Add comments to combination algorithms.

Author: trekhleb

src/algorithms/sets/combinations/combineWithRepetitions.js

@@ -4,21 +4,25 @@
  * @return {*[]}
  */
 export default function combineWithRepetitions(comboOptions, comboLength) {
+  // If the length of the combination is 1 then each element of the original array
+  // is a combination itself.
   if (comboLength === 1) {
     return comboOptions.map(comboOption => [comboOption]);
   }
 
   // Init combinations array.
   const combos = [];
 
-  // Eliminate characters one by one and concatenate them to
-  // combinations of smaller lengths.
+  // Remember characters one by one and concatenate them to combinations of smaller lengths.
+  // We don't extract elements here because the repetitions are allowed.
   comboOptions.forEach((currentOption, optionIndex) => {
+    // Generate combinations of smaller size.
     const smallerCombos = combineWithRepetitions(
       comboOptions.slice(optionIndex),
       comboLength - 1,
     );
 
+    // Concatenate currentOption with all combinations of smaller size.
     smallerCombos.forEach((smallerCombo) => {
       combos.push([currentOption].concat(smallerCombo));
     });

src/algorithms/sets/combinations/combineWithoutRepetitions.js

@@ -4,21 +4,25 @@
  * @return {*[]}
  */
 export default function combineWithoutRepetitions(comboOptions, comboLength) {
+  // If the length of the combination is 1 then each element of the original array
+  // is a combination itself.
   if (comboLength === 1) {
     return comboOptions.map(comboOption => [comboOption]);
   }
 
   // Init combinations array.
   const combos = [];
 
-  // Eliminate characters one by one and concatenate them to
-  // combinations of smaller lengths.
+  // Extract characters one by one and concatenate them to combinations of smaller lengths.
+  // We need to extract them because we don't want to have repetitions after concatenation.
   comboOptions.forEach((currentOption, optionIndex) => {
+    // Generate combinations of smaller size.
     const smallerCombos = combineWithoutRepetitions(
       comboOptions.slice(optionIndex + 1),
       comboLength - 1,
     );
 
+    // Concatenate currentOption with all combinations of smaller size.
     smallerCombos.forEach((smallerCombo) => {
       combos.push([currentOption].concat(smallerCombo));
     });