Simplify permutateWithRepetitions algorithm.

Author: trekhleb

src/algorithms/sets/combinations/combineWithRepetitions.js

@@ -13,9 +13,7 @@ export default function combineWithRepetitions(comboOptions, comboLength) {
 
   // Eliminate characters one by one and concatenate them to
   // combinations of smaller lengths.
-  for (let optionIndex = 0; optionIndex < comboOptions.length; optionIndex += 1) {
-    const currentOption = comboOptions[optionIndex];
-
+  comboOptions.forEach((currentOption, optionIndex) => {
     const smallerCombos = combineWithRepetitions(
       comboOptions.slice(optionIndex),
       comboLength - 1,
@@ -24,7 +22,7 @@ export default function combineWithRepetitions(comboOptions, comboLength) {
     smallerCombos.forEach((smallerCombo) => {
       combos.push([currentOption].concat(smallerCombo));
     });
-  }
+  });
 
   return combos;
 }

src/algorithms/sets/combinations/combineWithoutRepetitions.js

@@ -13,9 +13,7 @@ export default function combineWithoutRepetitions(comboOptions, comboLength) {
 
   // Eliminate characters one by one and concatenate them to
   // combinations of smaller lengths.
-  for (let optionIndex = 0; optionIndex <= (comboOptions.length - comboLength); optionIndex += 1) {
-    const currentOption = comboOptions[optionIndex];
-
+  comboOptions.forEach((currentOption, optionIndex) => {
     const smallerCombos = combineWithoutRepetitions(
       comboOptions.slice(optionIndex + 1),
       comboLength - 1,
@@ -24,7 +22,7 @@ export default function combineWithoutRepetitions(comboOptions, comboLength) {
     smallerCombos.forEach((smallerCombo) => {
       combos.push([currentOption].concat(smallerCombo));
     });
-  }
+  });
 
   return combos;
 }

src/algorithms/sets/permutations/__test__/permutateWithRepetitions.test.js

@@ -2,9 +2,6 @@ import permutateWithRepetitions from '../permutateWithRepetitions';
 
 describe('permutateWithRepetitions', () => {
   it('should permutate string with repetition', () => {
-    const permutations0 = permutateWithRepetitions([]);
-    expect(permutations0).toEqual([]);
-
     const permutations1 = permutateWithRepetitions(['A']);
     expect(permutations1).toEqual([
       ['A'],

src/algorithms/sets/permutations/__test__/permutateWithRepetitionsRecursive.test.js

@@ -1,42 +1,30 @@
 /**
  * @param {*[]} permutationOptions
+ * @param {number} permutationLength
  * @return {*[]}
  */
-export default function permutateWithRepetitions(permutationOptions) {
-  // There is no permutations for empty array.
-  if (!permutationOptions || permutationOptions.length === 0) {
-    return [];
+export default function permutateWithRepetitions(
+  permutationOptions,
+  permutationLength = permutationOptions.length,
+) {
+  if (permutationLength === 1) {
+    return permutationOptions.map(permutationOption => [permutationOption]);
   }
 
-  // There is only one permutation for the 1-element array.
-  if (permutationOptions.length === 1) {
-    return [permutationOptions];
-  }
-
-  // Let's create initial set of permutations.
-  let previousPermutations = permutationOptions.map(option => [option]);
-  let currentPermutations = [];
-  let permutationSize = 1;
-
-  // While the size of each permutation is less then or equal to options length...
-  while (permutationSize < permutationOptions.length) {
-    // Reset all current permutations.
-    currentPermutations = [];
+  // Init permutations array.
+  const permutations = [];
 
-    for (let permIndex = 0; permIndex < previousPermutations.length; permIndex += 1) {
-      for (let optionIndex = 0; optionIndex < permutationOptions.length; optionIndex += 1) {
-        let currentPermutation = previousPermutations[permIndex];
-        currentPermutation = currentPermutation.concat([permutationOptions[optionIndex]]);
-        currentPermutations.push(currentPermutation);
-      }
-    }
+  // Go through all options and join it to the smaller permutations.
+  permutationOptions.forEach((currentOption) => {
+    const smallerPermutations = permutateWithRepetitions(
+      permutationOptions,
+      permutationLength - 1,
+    );
 
-    // Make current permutations to be the previous ones.
-    previousPermutations = currentPermutations.slice(0);
-
-    // Increase permutation size counter.
-    permutationSize += 1;
-  }
+    smallerPermutations.forEach((smallerPermutation) => {
+      permutations.push([currentOption].concat(smallerPermutation));
+    });
+  });
 
-  return currentPermutations;
+  return permutations;
 }