Source: Error.js

const sumBy = (cb, arr) => {
  return arr.reduce((acc, next, i) => {
    return acc + cb(next, i)
  }, 0)
}

/**
 * Functions for calculating Network error.  The error is simply the difference
 * between the correct output and the actual output.
 * @namespace
 * @see Network
 * @type {object}
 */
const ERROR = {
  /**
   * @param {number[]} expected - Array of output values the Network should
   *   have produced.
   * @param {number[]} actual - Array of output values the Network actually
   *   produced.
   * @returns {number}
   */
  crossEntropy(expected, actual) {
    const sum = sumBy((a, i) => {
      return Math.log(a) * expected[i]
    }, actual)
    return -sum / actual.length
  },

  // These taken from: https://www.youtube.com/watch?v=U4BTzF3Wzt0

  /**
   * @param {number[]} expected - Array of output values the Network should
   *   have produced.
   * @param {number[]} actual - Array of output values the Network actually
   *   produced.
   * @returns {number}
   */
  meanSquared(expected, actual) {
    const sum = sumBy((a, i) => {
      return 0.5 * ((expected[i] - a) ** 2)
    }, actual)
    return sum / actual.length
  },

  /**
   * @param {number[]} expected - Array of output values the Network should
   *   have produced.
   * @param {number[]} actual - Array of output values the Network actually
   *   produced.
   * @returns {number}
   */
  rootMeanSquared(expected, actual) {
    return Math.sqrt(ERROR.meanSquared(expected, actual))
  },

  /**
   * @param {number[]} expected - Array of output values the Network should
   *   have produced.
   * @param {number[]} actual - Array of output values the Network actually
   *   produced.
   * @returns {number}
   */
  arcTan(expected, actual) {
    const sum = sumBy((a, i) => {
      return Math.atan(expected[i] - a)
    }, actual)
    return sum / actual.length
  },
}

export default ERROR