# mktime.awk --- convert a canonical date representation
#                into a timestamp
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

BEGIN    \
{
    # Initialize table of month lengths
    _tm_months[0,1] = _tm_months[1,1] = 31
    _tm_months[0,2] = 28; _tm_months[1,2] = 29
    _tm_months[0,3] = _tm_months[1,3] = 31
    _tm_months[0,4] = _tm_months[1,4] = 30
    _tm_months[0,5] = _tm_months[1,5] = 31
    _tm_months[0,6] = _tm_months[1,6] = 30
    _tm_months[0,7] = _tm_months[1,7] = 31
    _tm_months[0,8] = _tm_months[1,8] = 31
    _tm_months[0,9] = _tm_months[1,9] = 30
    _tm_months[0,10] = _tm_months[1,10] = 31
    _tm_months[0,11] = _tm_months[1,11] = 30
    _tm_months[0,12] = _tm_months[1,12] = 31
}
# decide if a year is a leap year
function _tm_isleap(year,    ret)
{
    ret = (year % 4 == 0 && year % 100 != 0) ||
            (year % 400 == 0)

    return ret
}
# convert a date into seconds
function _tm_addup(a,    total, yearsecs, daysecs,
                         hoursecs, i, j)
{
    hoursecs = 60 * 60
    daysecs = 24 * hoursecs
    yearsecs = 365 * daysecs

    total = (a[1] - 1970) * yearsecs

    # extra day for leap years
    for (i = 1970; i < a[1]; i++)
        if (_tm_isleap(i))
            total += daysecs

    j = _tm_isleap(a[1])
    for (i = 1; i < a[2]; i++)
        total += _tm_months[j, i] * daysecs

    total += (a[3] - 1) * daysecs
    total += a[4] * hoursecs
    total += a[5] * 60
    total += a[6]

    return total
}
# mktime --- convert a date into seconds,
#            compensate for time zone

function mktime(str,    res1, res2, a, b, i, j, t, diff)
{
    i = split(str, a, " ")    # don't rely on FS

    if (i != 6)
        return -1

    # force numeric
    for (j in a)
        a[j] += 0

    # validate
    if (a[1] < 1970 ||
        a[2] < 1 || a[2] > 12 ||
        a[3] < 1 || a[3] > 31 ||
        a[4] < 0 || a[4] > 23 ||
        a[5] < 0 || a[5] > 59 ||
        a[6] < 0 || a[6] > 60 )
            return -1

    res1 = _tm_addup(a)
    t = strftime("%Y %m %d %H %M %S", res1)

    if (_tm_debug)
        printf("(%s) -> (%s)\n", str, t) > "/dev/stderr"

    split(t, b, " ")
    res2 = _tm_addup(b)

    diff = res1 - res2

    if (_tm_debug)
        printf("diff = %d seconds\n", diff) > "/dev/stderr"

    res1 += diff

    return res1
}
BEGIN  {
    if (_tm_test) {
        printf "Enter date as yyyy mm dd hh mm ss: "
        getline _tm_test_date
    
        t = mktime(_tm_test_date)
        r = strftime("%Y %m %d %H %M %S", t)
        printf "Got back (%s)\n", r
    }
}
