When a pattern is over-engineering

// clock.tsexport interface Clock { now(): number }export const systemClock: Clock = { now: () => Date.now() }; // countdown.tsexport interface CountdownStrategy { remaining(startedAt: number, limitMs: number, now: number): number }export class LinearCountdown implements CountdownStrategy {  remaining(startedAt: number, limitMs: number, now: number) { return Math.max(0, startedAt + limitMs - now); }}export class ExtraTimeCountdown implements CountdownStrategy {  remaining(startedAt: number, limitMs: number, now: number) { return Math.max(0, startedAt + limitMs * 1.25 - now); }} // kv.ts (the vendor's key-value SDK, in the in-memory build it ships for tests)const buckets = new Map<string, Map<string, unknown>>();export class KvClient {  private rows: Map<string, unknown>;  constructor(bucket: string) { this.rows = buckets.get(bucket) ?? new Map(); buckets.set(bucket, this.rows); }  put(key: string, value: unknown) { this.rows.set(key, value); }  read(key: string) { return this.rows.get(key); }} // store.tsimport { KvClient } from './kv';export interface TimerStore {  save(id: string, startedAt: number, limitMs: number): void;  get(id: string): { startedAt: number; limitMs: number } | undefined;}export class KvStore implements TimerStore {  private kv = new KvClient('timers');  save(id: string, startedAt: number, limitMs: number) { this.kv.put(id, { startedAt, limitMs }); }  get(id: string) { return this.kv.read(id) as { startedAt: number; limitMs: number } | undefined; }} // timer.tsimport type { Clock } from './clock';import type { CountdownStrategy } from './countdown';import type { TimerStore } from './store';export class ExamTimer {  constructor(private clock: Clock, private countdown: CountdownStrategy, private store: TimerStore) {}  start(id: string, limitMs: number) { this.store.save(id, this.clock.now(), limitMs); }  remainingMs(id: string) {    const t = this.store.get(id);    return t ? this.countdown.remaining(t.startedAt, t.limitMs, this.clock.now()) : 0;  }  expired(id: string) { return this.remainingMs(id) === 0; }} // starts.ts (the invigilators' list)import type { TimerStore } from './store';export class StartLog {  constructor(private store: TimerStore) {}  startedAt(id: string) { return this.store.get(id)?.startedAt; }} // factory.tsimport { systemClock, type Clock } from './clock';import { LinearCountdown, type CountdownStrategy } from './countdown';import { KvStore } from './store';import { ExamTimer } from './timer';import { StartLog } from './starts';export class TimerFactory {  static create(clock: Clock = systemClock, countdown: CountdownStrategy = new LinearCountdown()) { return new ExamTimer(clock, countdown, new KvStore()); }  static startLog() { return new StartLog(new KvStore()); }} // index.tsimport type { Clock } from './clock';import { ExtraTimeCountdown } from './countdown';import { TimerFactory } from './factory';export const createTimer = (clock?: Clock) => TimerFactory.create(clock);export const createExtraTimeTimer = (clock?: Clock) => TimerFactory.create(clock, new ExtraTimeCountdown());export const startLog = () => TimerFactory.startLog(); // timer.test.tsimport { createTimer, createExtraTimeTimer, startLog } from './index';let now = 1_000_000;const clock = { now: () => now };const timer = createTimer(clock);const extra = createExtraTimeTimer(clock);const passed: string[] = [];const test = (name: string, ok: boolean) => { if (ok) passed.push(name); };timer.start('a-1', 60_000);extra.start('a-2', 60_000);test('starts full', timer.remainingMs('a-1') === 60_000);test('extra time runs longer', extra.remainingMs('a-2') === 75_000);test('start listed', startLog().startedAt('a-1') === 1_000_000);now += 59_000;test('counts down', timer.remainingMs('a-1') === 1_000);now += 2_000;test('runs out', timer.expired('a-1'));test('unknown attempt', timer.remainingMs('a-9') === 0);export { passed };

// stalls.tsexport const stallsLine = (seat: string, cents: number): string =>  `Stalls · seat ${seat} · £${(cents / 100).toFixed(2)} · booking fee included`; // circle.ts (copied from stalls.ts)export const circleLine = (seat: string, cents: number): string =>  `Circle · seat ${seat} · £${(cents / 100).toFixed(2)} · booking fee included`;