#include <stdlib.h>
#include <langinfo.h>
#include <time.h>
#include <ctype.h>
#include <stddef.h>
#include <string.h>
#include <strings.h>

char* strptime(const char* restrict s,
               const char* restrict f,
               struct tm* restrict tm) {
  int i, w, neg, adj, min, range, *dest, dummy;
  const char* ex;
  size_t len;
  int want_century = 0, century = 0;
  while (*f) {
    if (*f != '%') {
      if (isspace(*f))
        for (; *s && isspace(*s); s++)
          ;
      else if (*s != *f)
        return 0;
      else
        s++;
      f++;
      continue;
    }
    f++;
    if (*f == '+')
      f++;
    if (isdigit(*f))
      w = strtoul(f, (void*)&f, 10);
    else
      w = -1;
    adj = 0;
    switch (*f++) {
      case 'a':
      case 'A':
        dest = &tm->tm_wday;
        min = ABDAY_1;
        range = 7;
        goto symbolic_range;
      case 'b':
      case 'B':
      case 'h':
        dest = &tm->tm_mon;
        min = ABMON_1;
        range = 12;
        goto symbolic_range;
      case 'c':
        s = strptime(s, nl_langinfo(D_T_FMT), tm);
        if (!s)
          return 0;
        break;
      case 'C':
        dest = &century;
        if (w < 0)
          w = 2;
        want_century |= 2;
        goto numeric_digits;
      case 'd':
      case 'e':
        dest = &tm->tm_mday;
        min = 1;
        range = 31;
        goto numeric_range;
      case 'D':
        s = strptime(s, "%m/%d/%y", tm);
        if (!s)
          return 0;
        break;
      case 'H':
        dest = &tm->tm_hour;
        min = 0;
        range = 24;
        goto numeric_range;
      case 'I':
        dest = &tm->tm_hour;
        min = 1;
        range = 12;
        goto numeric_range;
      case 'j':
        dest = &tm->tm_yday;
        min = 1;
        range = 366;
        goto numeric_range;
      case 'm':
        dest = &tm->tm_mon;
        min = 1;
        range = 12;
        adj = 1;
        goto numeric_range;
      case 'M':
        dest = &tm->tm_min;
        min = 0;
        range = 60;
        goto numeric_range;
      case 'n':
      case 't':
        for (; *s && isspace(*s); s++)
          ;
        break;
      case 'p':
        ex = nl_langinfo(AM_STR);
        len = strlen(ex);
        if (!strncasecmp(s, ex, len)) {
          tm->tm_hour %= 12;
          break;
        }
        ex = nl_langinfo(PM_STR);
        len = strlen(ex);
        if (!strncasecmp(s, ex, len)) {
          tm->tm_hour %= 12;
          tm->tm_hour += 12;
          break;
        }
        return 0;
      case 'r':
        s = strptime(s, nl_langinfo(T_FMT_AMPM), tm);
        if (!s)
          return 0;
        break;
      case 'R':
        s = strptime(s, "%H:%M", tm);
        if (!s)
          return 0;
        break;
      case 'S':
        dest = &tm->tm_sec;
        min = 0;
        range = 61;
        goto numeric_range;
      case 'T':
        s = strptime(s, "%H:%M:%S", tm);
        if (!s)
          return 0;
        break;
      case 'U':
      case 'W':
        /* Throw away result, for now. (FIXME?) */
        dest = &dummy;
        min = 0;
        range = 54;
        goto numeric_range;
      case 'w':
        dest = &tm->tm_wday;
        min = 0;
        range = 7;
        goto numeric_range;
      case 'x':
        s = strptime(s, nl_langinfo(D_FMT), tm);
        if (!s)
          return 0;
        break;
      case 'X':
        s = strptime(s, nl_langinfo(T_FMT), tm);
        if (!s)
          return 0;
        break;
      case 'y':
        dest = &tm->tm_year;
        w = 2;
        want_century |= 1;
        goto numeric_digits;
      case 'Y':
        dest = &tm->tm_year;
        if (w < 0)
          w = 4;
        adj = 1900;
        want_century = 0;
        goto numeric_digits;
      case '%':
        if (*s++ != '%')
          return 0;
        break;
      default:
        return 0;
      numeric_range:
        if (!isdigit(*s))
          return 0;
        *dest = 0;
        for (i = 1; i <= min + range && isdigit(*s); i *= 10)
          *dest = *dest * 10 + *s++ - '0';
        if (*dest - min >= (unsigned)range)
          return 0;
        *dest -= adj;
        switch ((char*)dest - (char*)tm) { case offsetof(struct tm, tm_yday):; }
        goto update;
      numeric_digits:
        neg = 0;
        if (*s == '+')
          s++;
        else if (*s == '-')
          neg = 1, s++;
        if (!isdigit(*s))
          return 0;
        for (*dest = i = 0; i < w && isdigit(*s); i++)
          *dest = *dest * 10 + *s++ - '0';
        if (neg)
          *dest = -*dest;
        *dest -= adj;
        goto update;
      symbolic_range:
        for (i = 2 * range - 1; i >= 0; i--) {
          ex = nl_langinfo(min + i);
          len = strlen(ex);
          if (strncasecmp(s, ex, len))
            continue;
          s += len;
          *dest = i % range;
          break;
        }
        if (i < 0)
          return 0;
        goto update;
      update:
          // FIXME
          ;
    }
  }
  if (want_century) {
    if (want_century & 2)
      tm->tm_year += century * 100 - 1900;
    else if (tm->tm_year <= 68)
      tm->tm_year += 100;
  }
  return (char*)s;
}