parser.c

#include <ruby/ruby.h>
#include <time.h>
#include <unistd.h>

#define CONST_GET(scope, constant) rb_const_get(scope, rb_intern(constant))

VALUE day_secs;
VALUE cDateTime;
VALUE fNewBang;

uint64_t epoch_ajd_n, epoch_ajd_d;

// calculates local offset at a given time, including any DST variations.
size_t client_tzoffset(struct tm *given) {
    struct tm tm;
    uint64_t utc, local;
    memcpy(&tm, given, sizeof(tm));
    tm.tm_isdst = -1;
    local = mktime(&tm);
    gmtime_r(&local, &tm);
    tm.tm_isdst = -1;
    utc = mktime(&tm);
    return local-utc;
}

VALUE rb_datetime_parse_iso8601(VALUE self, VALUE str) {
    struct tm tm;
    uint64_t epoch, adjust, offset, tzoffset;
    const char* data = RSTRING_PTR(str);

    double usec = 0;
    char tzsign = 0;
    int tzhour  = 0, tzmin = 0;

    memset(&tm, 0, sizeof(struct tm));
    if (strchr(data, '.')) {
        sscanf(data, "%04d-%02d-%02d %02d:%02d:%02d%lf%c%02d:%02d",
            &tm.tm_year, &tm.tm_mon, &tm.tm_mday, &tm.tm_hour, &tm.tm_min, &tm.tm_sec,
            &usec, &tzsign, &tzhour, &tzmin);
    }
    else {
        sscanf(data, "%04d-%02d-%02d %02d:%02d:%02d%c%02d:%02d",
            &tm.tm_year, &tm.tm_mon, &tm.tm_mday, &tm.tm_hour, &tm.tm_min, &tm.tm_sec,
            &tzsign, &tzhour, &tzmin);
    }
    tm.tm_year  -= 1900;
    tm.tm_mon   -= 1;
    tm.tm_isdst = -1;
    if (tm.tm_mday > 0) {
        epoch  = mktime(&tm);
        adjust = client_tzoffset(&tm);
        offset = adjust;
        if (tzsign == '+' || tzsign == '-') {
            offset = tzsign == '+' ?
                  (time_t)tzhour *  3600 + (time_t)tzmin *  60
                : (time_t)tzhour * -3600 + (time_t)tzmin * -60;
        }
        VALUE ajd = rb_rational_new(ULONG2NUM(epoch_ajd_n + epoch + adjust - offset), day_secs);
        return rb_funcall(cDateTime, fNewBang, 3, ajd, rb_rational_new(INT2FIX(offset), day_secs), INT2NUM(2299161));
    }

    return Qnil;
}

void Init_parser(void) {
    rb_require("date");
    // astronomical julian date for 1970-01-01 00:00:00 is 2440587.5
    // epoch_ajd = Rational(epoch_ajd_n, epoch_ajd_d)
    epoch_ajd_d = 86400;
    epoch_ajd_n = (2440587L*2+1) * 43200L;
    day_secs    = INT2FIX(86400);
    fNewBang    = rb_intern("new!");
    cDateTime   = CONST_GET(rb_mKernel, "DateTime");
    rb_define_module_function(cDateTime, "parse_iso8601", RUBY_METHOD_FUNC(rb_datetime_parse_iso8601), 1);
}