♻️ refactor: Extract indices generator.

Author: make-github-pseudonymous-again

src/_permutations.js

@@ -0,0 +1,51 @@
+import assert from 'assert';
+import {_take} from '@iterable-iterator/slice';
+
+import {list} from '@iterable-iterator/list';
+import {
+	forwardRangeIterator,
+	backwardRangeIterator,
+} from '@iterable-iterator/range';
+
+/**
+ * Yields all k-permutations of {0, 1, ..., n-1}.
+ *
+ * @param {number} n
+ * @param {number} k
+ * @returns {IterableIterator<IterableIterator<number>>}
+ */
+export default function* _permutations(n, k) {
+	assert(Number.isInteger(k) && k >= 0);
+	if (k > n) return;
+
+	const indices = list(forwardRangeIterator(0, n, 1));
+	const cycles = list(backwardRangeIterator(n, n - k, -1));
+
+	yield _take(indices, k);
+
+	if (k === 0 || n === 0) return;
+
+	while (true) {
+		let i = k;
+
+		while (i--) {
+			--cycles[i];
+
+			if (cycles[i] === 0) {
+				// Could be costly
+				indices.push(indices.splice(i, 1)[0]);
+
+				cycles[i] = n - i;
+			} else {
+				const j = cycles[i];
+
+				[indices[i], indices[n - j]] = [indices[n - j], indices[i]];
+
+				yield _take(indices, k);
+				break;
+			}
+		}
+
+		if (i === -1) return;
+	}
+}

src/index.js

@@ -1 +1,2 @@
+export {default as _permutations} from './_permutations.js';
 export {default as permutations} from './permutations.js';

src/permutations.js

@@ -1,12 +1,7 @@
-import assert from 'assert';
-import {_take} from '@iterable-iterator/slice';
-
 import {list} from '@iterable-iterator/list';
-import {pick} from '@iterable-iterator/map';
-import {
-	forwardRangeIterator,
-	backwardRangeIterator,
-} from '@iterable-iterator/range';
+import {map, pick} from '@iterable-iterator/map';
+
+import _permutations from './_permutations.js';
 
 /**
  * Yields all permutations of each possible choice of <code>r</code> elements
@@ -24,48 +19,12 @@ import {
  * @param {number} r - The size of the permutations to generate.
  * @returns {IterableIterator}
  */
-export default function* permutations(iterable, r) {
-	assert(Number.isInteger(r) && r >= 0);
+const permutations = (iterable, r) => {
 	const pool = list(iterable);
+	return map(
+		(indices) => list(pick(pool, indices)),
+		_permutations(pool.length, r),
+	);
+};
 
-	const length = pool.length;
-
-	if (r > length) {
-		return;
-	}
-
-	const indices = list(forwardRangeIterator(0, length, 1));
-	const cycles = list(backwardRangeIterator(length, length - r, -1));
-
-	yield list(pick(pool, _take(indices, r)));
-
-	if (r === 0 || length === 0) {
-		return;
-	}
-
-	while (true) {
-		let i = r;
-
-		while (i--) {
-			--cycles[i];
-
-			if (cycles[i] === 0) {
-				// Could be costly
-				indices.push(indices.splice(i, 1)[0]);
-
-				cycles[i] = length - i;
-			} else {
-				const j = cycles[i];
-
-				[indices[i], indices[length - j]] = [indices[length - j], indices[i]];
-
-				yield list(pick(pool, _take(indices, r)));
-				break;
-			}
-		}
-
-		if (i === -1) {
-			return;
-		}
-	}
-}
+export default permutations;