Pairwise

pairwise.js

function pairwise(arr, arg) {
  let sum = 0;

  let arrIndexes = arr.map((v, i) => i);
  arrIndexes.sort((a, b) => {
    let s = arr[a] - arr[b];
    if (s === 0)
      s = b - a;
    return s;
  });
  
  let middle = Math.ceil(arg/2) || 1;

  let j = arrIndexes.findIndex((v) => arr[v] >= middle);
  if (j === -1)
    return 0;
  
  // If there are a bunch of numbers arg/2 set pointer correctly
  if (arr[arrIndexes[j]] * 2 === arg) {
    let k = j;
    for (k = j; k < arrIndexes.length && arr[arrIndexes[k]] === middle; k++) {}

    let totalNumbers = k - j;
    if (totalNumbers % 2 === 1)
      --totalNumbers;

    j = k - totalNumbers/2;
  }

  let i = j-1;

  while (i >= 0 && j < arrIndexes.length) {
    let currentSum = arr[arrIndexes[i]] + arr[arrIndexes[j]];
    if (currentSum === arg) {
      sum += arrIndexes[i];
      sum += arrIndexes[j];
      i--;
      j++;
    } else if (currentSum > arg) {
      i--;
    } else
      j++;  
  }

  return sum;
  
}

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