From 65f08db5deb8737ee0a9cf89518c9a99c1fccfb0 Mon Sep 17 00:00:00 2001
From: Oleksii Trekhleb <trehleb@gmail.com>
Date: Thu, 28 Jun 2018 14:05:58 +0300
Subject: [PATCH] Simplify combineWithRepetitions function.

---
 .../combinations/combineWithRepetitions.js    | 44 ++++++++-----------
 .../combinations/combineWithoutRepetitions.js | 11 ++---
 2 files changed, 24 insertions(+), 31 deletions(-)

diff --git a/src/algorithms/sets/combinations/combineWithRepetitions.js b/src/algorithms/sets/combinations/combineWithRepetitions.js
index 0d2ef3c1..a95d2857 100644
--- a/src/algorithms/sets/combinations/combineWithRepetitions.js
+++ b/src/algorithms/sets/combinations/combineWithRepetitions.js
@@ -1,38 +1,30 @@
 /**
- * @param {*[]} combinationOptions
- * @param {number} combinationLength
+ * @param {*[]} comboOptions
+ * @param {number} comboLength
  * @return {*[]}
  */
-
-export default function combineWithRepetitions(combinationOptions, combinationLength) {
-  // If combination length equal to 0 then return empty combination.
-  if (combinationLength === 0) {
-    return [[]];
-  }
-
-  // If combination options are empty then return "no-combinations" array.
-  if (combinationOptions.length === 0) {
-    return [];
+export default function combineWithRepetitions(comboOptions, comboLength) {
+  if (comboLength === 1) {
+    return comboOptions.map(comboOption => [comboOption]);
   }
 
   // Init combinations array.
   const combos = [];
 
-  // Find all shorter combinations and attach head to each of those.
-  const headCombo = [combinationOptions[0]];
-  const shorterCombos = combineWithRepetitions(combinationOptions, combinationLength - 1);
+  // 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];
 
-  for (let combinationIndex = 0; combinationIndex < shorterCombos.length; combinationIndex += 1) {
-    const combo = headCombo.concat(shorterCombos[combinationIndex]);
-    combos.push(combo);
+    const smallerCombos = combineWithRepetitions(
+      comboOptions.slice(optionIndex),
+      comboLength - 1,
+    );
+
+    smallerCombos.forEach((smallerCombo) => {
+      combos.push([currentOption].concat(smallerCombo));
+    });
   }
 
-  // Let's shift head to the right and calculate all the rest combinations.
-  const combinationsWithoutHead = combineWithRepetitions(
-    combinationOptions.slice(1),
-    combinationLength,
-  );
-
-  // Join all combinations and return them.
-  return combos.concat(combinationsWithoutHead);
+  return combos;
 }
diff --git a/src/algorithms/sets/combinations/combineWithoutRepetitions.js b/src/algorithms/sets/combinations/combineWithoutRepetitions.js
index a6f50ec0..8f712b51 100644
--- a/src/algorithms/sets/combinations/combineWithoutRepetitions.js
+++ b/src/algorithms/sets/combinations/combineWithoutRepetitions.js
@@ -8,20 +8,21 @@ export default function combineWithoutRepetitions(comboOptions, comboLength) {
     return comboOptions.map(comboOption => [comboOption]);
   }
 
+  // Init combinations array.
   const combos = [];
 
   // Eliminate characters one by one and concatenate them to
-  // combinations of smaller lengths.s
-  for (let letterIndex = 0; letterIndex <= (comboOptions.length - comboLength); letterIndex += 1) {
-    const currentLetter = comboOptions[letterIndex];
+  // combinations of smaller lengths.
+  for (let optionIndex = 0; optionIndex <= (comboOptions.length - comboLength); optionIndex += 1) {
+    const currentOption = comboOptions[optionIndex];
 
     const smallerCombos = combineWithoutRepetitions(
-      comboOptions.slice(letterIndex + 1),
+      comboOptions.slice(optionIndex + 1),
       comboLength - 1,
     );
 
     smallerCombos.forEach((smallerCombo) => {
-      combos.push([currentLetter].concat(smallerCombo));
+      combos.push([currentOption].concat(smallerCombo));
     });
   }