export class Timer {
  private readonly start: Date;
  private readonly markers: Record<string, Date | undefined> =
    Object.create(null);

  constructor(private readonly now = () => new Date()) {
    this.start = this.now();
  }
  public humanReadableElapsed(sinceMarker?: string): string {
    const elapsedSeconds = this.elapsedSeconds(sinceMarker);
    return new Intl.ListFormat('en').format(
      [
        Timer.humanReadableElapsedMinutes(elapsedSeconds),
        Timer.humanReadableElapsedSeconds(elapsedSeconds),
      ].filter(Boolean),
    );
  }

  public elapsedSeconds(sinceMarker?: string): number {
    const elapsedMs = this.elapsedMs(sinceMarker);
    return Math.floor(elapsedMs / 1000);
  }

  public elapsedMs(sinceMarker?: string): number {
    const marker = sinceMarker && this.markers[sinceMarker];
    if (marker) {
      return this.now().getTime() - marker.getTime();
    } else {
      return this.now().getTime() - this.start.getTime();
    }
  }

  public mark(name: string): void {
    this.markers[name] = this.now();
  }

  private static humanReadableElapsedSeconds(elapsedSeconds: number) {
    const restSeconds = elapsedSeconds % 60;
    return this.formatTime('second', restSeconds);
  }

  private static humanReadableElapsedMinutes(elapsedSeconds: number) {
    const elapsedMinutes = Math.floor(elapsedSeconds / 60);
    if (elapsedMinutes === 0) {
      return '';
    } else {
      return this.formatTime('minute', elapsedMinutes);
    }
  }

  private static formatTime(word: 'minute' | 'second', elapsed: number) {
    return elapsed.toLocaleString('en', {
      unit: word,
      style: 'unit',
      unitDisplay: 'long',
    });
  }
}