/**
 * Copyright 2015 CANAL+ Group
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import config from "../../../config";
import EWMA from "./ewma";

/**
 * Calculate a mean bandwidth based on the bytes downloaded and the amount
 * of time needed to do so.
 * @class BandwidthEstimator
 */
export default class BandwidthEstimator {
  /**
   * Slowly-moving exponentially weighted moving average.
   * It reacts fastly to changing network conditions such as sudden slowing/bursts.
   */
  private _fastEWMA: EWMA;
  /** Slowly-moving exponentially weighted moving average. */
  private _slowEWMA: EWMA;
  /** Amount of bytes currently used to produce a bandwidth estimate. */
  private _bytesSampled: number;

  constructor() {
    const { ABR_FAST_EMA, ABR_SLOW_EMA } = config.getCurrent();
    this._fastEWMA = new EWMA(ABR_FAST_EMA);
    this._slowEWMA = new EWMA(ABR_SLOW_EMA);
    this._bytesSampled = 0;
  }

  /**
   * Takes a bandwidth sample.
   * @param {number} durationInMs - The amount of time, in milliseconds, for a
   * particular request.
   * @param {number} numberOfBytes - The total number of bytes transferred in
   * that request.
   */
  public addSample(durationInMs: number, numberOfBytes: number): void {
    const { ABR_MINIMUM_CHUNK_SIZE } = config.getCurrent();
    if (numberOfBytes < ABR_MINIMUM_CHUNK_SIZE) {
      return;
    }

    const bandwidth = (numberOfBytes * 8000) / durationInMs;
    const weight = durationInMs / 1000;
    this._bytesSampled += numberOfBytes;

    this._fastEWMA.addSample(weight, bandwidth);
    this._slowEWMA.addSample(weight, bandwidth);
  }

  /**
   * Get estimate of the bandwidth, in bits per seconds.
   * @returns {Number|undefined}
   */
  public getEstimate(): number | undefined {
    const { ABR_MINIMUM_TOTAL_BYTES } = config.getCurrent();
    if (this._bytesSampled < ABR_MINIMUM_TOTAL_BYTES) {
      return undefined;
    }

    // Take the minimum of these two estimates.
    // This should have the effect of adapting down quickly, but up more slowly.
    return Math.min(this._fastEWMA.getEstimate(), this._slowEWMA.getEstimate());
  }

  /** Reset the bandwidth estimation. */
  public reset(): void {
    const { ABR_FAST_EMA, ABR_SLOW_EMA } = config.getCurrent();
    this._fastEWMA = new EWMA(ABR_FAST_EMA);
    this._slowEWMA = new EWMA(ABR_SLOW_EMA);
    this._bytesSampled = 0;
  }
}