feat(checkout): synchronize purchase countdown

src/app/core/services/checkout-countdown.service.spec.ts

@@ -0,0 +1,57 @@
+import { TestBed } from '@angular/core/testing';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import { CheckoutCountdownService } from './checkout-countdown.service';
+
+describe('CheckoutCountdownService', () => {
+  let service: CheckoutCountdownService;
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2026-08-27T12:00:00.000Z'));
+    TestBed.configureTestingModule({ providers: [CheckoutCountdownService] });
+    service = TestBed.inject(CheckoutCountdownService);
+  });
+
+  afterEach(() => {
+    service.clear();
+    vi.useRealTimers();
+  });
+
+  it('counts down from the checkout server timing without depending on the client clock', () => {
+    service.synchronize({
+      expires_at: '2026-08-27T15:10:00.000Z',
+      expires_in_seconds: 600,
+      server_time: '2026-08-27T15:00:00.000Z',
+    });
+
+    expect(service.remainingSeconds()).toBe(600);
+
+    vi.advanceTimersByTime(1_000);
+
+    expect(service.remainingSeconds()).toBe(599);
+  });
+
+  it('recalculates against the deadline after a delayed browser interval', () => {
+    service.synchronize({
+      expires_at: null,
+      expires_in_seconds: 10,
+      server_time: '2026-08-27T15:00:00.000Z',
+    });
+
+    vi.setSystemTime(new Date('2026-08-27T12:00:07.000Z'));
+    vi.advanceTimersByTime(1_000);
+
+    expect(service.remainingSeconds()).toBe(2);
+  });
+
+  it('clears the countdown when checkout has no expiration', () => {
+    service.synchronize({
+      expires_at: null,
+      expires_in_seconds: null,
+      server_time: '2026-08-27T15:00:00.000Z',
+    });
+
+    expect(service.remainingSeconds()).toBeNull();
+  });
+});

src/app/core/services/checkout-countdown.service.ts

@@ -0,0 +1,87 @@
+import { Injectable, OnDestroy, signal } from '@angular/core';
+
+export interface CheckoutTiming {
+  expires_at: string | null;
+  expires_in_seconds: number | null;
+  server_time: string;
+}
+
+@Injectable({
+  providedIn: 'root',
+})
+export class CheckoutCountdownService implements OnDestroy {
+  private readonly remainingSecondsState = signal<number | null>(null);
+  private deadlineMs: number | null = null;
+  private intervalId: ReturnType<typeof setInterval> | null = null;
+
+  readonly remainingSeconds = this.remainingSecondsState.asReadonly();
+
+  synchronize(timing: CheckoutTiming): void {
+    const remainingSeconds = this.resolveRemainingSeconds(timing);
+
+    if (remainingSeconds === null) {
+      this.clear();
+      return;
+    }
+
+    this.stopInterval();
+    this.deadlineMs = Date.now() + remainingSeconds * 1_000;
+    this.updateRemainingSeconds();
+
+    if (remainingSeconds > 0) {
+      this.intervalId = setInterval(() => this.updateRemainingSeconds(), 1_000);
+    }
+  }
+
+  clear(): void {
+    this.stopInterval();
+    this.deadlineMs = null;
+    this.remainingSecondsState.set(null);
+  }
+
+  ngOnDestroy(): void {
+    this.clear();
+  }
+
+  private resolveRemainingSeconds(timing: CheckoutTiming): number | null {
+    const expiresAt = timing.expires_at ? Date.parse(timing.expires_at) : Number.NaN;
+    const serverTime = Date.parse(timing.server_time);
+
+    if (Number.isFinite(expiresAt) && Number.isFinite(serverTime)) {
+      return Math.max(0, Math.ceil((expiresAt - serverTime) / 1_000));
+    }
+
+    if (
+      typeof timing.expires_in_seconds === 'number' &&
+      Number.isFinite(timing.expires_in_seconds)
+    ) {
+      return Math.max(0, Math.ceil(timing.expires_in_seconds));
+    }
+
+    if (Number.isFinite(expiresAt)) {
+      return Math.max(0, Math.ceil((expiresAt - Date.now()) / 1_000));
+    }
+
+    return null;
+  }
+
+  private updateRemainingSeconds(): void {
+    if (this.deadlineMs === null) {
+      return;
+    }
+
+    const remainingSeconds = Math.max(0, Math.ceil((this.deadlineMs - Date.now()) / 1_000));
+    this.remainingSecondsState.set(remainingSeconds);
+
+    if (remainingSeconds === 0) {
+      this.stopInterval();
+    }
+  }
+
+  private stopInterval(): void {
+    if (this.intervalId !== null) {
+      clearInterval(this.intervalId);
+      this.intervalId = null;
+    }
+  }
+}