signal-desktop/ts/util/BackOff.ts

// Copyright 2021 Signal Messenger, LLC
// SPDX-License-Identifier: AGPL-3.0-only

const SECOND = 1000;

export const FIBONACCI_TIMEOUTS: ReadonlyArray<number> = [
  1 * SECOND,
  2 * SECOND,
  3 * SECOND,
  5 * SECOND,
  8 * SECOND,
  13 * SECOND,
  21 * SECOND,
  34 * SECOND,
  55 * SECOND,
];

export type BackOffOptionsType = Readonly<{
  jitter?: number;

  // Testing
  random?: () => number;
}>;

const DEFAULT_RANDOM = () => Math.random();

export class BackOff {
  private count = 0;

  constructor(
    private readonly timeouts: ReadonlyArray<number>,
    private readonly options: BackOffOptionsType = {}
  ) {}

  public get(): number {
    let result = this.timeouts[this.count];
    const { jitter = 0, random = DEFAULT_RANDOM } = this.options;

    // Do not apply jitter larger than the timeout value. It is supposed to be
    // activated for longer timeouts.
    if (jitter < result) {
      result += random() * jitter;
    }
    return result;
  }

  public getAndIncrement(): number {
    const result = this.get();
    if (!this.isFull()) {
      this.count += 1;
    }

    return result;
  }

  public reset(): void {
    this.count = 0;
  }

  public isFull(): boolean {
    return this.count === this.timeouts.length - 1;
  }

  public getIndex(): number {
    return this.count;
  }
}