/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2006 Poul-Henning Kamp
 * 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 AUTHOR 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 AUTHOR 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.
 *
 * Convert MS-DOS FAT format timestamps to and from unix timespecs
 *
 * FAT filestamps originally consisted of two 16 bit integers, encoded like
 * this:
 *
 *      yyyyyyymmmmddddd (year - 1980, month, day)
 *
 *      hhhhhmmmmmmsssss (hour, minutes, seconds divided by two)
 *
 * Subsequently even Microsoft realized that files could be accessed in less
 * than two seconds and a byte was added containing:
 *
 *      sfffffff         (second mod two, 100ths of second)
 *
 * FAT timestamps are in the local timezone, with no indication of which
 * timezone much less if daylight savings time applies.
 *
 * Later on again, in Windows NT, timestamps were defined relative to GMT.
 *
 * Purists will point out that UTC replaced GMT for such uses around
 * half a century ago, already then.  Ironically "NT" was an abbreviation of 
 * "New Technology".  Anyway...
 *
 * The 'utc' argument determines if the resulting FATTIME timestamp
 * should be on the UTC or local timezone calendar.
 *
 * The conversion functions below cut time into four-year leap-year
 * cycles rather than single years and uses table lookups inside those
 * cycles to get the months and years sorted out.
 *
 * Obviously we cannot calculate the correct table index going from
 * a posix seconds count to Y/M/D, but we can get pretty close by
 * dividing the daycount by 32 (giving a too low index), and then
 * adjusting upwards a couple of steps if necessary.
 *
 * FAT timestamps have 7 bits for the year and starts at 1980, so
 * they can represent up to 2107 which means that the non-leap-year
 * 2100 must be handled.
 */

#include <sys/param.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/clock.h>

#ifdef TEST_DRIVER
/* stub for testing */
#define utc_offset() 0
#endif

#define DAY     (24 * 60 * 60)  /* Length of day in seconds */
#define YEAR    365             /* Length of normal year */
#define LYC     (4 * YEAR + 1)  /* Length of 4 year leap-year cycle */
#define T1980   (10 * 365 + 2)  /* Days from 1970 to 1980 */
#define T2108   (138 * 365 + 33) /* Days from 1970 to 2108 */

/* End of month is N days from start of (normal) year */
#define JAN     31
#define FEB     (JAN + 28)
#define MAR     (FEB + 31)
#define APR     (MAR + 30)
#define MAY     (APR + 31)
#define JUN     (MAY + 30)
#define JUL     (JUN + 31)
#define AUG     (JUL + 31)
#define SEP     (AUG + 30)
#define OCT     (SEP + 31)
#define NOV     (OCT + 30)
#define DEC     (NOV + 31)

/* Table of months in a 4 year leap-year cycle */

#define ENC(y,m)        (((y) << 9) | ((m) << 5))

static const struct {
        uint16_t        days;   /* month start in days relative to cycle */
        uint16_t        coded;  /* encoded year + month information */
} mtab[48] = {
        {   0 + 0 * YEAR,     ENC(0, 1)  },

        { JAN + 0 * YEAR,     ENC(0, 2)  }, { FEB + 0 * YEAR + 1, ENC(0, 3)  },
        { MAR + 0 * YEAR + 1, ENC(0, 4)  }, { APR + 0 * YEAR + 1, ENC(0, 5)  },
        { MAY + 0 * YEAR + 1, ENC(0, 6)  }, { JUN + 0 * YEAR + 1, ENC(0, 7)  },
        { JUL + 0 * YEAR + 1, ENC(0, 8)  }, { AUG + 0 * YEAR + 1, ENC(0, 9)  },
        { SEP + 0 * YEAR + 1, ENC(0, 10) }, { OCT + 0 * YEAR + 1, ENC(0, 11) },
        { NOV + 0 * YEAR + 1, ENC(0, 12) }, { DEC + 0 * YEAR + 1, ENC(1, 1)  },

        { JAN + 1 * YEAR + 1, ENC(1, 2)  }, { FEB + 1 * YEAR + 1, ENC(1, 3)  },
        { MAR + 1 * YEAR + 1, ENC(1, 4)  }, { APR + 1 * YEAR + 1, ENC(1, 5)  },
        { MAY + 1 * YEAR + 1, ENC(1, 6)  }, { JUN + 1 * YEAR + 1, ENC(1, 7)  },
        { JUL + 1 * YEAR + 1, ENC(1, 8)  }, { AUG + 1 * YEAR + 1, ENC(1, 9)  },
        { SEP + 1 * YEAR + 1, ENC(1, 10) }, { OCT + 1 * YEAR + 1, ENC(1, 11) },
        { NOV + 1 * YEAR + 1, ENC(1, 12) }, { DEC + 1 * YEAR + 1, ENC(2, 1)  },

