maia-util/es6/intersection_hash.js

Utility math and music functions supporting various applications by Music Artificial Intelligence Algorithms, Inc.

/**
 * This function returns the intersection of two multidimensional numeric,
 * arrays, assuming both are already in increasing lexicographic order to
 * achieve O(n) computational complexity.
 *
 * @author Tom Collins
 * @comment 30th June 2022
 * @param {number[]} arr - An array.
 * @param {number[]} obj - JavaScript object representing a hash of a large
 * array.
 * @param {(boolean|number)} index - Indicates whether to record indices of arr
 * as belonging to the intersection. Takes value true if recording indices from
 * arr and false otherwise.
 * @return {number[]} An array.
 *
 * @example
 *     intersection([[1, 4], [2, -2]], [[-2, 3], [2, -2], [4, 1]])
 * →
 * [[2, -2]]
 */
export default function intersection_hash(arr, obj, index = false){
  const ans = []
  let indices
  if (index){
    indices = []
  }
  let i = 0 // Counter for arr.
  while (i < arr.length){
    const hash = arr[i].map(function(val){
      // Calculate value rounded to 5 d.p. and convert to string.
      return (Math.round(100000*(val))/100000).toString()
    }).toString()
    if (obj[hash] !== undefined){
      ans.push(arr[i])
      // Handle recording of indices.
      if (index){
        indices.push(i)
      }
    }
    i++
  }
  if (index){
    return [ans, indices]
  }
  else {
    return ans
  }
}