// https://www.freecodecamp.org/learn/coding-interview-prep/algorithms/pairwise

function pairwise(arr, sum) {
  const arr2 = arr.map((num, index) => ({ num: num, index: index }));
  arr2.sort(({ num: numA, index: indexA }, { num: numB, index: indexB }) => {
    if (numA === numB) {
      return indexA - indexB;
    } else {
      return numA - numB;
    }
  });

  let [i, reverseI, pairsIndexSum] = [0, arr2.length - 1, 0];
  while (i < reverseI) {
    if (arr2[i].num + arr2[reverseI].num === sum) {
      if (arr2[i].num === arr2[reverseI].num) {
        pairsIndexSum += arr2[i++].index + arr2[i++].index;
      } else {
        pairsIndexSum += arr2[i++].index + arr2[reverseI--].index;
      }
    } else if (arr2[i].num + arr2[reverseI].num < sum) {
      i++;
    } else {
      reverseI--;
    }
  }
  return pairsIndexSum;
}

console.log(pairwise([1, 1, 1], 2));