, "%+.2d%.2d", (tm->__tm_gmtoff) / 3600,
                    abs(tm->__tm_gmtoff % 3600) / 60);
      return *s;
    case 'Z':
      if (tm->tm_isdst < 0) {
        *l = 0;
        return "";
      }
      fmt = __tm_to_tzname(tm);
      goto string;
    case '%':
      *l = 1;
      return "%";
    default:
      return 0;
  }
number:
  *l = snprintf(*s, sizeof *s, "%0*lld", width, 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];
  char* p;
  const char* t;
  int plus;
  unsigned long width;
  for (l = 0; l < n; f++) {
    if (!*f) {
      s[l] = 0;
      return l;
    }
    if (*f != '%') {
      s[l++] = *f;
      continue;
    }
    f++;
    if ((plus = (*f == '+')))
      f++;
    width = strtoul(f, &p, 10);
    if (*p == 'C' || *p == 'F' || *p == 'G' || *p == 'Y') {
      if (!width && p != f)
        width = 1;
    } else {
      width = 0;
    }
    f = p;
    if (*f == 'E' || *f == 'O')
      f++;
    t = __strftime_fmt_1(&buf, &k, *f, tm, loc);
    if (!t)
      break;
    if (width) {
      for (; *t == '+' || *t == '-' || (*t == '0' && t[1]); t++, k--)
        ;
      width--;
      if (plus && tm->tm_year >= 10000 - 1900)
        s[l++] = '+';
      else if (tm->tm_year < -1900)
        s[l++] = '-';
      else
        width++;
      for (; width > k && l < n; width--)
        s[l++] = '0';
    }
    if (k > n - l)
      k = n - l;
    memcpy(s + l, t, k);
    l += k;
  }
  if (n) {
    if (l == n)
      l = n - 1;
    s[l] = 0;
  }
  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, CURRENT_LOCALE);
}

weak_alias(__strftime_l, strftime_l);