websocket-ts/src/backoff/exponentialbackoff.ts

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import { Backoff } from "./backoff";

/**
 * ExponentialBackoff increases the backoff-time exponentially.
 * An optional maximum can be provided as an upper bound to the
 * exponent and thus to the returned backoff.
 *
 * The series can be described as ('i' is the current step/retry):
 *    backoff = base * 2^i                | without bound
 *    backoff = base * 2^min(i, expMax)   | with bound
 *
 * Example:
 *
 * 1) Without bound:
 *   base = 1000, expMax = undefined
 *   backoff = 1000 * 2^0 = 1000 // first retry
 *   backoff = 1000 * 2^1 = 2000 // second retry
 *   backoff = 1000 * 2^2 = 4000 // ...doubles with every retry
 *   backoff = 1000 * 2^3 = 8000
 *   backoff = 1000 * 2^4 = 16000
 *   ... // and so on
 *
 * 2) With bound:
 *  base = 1000, expMax = 3
 *  backoff = 1000 * 2^0 = 1000 // first retry
 *  backoff = 1000 * 2^1 = 2000 // second retry
 *  backoff = 1000 * 2^2 = 4000 // third retry
 *  backoff = 1000 * 2^3 = 8000 // maximum reached, don't increase further
 *  backoff = 1000 * 2^3 = 8000
 *  backoff = 1000 * 2^3 = 8000
 *  ... // and so on
 */
export class ExponentialBackoff implements Backoff {
  private readonly base: number;
  private readonly expMax?: number;
  private i: number;
  private _retries: number = 0;

  /**
   * Creates a new ExponentialBackoff.
   * @param base the base of the exponentiation
   * @param expMax the maximum exponent, no bound if undefined
   */
  constructor(base: number, expMax?: number) {
    if (!Number.isInteger(base) || base < 0) {
      throw new Error("Base must be a positive integer or zero");
    }
    if (expMax !== undefined && (!Number.isInteger(expMax) || expMax < 0)) {
      throw new Error("ExpMax must be a undefined, a positive integer or zero");
    }

    this.base = base;
    this.expMax = expMax;
    this.i = 0;
  }

  get retries() {
    return this._retries;
  }

  get current(): number {
    return this.base * Math.pow(2, this.i);
  }

  next(): number {
    this._retries++;
    this.i =
      this.expMax === undefined
        ? this.i + 1
        : Math.min(this.i + 1, this.expMax);
    return this.current;
  }

  reset(): void {
    this._retries = 0;
    this.i = 0;
  }
}