        { JAN + 2 * YEAR + 1, ENC(2, 2)  }, { FEB + 2 * YEAR + 1, ENC(2, 3)  },
        { MAR + 2 * YEAR + 1, ENC(2, 4)  }, { APR + 2 * YEAR + 1, ENC(2, 5)  },
        { MAY + 2 * YEAR + 1, ENC(2, 6)  }, { JUN + 2 * YEAR + 1, ENC(2, 7)  },
        { JUL + 2 * YEAR + 1, ENC(2, 8)  }, { AUG + 2 * YEAR + 1, ENC(2, 9)  },
        { SEP + 2 * YEAR + 1, ENC(2, 10) }, { OCT + 2 * YEAR + 1, ENC(2, 11) },
        { NOV + 2 * YEAR + 1, ENC(2, 12) }, { DEC + 2 * YEAR + 1, ENC(3, 1)  },

        { JAN + 3 * YEAR + 1, ENC(3, 2)  }, { FEB + 3 * YEAR + 1, ENC(3, 3)  },
        { MAR + 3 * YEAR + 1, ENC(3, 4)  }, { APR + 3 * YEAR + 1, ENC(3, 5)  },
        { MAY + 3 * YEAR + 1, ENC(3, 6)  }, { JUN + 3 * YEAR + 1, ENC(3, 7)  },
        { JUL + 3 * YEAR + 1, ENC(3, 8)  }, { AUG + 3 * YEAR + 1, ENC(3, 9)  },
        { SEP + 3 * YEAR + 1, ENC(3, 10) }, { OCT + 3 * YEAR + 1, ENC(3, 11) },
        { NOV + 3 * YEAR + 1, ENC(3, 12) }
};

void
timespec2fattime(const struct timespec *tsp, int utc, uint16_t *ddp,
    uint16_t *dtp, uint8_t *dhp)
{
        time_t t1;
        unsigned t2, l, m;

        t1 = tsp->tv_sec;
        if (!utc)
                t1 -= utc_offset();

        if (dhp != NULL)
                *dhp = (tsp->tv_sec & 1) * 100 + tsp->tv_nsec / 10000000;
        if (dtp != NULL) {
                *dtp = (t1 / 2) % 30;
                *dtp |= ((t1 / 60) % 60) << 5;
                *dtp |= ((t1 / 3600) % 24) << 11;
        }
        if (ddp != NULL) {
                t2 = t1 / DAY;
                if (t2 < T1980) {
                        /* Impossible date, truncate to 1980-01-01 */
                        *ddp = 0x0021;
                } else {
                        t2 -= T1980;

                        /* 2100 is not a leap year */
                        if (t2 >= ((2100 - 1980) / 4 * LYC + FEB))
                                t2++;

                        /* Account for full leapyear cycles */
                        l = t2 / LYC;
                        *ddp = (l * 4) << 9;
                        t2 -= l * LYC;

                        /* Find approximate table entry */
                        m = t2 / 32;

                        /* Find correct table entry */
                        while (m < 47 && mtab[m + 1].days <= t2)
                                m++;

                        /* Get year + month from the table */
                        *ddp += mtab[m].coded;

                        /* And apply the day in the month */
                        t2 -= mtab[m].days - 1;
                        *ddp |= t2;
                }
        }
}

/*
 * Table indexed by the bottom two bits of year + four bits of the month
 * from the FAT timestamp, returning number of days into 4 year long
 * leap-year cycle
 */

