Add editable countdown timer with deadline-based timing

Co-authored-by: Shelley <shelley@exe.dev>

site/countdown/timer.mjs

@@ -0,0 +1,64 @@
+// 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 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);
+    }
+}