peerreynders/permutations.html

## permutations.html
<!DOCTYPE html>
<html lang="en-us">
  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>itertools.permutations in vanilla JavaScript</title>
  </head>
  <body>
    <p>Check Developer Tools Console (Chrome) or Web Developer Web Console (Firefox) </p>
    <script>
     // Written trying to decipher what
     //
     // https://docs.python.org/3/library/itertools.html#itertools.permutations
     //
     // is doing.

     function* permutations(iterable, m) {
       const elements = Array.from(iterable);
       const n = elements.length;
       const k = m === undefined ? n : m;
       if (k > n) return;

       let indices = Array.from({ length: n }, (_v, i) => i);
       const kConfig = { length: k };
       const cycles = Array.from(kConfig, (_v, i) => n - i);

       const fromElement = (_v, i) => elements[indices[i]];
       const makePermutation = () => Array.from(kConfig, fromElement);

       yield makePermutation();
       if (n < 1) return;

       let repeat = false;
       do {
         repeat = false;

         // offset = cycles[target]
         // target - index to swap into
         // offset - position (from the end) to swap out of
         // source = n - offset
         // source - index to swap out of
         //
         // for loop only executes until the next value is
         // yielded - then it starts over.
         // When it runs to completion without yielding
         // a value the generator is complete.

         for (let target = k - 1; target >= 0; target -= 1) {
           cycles[target] -= 1;
           const offset = cycles[target];
           if (offset > 0) {
             // swap `target` with `-offset`
             const source = n - offset;
             const used = indices[target];
             indices[target] = indices[source];
             indices[source] = used;

             yield makePermutation();
             repeat = true;
             break;
           } else {
             // move `target` to the end
             // and reset the swap out offset
             indices = indices
               .slice(0, target)
               .concat(indices.slice(target + 1), [indices[target]]);
             cycles[target] = n - target;
           }
         }
       } while (repeat);
     }

     for (const p of permutations('ABCD', 2)) {
       console.log(p.join(''));
     }

     function* range(...args) {
       const [start, end, step] =
         args.length === 1
         ? [0, args[0], 1]
         : args.length === 2
         ? [args[0], args[1], 1]
         : args;

       for (let i = start; i < end; i += step) yield i;
     }

     for (const p of permutations(range(3))) {
       console.log(p.join(''));
     }
    </script>
  </body>
</html>
	<!DOCTYPE html>
	<html lang="en-us">
	<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<meta http-equiv="X-UA-Compatible" content="IE=edge">
	<title>itertools.permutations in vanilla JavaScript</title>
	</head>
	<body>
	<p>Check Developer Tools Console (Chrome) or Web Developer Web Console (Firefox) </p>
	<script>
	// Written trying to decipher what
	//
	// https://docs.python.org/3/library/itertools.html#itertools.permutations
	//
	// is doing.

	function* permutations(iterable, m) {
	const elements = Array.from(iterable);
	const n = elements.length;
	const k = m === undefined ? n : m;
	if (k > n) return;

	let indices = Array.from({ length: n }, (_v, i) => i);
	const kConfig = { length: k };
	const cycles = Array.from(kConfig, (_v, i) => n - i);

	const fromElement = (_v, i) => elements[indices[i]];
	const makePermutation = () => Array.from(kConfig, fromElement);

	yield makePermutation();
	if (n < 1) return;

	let repeat = false;
	do {
	repeat = false;

	// offset = cycles[target]
	// target - index to swap into
	// offset - position (from the end) to swap out of
	// source = n - offset
	// source - index to swap out of
	//
	// for loop only executes until the next value is
	// yielded - then it starts over.
	// When it runs to completion without yielding
	// a value the generator is complete.

	for (let target = k - 1; target >= 0; target -= 1) {
	cycles[target] -= 1;
	const offset = cycles[target];
	if (offset > 0) {
	// swap `target` with `-offset`
	const source = n - offset;
	const used = indices[target];
	indices[target] = indices[source];
	indices[source] = used;

	yield makePermutation();
	repeat = true;
	break;
	} else {
	// move `target` to the end
	// and reset the swap out offset
	indices = indices
	.slice(0, target)
	.concat(indices.slice(target + 1), [indices[target]]);
	cycles[target] = n - target;
	}
	}
	} while (repeat);
	}

	for (const p of permutations('ABCD', 2)) {
	console.log(p.join(''));
	}

	function* range(...args) {
	const [start, end, step] =
	args.length === 1
	? [0, args[0], 1]
	: args.length === 2
	? [args[0], args[1], 1]
	: args;

	for (let i = start; i < end; i += step) yield i;
	}

	for (const p of permutations(range(3))) {
	console.log(p.join(''));
	}
	</script>
	</body>
	</html>