#define DCOD(m, y, l)   ((m) + YEAR * (y) + (l))
static const uint16_t daytab[64] = {
        0,               DCOD(  0, 0, 0), DCOD(JAN, 0, 0), DCOD(FEB, 0, 1),
        DCOD(MAR, 0, 1), DCOD(APR, 0, 1), DCOD(MAY, 0, 1), DCOD(JUN, 0, 1),
        DCOD(JUL, 0, 1), DCOD(AUG, 0, 1), DCOD(SEP, 0, 1), DCOD(OCT, 0, 1),
        DCOD(NOV, 0, 1), DCOD(DEC, 0, 1), 0,               0,
        0,               DCOD(  0, 1, 1), DCOD(JAN, 1, 1), DCOD(FEB, 1, 1),
        DCOD(MAR, 1, 1), DCOD(APR, 1, 1), DCOD(MAY, 1, 1), DCOD(JUN, 1, 1),
        DCOD(JUL, 1, 1), DCOD(AUG, 1, 1), DCOD(SEP, 1, 1), DCOD(OCT, 1, 1),
        DCOD(NOV, 1, 1), DCOD(DEC, 1, 1), 0,               0,
        0,               DCOD(  0, 2, 1), DCOD(JAN, 2, 1), DCOD(FEB, 2, 1),
        DCOD(MAR, 2, 1), DCOD(APR, 2, 1), DCOD(MAY, 2, 1), DCOD(JUN, 2, 1),
        DCOD(JUL, 2, 1), DCOD(AUG, 2, 1), DCOD(SEP, 2, 1), DCOD(OCT, 2, 1),
        DCOD(NOV, 2, 1), DCOD(DEC, 2, 1), 0,               0,
        0,               DCOD(  0, 3, 1), DCOD(JAN, 3, 1), DCOD(FEB, 3, 1),
        DCOD(MAR, 3, 1), DCOD(APR, 3, 1), DCOD(MAY, 3, 1), DCOD(JUN, 3, 1),
        DCOD(JUL, 3, 1), DCOD(AUG, 3, 1), DCOD(SEP, 3, 1), DCOD(OCT, 3, 1),
        DCOD(NOV, 3, 1), DCOD(DEC, 3, 1), 0,               0
};

void
fattime2timespec(unsigned dd, unsigned dt, unsigned dh, int utc,
    struct timespec *tsp)
{
        unsigned day;

        /* Unpack time fields */
        tsp->tv_sec = (dt & 0x1f) << 1;
        tsp->tv_sec += ((dt & 0x7e0) >> 5) * 60;
        tsp->tv_sec += ((dt & 0xf800) >> 11) * 3600;
        tsp->tv_sec += dh / 100;
        tsp->tv_nsec = (dh % 100) * 10000000;

        /* Day of month */
        day = (dd & 0x1f) - 1;

        /* Full leap-year cycles */
        day += LYC * ((dd >> 11) & 0x1f);

        /* Month offset from leap-year cycle */
        day += daytab[(dd >> 5) & 0x3f];

        /* 2100 is not a leap year */
        if (day >= ((2100 - 1980) / 4 * LYC + FEB))
                day--;

        /* Align with time_t epoch */
        day += T1980;

        tsp->tv_sec += (time_t) DAY * day;
        if (!utc)
                tsp->tv_sec += utc_offset();
}

#ifdef TEST_DRIVER

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int
main(int argc __unused, char **argv __unused)
{
        int i;
        struct timespec ts;
        struct tm tm;
        double a;
        uint16_t d, t;
        uint8_t p;
        char buf[100];

        for (i = 0; i < 10000; i++) {
                do {
                        /*
                         * 32-bits gets us to 2106-02-07 06:28:15, but we
                         * need to get to the end of 2107.  So, we generate
                         * a 36-bit second count to get us way past 2106.
                         */
                        ts.tv_sec = ((time_t) arc4random() << 4) ^ arc4random();
                } while ((ts.tv_sec < T1980 * 86400) || (ts.tv_sec >= T2108 * 86400ull));

                ts.tv_nsec = random() % 1000000000;

                printf("%10jd.%03ld -- ", (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);

                gmtime_r(&ts.tv_sec, &tm);
                strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
                printf("%s -- ", buf);

                a = ts.tv_sec + ts.tv_nsec * 1e-9;
                d = t = p = 0;
                timespec2fattime(&ts, 1, &d, &t, &p);
                printf("%04x %04x %02x -- ", d, t, p);
                printf("%3d %02d %02d %02d %02d %02d -- ",
                    ((d >> 9)  & 0x7f) + 1980,
                    (d >> 5)  & 0x0f,
                    (d >> 0)  & 0x1f,
                    (t >> 11) & 0x1f,
                    (t >> 5)  & 0x3f,
                    ((t >> 0)  & 0x1f) * 2);

                ts.tv_sec = ts.tv_nsec = 0;
                fattime2timespec(d, t, p, 1, &ts);
                printf("%10jd.%03ld == ", (intmax_t) ts.tv_sec, ts.tv_nsec / 1000000);
                gmtime_r(&ts.tv_sec, &tm);
                strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
                printf("%s -- ", buf);
                a -= ts.tv_sec + ts.tv_nsec * 1e-9;
                printf("%.3f", a);
                printf("\n");
        }
        return (0);
}

#endif /* TEST_DRIVER */