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

const char* __nl_langinfo(nl_item);

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*);

const char* __strftime_fmt_1(char (*s)[100], size_t* l, int f, const struct tm* tm) {
    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, "%+.2ld%.2ld", (tm->__tm_gmtoff) / 3600,
                      labs(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(item);
string:
    *l = strlen(fmt);
    return fmt;
nl_strftime:
    fmt = __nl_langinfo(item);
recu_strftime:
    *l = strftime(*s, sizeof *s, fmt, tm);
    if (!*l)
        return 0;
    return *s;
}

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