/*
 * posix_time.c — minimal POSIX <time.h> implementation on top of
 * getdatetime() (ESTEX SYSTIME $21).
 *
 * SDCC's z80.lib bundles time/localtime/mktime AND _RtcRead in a
 * single time.rel module, so the user can't override _RtcRead from a
 * separate object — overriding triggers a "multiple definition"
 * linker error.  We sidestep that by implementing the whole POSIX time
 * API ourselves; the linker then never pulls SDCC's time.rel.
 *
 * The epoch is Unix (1970-01-01 00:00:00).  No timezone support —
 * gmtime and localtime are identical.  No DST.
 */

#include <stdio.h>
#include <time.h>

static const unsigned char mdays[12] = {
    31,28,31,30,31,30,31,31,30,31,30,31
};

static const char *const dnames[7] = {
    "Sun","Mon","Tue","Wed","Thu","Fri","Sat"
};
static const char *const mnames[12] = {
    "Jan","Feb","Mar","Apr","May","Jun",
    "Jul","Aug","Sep","Oct","Nov","Dec"
};

static int is_leap(unsigned int y)
{
    return (y % 4 == 0 && y % 100 != 0) || (y % 400 == 0);
}

/* Days elapsed from 1970-01-01 to (y, 1, 1). */
static unsigned long year_days(unsigned int y)
{
    unsigned long d = 0;
    for (unsigned int i = 1970; i < y; i++)
        d += is_leap(i) ? 366 : 365;
    return d;
}

/* Days from Jan 1 to month start (1-based month input). */
static unsigned int month_days(unsigned int y, unsigned int m)
{
    unsigned int d = 0;
    for (unsigned int i = 0; i < m - 1; i++) d += mdays[i];
    if (m > 2 && is_leap(y)) d++;
    return d;
}

time_t time(time_t *t)
{
    datetime_t dt;
    getdatetime(&dt);
    unsigned long days = year_days(dt.year)
                       + month_days(dt.year, dt.month)
                       + (dt.day - 1);
    time_t epoch = days * 86400UL
                 + (unsigned long)dt.hour   * 3600UL
                 + (unsigned long)dt.minute * 60UL
                 + dt.second;
    if (t) *t = epoch;
    return epoch;
}

/* localtime and gmtime share one static buffer — caller copies if
 * needed across further calls (matches POSIX behaviour). */
static struct tm tm_buf;

struct tm *gmtime(time_t *timep)
{
    unsigned long sec = *timep;
    tm_buf.tm_sec  = (unsigned char)(sec % 60);  sec /= 60;
    tm_buf.tm_min  = (unsigned char)(sec % 60);  sec /= 60;
    tm_buf.tm_hour = (unsigned char)(sec % 24);  sec /= 24;
    /* sec is now days since 1970-01-01 (Thursday). */
    tm_buf.tm_wday = (unsigned char)((4 + sec) % 7);
    /* find year */
    unsigned int y = 1970;
    unsigned long days = sec;
    while (days >= (unsigned long)(is_leap(y) ? 366 : 365)) {
        days -= is_leap(y) ? 366 : 365;
        y++;
    }
    tm_buf.tm_year = (int)y - 1900;
    tm_buf.tm_yday = (int)days;
    /* find month/day */
    unsigned int m = 0;
    while (m < 12) {
        unsigned int dim = mdays[m] + ((m == 1) && is_leap(y) ? 1u : 0u);
        if (days < dim) break;
        days -= dim;
        m++;
    }
    tm_buf.tm_mon  = (unsigned char)m;
    tm_buf.tm_mday = (unsigned char)(days + 1);
    tm_buf.tm_isdst = 0;
    tm_buf.tm_hundredth = 0;
    return &tm_buf;
}

struct tm *localtime(time_t *timep)
{
    return gmtime(timep);   /* no timezone */
}

time_t mktime(struct tm *tm)
{
    unsigned int y = (unsigned int)(tm->tm_year + 1900);
    unsigned long days = year_days(y)
                       + month_days(y, (unsigned int)tm->tm_mon + 1)
                       + (unsigned int)(tm->tm_mday - 1);
    time_t epoch = days * 86400UL
                 + (unsigned long)tm->tm_hour * 3600UL
                 + (unsigned long)tm->tm_min  * 60UL
                 + tm->tm_sec;
    /* Backfill wday/yday so callers can inspect them. */
    tm->tm_wday = (unsigned char)((4 + days) % 7);
    tm->tm_yday = (int)month_days(y, (unsigned int)tm->tm_mon + 1)
                + (tm->tm_mday - 1);
    return epoch;
}

/* "Day Mon DD HH:MM:SS YYYY\n" — 25 chars + NUL. */
static char asctime_buf[26];

char *asctime(struct tm *tm)
{
    sprintf(asctime_buf, "%s %s %2d %02d:%02d:%02d %d\n",
            dnames[tm->tm_wday % 7],
            mnames[tm->tm_mon  % 12],
            tm->tm_mday,
            tm->tm_hour, tm->tm_min, tm->tm_sec,
            tm->tm_year + 1900);
    return asctime_buf;
}

char *ctime(time_t *timep)
{
    return asctime(localtime(timep));
}