/**
 * 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.
 */

/**
 * Return "Buffer Levels" which are steps of available buffers from which we
 * are normally able switch safely to the next available bitrate.
 * (Following an algorithm close to BOLA)
 * @param {Array.<number>} bitrates - All available bitrates, __sorted__ in
 * ascending order.
 * @returns {Array.<number>}
 */
export default function getBufferLevels(bitrates: number[]): number[] {
  const logs = bitrates.map((b) => Math.log(b / bitrates[0]));
  const utilities = logs.map((l) => l - logs[0] + 1); // normalize
  const gp = (utilities[utilities.length - 1] - 1) / (bitrates.length * 2 + 10);
  const Vp = 1 / gp;

  return bitrates.map((_, i) => minBufferLevelForBitrate(i));

  /**
   * Get minimum buffer we should keep ahead to pick this bitrate.
   * @param {number} index
   * @returns {number}
   */
  function minBufferLevelForBitrate(index: number): number {
    if (index === 0) {
      return 0;
    }
    const boundedIndex = Math.min(Math.max(1, index), bitrates.length - 1);
    if (bitrates[boundedIndex] === bitrates[boundedIndex - 1]) {
      return minBufferLevelForBitrate(index - 1);
    }
    return (
      Vp *
        (gp +
          (bitrates[boundedIndex] * utilities[boundedIndex - 1] -
            bitrates[boundedIndex - 1] * utilities[boundedIndex]) /
            (bitrates[boundedIndex] - bitrates[boundedIndex - 1])) +
      4
    );
  }
}