datetime.nula

// Nulang standard library — DateTime type and operations.
// Import: `import stdlib::datetime`

/// A point in time represented as calendar fields.
pub type DateTime = {
    year: Int,
    month: Int,
    day: Int,
    hour: Int,
    minute: Int,
    second: Int,
}

// ── Helpers ────────────────────────────────────────────────────────────────

/// True when y is a leap year (Gregorian calendar).
fn is_leap(y: Int) -> Bool {
    (y % 4 == 0 && y % 100 != 0) || y % 400 == 0
}

/// Number of days in the given year.
fn days_in_year(y: Int) -> Int {
    if is_leap(y) then 366 else 365
}

/// Days per month (non-leap index 0 = January).
fn month_days(month: Int, leap: Bool) -> Int {
    if month == 1 then 31
    else if month == 2 then { if leap then 29 else 28 }
    else if month == 3 then 31
    else if month == 4 then 30
    else if month == 5 then 31
    else if month == 6 then 30
    else if month == 7 then 31
    else if month == 8 then 31
    else if month == 9 then 30
    else if month == 10 then 31
    else if month == 11 then 30
    else if month == 12 then 31
    else 0
}

// ── Conversion ──────────────────────────────────────────────────────────────

/// Convert a Unix timestamp (seconds since epoch) to a DateTime.
///
/// Handles timestamps before 1970 (negative values) via floor division.
/// The conversion is accurate for all dates in the Gregorian calendar.
/// Leap seconds are not accounted for.
fn unix_to_datetime(ts: Int) -> DateTime {
    // Seconds within the day.
    let sec_of_day = ts % 86400;
    var sec = sec_of_day;
    // Handle negative remainders for timestamps before epoch.
    if sec < 0 then { sec = sec + 86400 } else {};
    let hour = sec / 3600;
    let minute = (sec % 3600) / 60;
    let second = sec % 60;

    // Days since epoch (floor division for negative timestamps).
    var days = ts / 86400;
    if ts < 0 && ts % 86400 != 0 then { days = days - 1 } else {};

    // Walk forward/backward through years.
    var year = 1970;
    if days >= 0 then {
        while days >= days_in_year(year) {
            days = days - days_in_year(year);
            year = year + 1
        }
    } else {
        while days < 0 {
            year = year - 1;
            days = days + days_in_year(year)
        }
    };

    // days is now day-of-year (0-indexed).
    let leap = is_leap(year);
    var month = 1;
    while days >= month_days(month, leap) {
        days = days - month_days(month, leap);
        month = month + 1
    };

    { year: year, month: month, day: days + 1, hour: hour, minute: minute, second: second }
}

// ── Public API ─────────────────────────────────────────────────────────────

/// Return the current system time as a DateTime.
///
/// Calls `perform Time.now()` to obtain a Unix timestamp (seconds since
/// 1970-01-01 00:00:00 UTC) and delegates to `unix_to_datetime`.
pub fn now() -> DateTime {
    let ts = perform Time.now();
    unix_to_datetime(ts)
}

/// Create a DateTime from individual fields.
pub fn new(year: Int, month: Int, day: Int, hour: Int, minute: Int, second: Int) -> DateTime {
    { year: year, month: month, day: day, hour: hour, minute: minute, second: second }
}

/// Check if a DateTime represents a valid calendar date.
/// Does basic range validation on each field.
pub fn is_valid(dt: DateTime) -> Bool {
    dt.year >= 0 &&
    dt.month >= 1 && dt.month <= 12 &&
    dt.day >= 1 && dt.day <= 31 &&
    dt.hour >= 0 && dt.hour <= 23 &&
    dt.minute >= 0 && dt.minute <= 59 &&
    dt.second >= 0 && dt.second <= 59
}