webtools/site/countdown/timer.mjs

// BigInt keeps entered integers exact, even for very large countdowns.
export function integer(value) {
    return /^\d+$/.test(String(value)) ? BigInt(value) : 0n;
}

export function splitSeconds(seconds) {
    return {
        minutes: String(seconds / 60n),
        seconds: String(seconds % 60n).padStart(2, '0'),
    };
}

export function adjustTime(minutes, seconds, field, delta) {
    const total = integer(minutes) * 60n + integer(seconds);
    if (field === 'minutes') {
        const adjusted = total / 60n + delta;
        return splitSeconds((adjusted < 0n ? 0n : adjusted) * 60n + total % 60n);
    }
    const adjusted = total + delta;
    return splitSeconds(adjusted < 0n ? 0n : adjusted);
}

export class Countdown {
    constructor(now = Date.now) {
        this.now = now;
        this.state = 'idle';
        this.finished = false;
        this.duration = 0n;
        this.remaining = 0n;
        this.deadline = 0n;
    }

    edit(minutes, seconds) {
        if (this.state === 'running') return;
        this.duration = (integer(minutes) * 60n + integer(seconds)) * 1000n;
        this.remaining = this.duration;
        this.state = 'idle';
        this.finished = false;
    }

    update() {
        if (this.state !== 'running') return;
        const delta = this.deadline - BigInt(this.now());
        this.remaining = delta > 0n ? delta : 0n;
        if (this.remaining === 0n) {
            this.state = 'idle';
            this.finished = true;
        }
    }

    toggle() {
        if (this.state === 'running') {
            this.update();
            if (!this.finished) this.state = 'paused';
            return;
        }
        if (this.finished) this.remaining = this.duration;
        this.finished = false;
        this.deadline = BigInt(this.now()) + this.remaining;
        this.state = 'running';
        this.update();
    }

    reset() {
        this.remaining = this.duration;
        this.state = 'idle';
        this.finished = false;
    }

    display() {
        // Round up so the timer only displays zero once its deadline is reached.
        return splitSeconds((this.remaining + 999n) / 1000n);
    }
}