shootismoke/backend

import timezones from 'timezones.json';

/**
 * Math modulo. Javascript sometimes feels like a very cruel joke.
 *
 * @see https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers
 * @example
 * ```typescript
 * -13 % 64; // -13
 * mod(-13, 64); // 51
 * ```
 */
function mod(n: number, m: number): number {
    return ((n % m) + m) % m;
}

/**
 * Is DST observed right now?
 *
 * @see https://stackoverflow.com/questions/11887934/how-to-check-if-the-dst-daylight-saving-time-is-in-effect-and-if-it-is-whats
 */
function isDstObserved(now = new Date()): boolean {
    const jan = new Date(now.getFullYear(), 0, 1);
    const jul = new Date(now.getFullYear(), 6, 1);

    const stdTimezoneOffset = Math.max(
        jan.getTimezoneOffset(),
        jul.getTimezoneOffset()
    );

    return now.getTimezoneOffset() < stdTimezoneOffset;
}

/**
 * Find all the timezones whose current hour is `hour`.
 *
 * @param hour - The hour we would like to find the timezones.
 * @param now - The base hour, for example use `new Date()` to find the
 * timzones at `hour` right now.
 */
export function findTimezonesAt(hour: number, now: Date): string[] {
    // Get offet in the [-10, 13] range
    let offset = mod(hour - now.getUTCHours(), 24);
    if (offset > 13) {
        offset -= 24;
    }

    // Is DST currently observed?
    const isDstNow = isDstObserved();

    const tzs = timezones.filter(
        (tz) =>
            offset ===
            (tz.isdst && !isDstNow
                ? Math.floor(tz.offset - 1) // Now is not DST, we remove an hour to this tz that's following DST
                : Math.floor(tz.offset))
    );

    return tzs.map(({ utc }) => utc).flat();
}