Question

What is a fast and simple implementation of interleave:

console.log( interleave([1,2,3,4,5,6]      ,2) ); // [1,4,2,5,3,6]
console.log( interleave([1,2,3,4,5,6,7,8]  ,2) ); // [1,5,2,6,3,7,4,8]
console.log( interleave([1,2,3,4,5,6]      ,3) ); // [1,3,5,2,4,6]
console.log( interleave([1,2,3,4,5,6,7,8,9],3) ); // [1,4,7,2,5,8,3,6,9]

This mimics taking the array and splitting it into n equal parts, and then shifting items off the front of each partial array in sequence. (n=2 simulates a perfect halving and single shuffle of a deck of cards.)

I don’t much care exactly what happens when the number of items in the array is not evenly divisible by n. Reasonable answers might either interleave the leftovers, or even “punt” and throw them all onto the end.

Answer 1

Since I’ve been pushed to add my own answer early (edited to fix bugs noted by RobG):

function interleave(items,parts){
  var stride = Math.ceil( items.length / parts ) || 1;
  var result = [], len=items.length;
  for (var i=0;i<stride;++i){
    for (var j=i;j<len;j+=stride){
      result.push(items[j]);
    }
  }
  return result;
}