Permutations via Heap's algorithm with a mystery comma

Question

I have spent the whole day (finally) wrapping my head around a permutation algorithm in practice for an admissions application on Friday. Heap\'s algorithm seemed most simple an

Answer 1

Updated answer since Jan-2018: The accepted answer is absolutely correct, but js has evolved since then. And with it comes some new features, 2 of which could help this answer.

Array destructuring:

let [a, b] = [1, 2]; // a=1, b=2

Generators:

function *foo {
  yield 1;
  yield 2;
  yield 3;
}
const bar = foo();
bar.next(); // 1
bar.next(); // 2
bar.next(); // 3

With this we can implement the Heap's algorithm like this:

function *heaps(arr, n) {
  if (n === undefined) n = arr.length;
  if (n <= 1) yield arr;
  else {
    for (let i = 0; i < n - 1; i++) {
      yield *heaps(arr, n-1);
      if (n % 2 === 0) [arr[n-1], arr[i]] = [arr[i], arr[n-1]];
      else             [arr[n-1], arr[0]] = [arr[0], arr[n-1]];
    }
    yield *heaps(arr, n-1);
  }
}


for (let a of heaps([1, 2, 3, 4])) {
  console.log(`[${a.join(', ')}]`);
}

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