FreeCodeCamp => Algorithms: Pairwise

fcc-algo-pairwise.js

/**
 * @param {number} prev
 * @param {number} next
 * @return {number} the sum of prev and next
 */
const add = (prev, next) => prev + next;

/**
 * Generate index pair combinations, arr.length choose 2
 * @param {array} arr
 * @return {array} new array containing each index pair combination
 *     as a subarray
 */
const getIndexCombos = (arr) => {
  let res = [];

  arr.forEach((val, index) => {
    let pairs = [];
    
    for (let i = index + 1, len = arr.length; i < len; i++) {
      pairs.push([index, i]);
    }
    
    res = res.concat(pairs);
  });

  return res;
};


/**
 * @param {array} arr
 * @param {number} sum
 * @return {number} sum of arr indices such that the arr element pairs sum equal
 *   the sum parameter
 */
function pairwise(arr, arg) {
  if (!arr.length) return 0;

  // Generate combinations, arr.length choose 2
  const combinations = getIndexCombos(arr)
    // get sum of each combo
    .map((val, ind) => ({
      sum: arr[val[0]] + arr[val[1]],
      pair: val,
    }));

  let usedIndex = new Set();
  let sums = combinations
    .filter((val) => {
      // filter to match arg
      if (val.sum !== arg) return false;
      // then filter index re-use
      if (usedIndex.has(val.pair[0])) return false;
      if (usedIndex.has(val.pair[1])) return false;
      usedIndex.add(val.pair[0]);
      usedIndex.add(val.pair[1]);

      return true;
    })
    .map((val) =>
      // sum indices in each pair
      val.pair.reduce(add)
    );

  return sums.reduce(add);
}

pairwise([1, 4, 2, 3, 0, 5], 7);