feat: pure session clock with pause/resume

src/core/clock.js

@@ -0,0 +1,28 @@
+export function initialClock() {
+  return { startedAt: null, pausedAt: null, pausedAccumMs: 0 };
+}
+
+export function start(state, now) {
+  if (state.startedAt != null) return state;
+  return { startedAt: now, pausedAt: null, pausedAccumMs: 0 };
+}
+
+export function pause(state, now) {
+  if (state.startedAt == null || state.pausedAt != null) return state;
+  return { ...state, pausedAt: now };
+}
+
+export function resume(state, now) {
+  if (state.pausedAt == null) return state;
+  return { ...state, pausedAt: null, pausedAccumMs: state.pausedAccumMs + (now - state.pausedAt) };
+}
+
+export function elapsedMs(state, now) {
+  if (state.startedAt == null) return 0;
+  const end = state.pausedAt != null ? state.pausedAt : now;
+  return Math.max(0, end - state.startedAt - state.pausedAccumMs);
+}
+
+export function isRunning(state) { return state.startedAt != null && state.pausedAt == null; }
+
+export function isPaused(state) { return state.pausedAt != null; }

tests/core/clock.test.js

@@ -0,0 +1,30 @@
+import { describe, it, expect } from 'vitest';
+import { initialClock, start, pause, resume, elapsedMs, isRunning, isPaused } from '../../src/core/clock.js';
+
+describe('session clock', () => {
+  it('reads zero before start', () => {
+    const c = initialClock();
+    expect(elapsedMs(c, 1000)).toBe(0);
+    expect(isRunning(c)).toBe(false);
+  });
+
+  it('measures elapsed from start', () => {
+    const c = start(initialClock(), 1000);
+    expect(isRunning(c)).toBe(true);
+    expect(elapsedMs(c, 61000)).toBe(60000); // 1 minute
+  });
+
+  it('freezes while paused and excludes paused time after resume', () => {
+    let c = start(initialClock(), 0);
+    c = pause(c, 60000);            // paused at 1:00
+    expect(isPaused(c)).toBe(true);
+    expect(elapsedMs(c, 120000)).toBe(60000); // frozen at 1:00 while paused
+    c = resume(c, 180000);         // resumed after 2 min paused
+    expect(elapsedMs(c, 240000)).toBe(120000); // 4 min wall − 2 min paused = 2 min
+  });
+
+  it('ignores double start', () => {
+    const c = start(start(initialClock(), 100), 5000);
+    expect(elapsedMs(c, 1100)).toBe(1000);
+  });
+});