interweave-stretch.js

// interweave two arrays with stretching; i.e. if one is shorter than the other, its elements are
// distributed to cover most of the final array

const interweave = (A, B) => {
  // create [0..1] index of each item
  const map = array => array.map((val, i) => ({
    array,
    val,
    index: array.length === 1 ? 0.5 : i / (array.length - 1)
  }));
  const a = map(A);
  const b = map(B);
  // build result by taking lowest index item; alternating in case of equality
  const res = [];
  let last = {
    array: A.length > B.length ? B : A
  };
  while (a.length > 0 || b.length > 0) {
    let next = null;
    let nextA = a[0];
    let nextB = b[0];
    if (!nextA) {
      next = b.shift();
    } else if (!nextB) {
      next = a.shift();
    } else if (nextA.index < nextB.index) {
      next = a.shift();
    } else if (nextA.index > nextB.index) {
      next = b.shift();
    } else {
      next = last.array === B ? a.shift() : b.shift();
    }
    res.push(next);
    last = next;
  }
  return res.map(it => it.val);
};

console.log(interweave([1],[10,20,30,40]));
// [10, 20, 1, 30, 40]

console.log(interweave([1,2],[10,20,30,40]));
// [10, 1, 20, 30, 2, 40]