timestamp_tm.c

/*
 * Copyright (c) 2014 Christian Hansen <chansen@cpan.org>
 * <https://github.com/chansen/c-timestamp>
 * All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met: 
 * 
 * 1. Redistributions of source code must retain the above copyright notice, this
 *    list of conditions and the following disclaimer. 
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution. 
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
#include <stddef.h>
#include <time.h>
#include "timestamp.h"

static const uint16_t DayOffset[13] = {
    0, 306, 337, 0, 31, 61, 92, 122, 153, 184, 214, 245, 275
};

/* Rata Die algorithm by Peter Baum */

static void
rdn_to_struct_tm(uint32_t rdn, struct tm *tmp) {
    uint32_t Z, H, A, B;
    uint16_t C, y, m, d;

    Z = rdn + 306;
    H = 100 * Z - 25;
    A = H / 3652425;
    B = A - (A >> 2);
    y = (100 * B + H) / 36525;
    C = B + Z - (1461 * y >> 2);
    m = (535 * C + 48950) >> 14;
    if (m > 12)
        d = C - 306, y++, m -= 12;
    else
        d = C + 59 + ((y & 3) == 0 && (y % 100 != 0 || y % 400 == 0));

    tmp->tm_mday = C - DayOffset[m];    /* Day of month [1,31]           */
    tmp->tm_mon  = m - 1;               /* Month of year [0,11]          */
    tmp->tm_year = y - 1900;            /* Years since 1900              */
    tmp->tm_wday = rdn % 7;             /* Day of week [0,6] (Sunday =0) */
    tmp->tm_yday = d - 1;               /* Day of year [0,365]           */
}

#define RDN_OFFSET INT64_C(62135683200)  /* 1970-01-01T00:00:00 */

static struct tm *
timestamp_to_tm(const timestamp_t *tsp, struct tm *tmp, const bool local) {
    uint64_t sec;
    uint32_t rdn, sod;

    if (!timestamp_valid(tsp))
        return NULL;

    sec = tsp->sec + RDN_OFFSET;
    if (local)
        sec += tsp->offset * 60;
    rdn = sec / 86400;
    sod = sec % 86400;

    rdn_to_struct_tm(rdn, tmp);
    tmp->tm_sec  = sod % 60; sod /= 60;
    tmp->tm_min  = sod % 60; sod /= 60;
    tmp->tm_hour = sod;
    return tmp;
}

struct tm *
timestamp_to_tm_local(const timestamp_t *tsp, struct tm *tmp) {
    return timestamp_to_tm(tsp, tmp, true);
}

struct tm *
timestamp_to_tm_utc(const timestamp_t *tsp, struct tm *tmp) {
    return timestamp_to_tm(tsp, tmp, false);
}