array-inversion-count.js

function merge (left, right) {
  let resultArray = [], leftIndex = 0, rightIndex = 0, counter = 0;
  let existing_c = left[1] + right[1];
  left = left[0];
  right = right[0];
  // We will concatenate values into the resultArray in order
  while (leftIndex < left.length && rightIndex < right.length) {
    if (left[leftIndex] <= right[rightIndex]) {
      resultArray.push(left[leftIndex]);
      leftIndex++; // move left array cursor
    } else {
      resultArray.push(right[rightIndex]);
      rightIndex++; // move right array cursor
      counter += (left.length - leftIndex)
    }
  }

  // We need to concat here because there will be one element remaining
  // from either left OR the right
  return [resultArray.concat(left.slice(leftIndex)).concat(right.slice(rightIndex)),
          existing_c + counter]
}


function mergeSort (unsortedArray) {
  // No need to sort the array if the array only has one element or empty
  if (unsortedArray.length <= 1) {
    return [unsortedArray, 0];
  }
  // In order to divide the array in half, we need to figure out the middle
  const middle = Math.floor(unsortedArray.length / 2);

  // This is where we will be dividing the array into left and right
  const left = unsortedArray.slice(0, middle);
  const right = unsortedArray.slice(middle);

  // Using recursion to combine the left and right
  return merge(
    mergeSort(left), mergeSort(right)
  );
}

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