From bf5d7b338ccefd4d1449a2490d27af179b40339b Mon Sep 17 00:00:00 2001
From: Robert Taussig <robert.taussig@gmail.com>
Date: Sun, 27 May 2018 16:11:13 -0400
Subject: [PATCH] Add in-place sort to QuickSort.js (#16)

* Add in-place sort to QuickSort.js

* Fix linting errors and clean up comments

* Change implementation to address lint errors

* Trailing space and undefined variable

* Create own class for in-place quicksort and use tests

* Add trailing space at end of file

* Fix placement of visitedCallback, explain itial destructuring
---
 .../sorting/quick-sort/QuickSort.js           |  2 +-
 .../sorting/quick-sort/QuickSortInPlace.js    | 59 +++++++++++++++++++
 .../__test__/QuickSortInPlace.test.js         | 56 ++++++++++++++++++
 3 files changed, 116 insertions(+), 1 deletion(-)
 create mode 100644 src/algorithms/sorting/quick-sort/QuickSortInPlace.js
 create mode 100644 src/algorithms/sorting/quick-sort/__test__/QuickSortInPlace.test.js

diff --git a/src/algorithms/sorting/quick-sort/QuickSort.js b/src/algorithms/sorting/quick-sort/QuickSort.js
index e1eb8558..8a9c113f 100644
--- a/src/algorithms/sorting/quick-sort/QuickSort.js
+++ b/src/algorithms/sorting/quick-sort/QuickSort.js
@@ -5,7 +5,7 @@ export default class QuickSort extends Sort {
     // Clone original array to prevent it from modification.
     const array = [...originalArray];
 
-    // If array has less then or equal to one elements then it is already sorted.
+    // If array has less than or equal to one elements then it is already sorted.
     if (array.length <= 1) {
       return array;
     }
diff --git a/src/algorithms/sorting/quick-sort/QuickSortInPlace.js b/src/algorithms/sorting/quick-sort/QuickSortInPlace.js
new file mode 100644
index 00000000..fc7bb67d
--- /dev/null
+++ b/src/algorithms/sorting/quick-sort/QuickSortInPlace.js
@@ -0,0 +1,59 @@
+import Sort from '../Sort';
+
+export default class QuickSortInPlace extends Sort {
+  /* Sorting in place avoids unnecessary use of additional memory, but modifies input array.
+   *
+   * This process is difficult to describe, but much clearer with a visualization:
+   * http://www.algomation.com/algorithm/quick-sort-visualization
+   */
+  sort(originalArray, inputLow, inputHigh) {
+    // Destructures array on initial passthrough, and then sorts in place.
+    const array = inputLow === undefined ? [...originalArray] : originalArray;
+    // Partition array segment and return index of last swap
+    const partition = (l, h) => {
+      const swap = (left, right) => {
+        const tempVariable = array[left];
+        array[left] = array[right];
+        array[right] = tempVariable;
+      };
+
+      const pivot = array[h];
+      this.callbacks.visitingCallback(array[pivot]);
+      let firstRunner = l - 1;
+
+      for (let secondRunner = l; secondRunner < h; secondRunner += 1) {
+        if (this.comparator.lessThan(array[secondRunner], pivot)) {
+          firstRunner += 1;
+          swap(firstRunner, secondRunner);
+        }
+      }
+
+      if (this.comparator.lessThan(pivot, array[firstRunner + 1])) {
+        swap(firstRunner + 1, h);
+      }
+
+      return firstRunner + 1;
+    };
+
+    /*
+     * While we can use a default parameter to set `low` to 0, we would
+     * still have to set `high`'s default within the function as we
+     * don't have access to `array.length - 1` when declaring paramaters
+     */
+    const low = inputLow === undefined ? 0 : inputLow;
+    const high = inputHigh === undefined ? array.length - 1 : inputHigh;
+
+    // Base case is when low and high converge
+    if (low < high) {
+      const partitionIndex = partition(low, high);
+      /*
+       * `partition()` swaps elements of the array based on their comparison to the `hi` parameter,
+       * and then returns the index where swapping is no longer necessary, which can be best thought
+       * of as the pivot used to split an array in a non-in-place quicksort
+       */
+      this.sort(array, low, partitionIndex - 1);
+      this.sort(array, partitionIndex + 1, high);
+    }
+    return array;
+  }
+}
diff --git a/src/algorithms/sorting/quick-sort/__test__/QuickSortInPlace.test.js b/src/algorithms/sorting/quick-sort/__test__/QuickSortInPlace.test.js
new file mode 100644
index 00000000..6e78ef29
--- /dev/null
+++ b/src/algorithms/sorting/quick-sort/__test__/QuickSortInPlace.test.js
@@ -0,0 +1,56 @@
+import QuickSortInPlace from '../QuickSortInPlace';
+import {
+  equalArr,
+  notSortedArr,
+  reverseArr,
+  sortedArr,
+  SortTester,
+} from '../../SortTester';
+
+// Complexity constants.
+const SORTED_ARRAY_VISITING_COUNT = 19;
+const NOT_SORTED_ARRAY_VISITING_COUNT = 12;
+const REVERSE_SORTED_ARRAY_VISITING_COUNT = 19;
+const EQUAL_ARRAY_VISITING_COUNT = 19;
+
+describe('QuickSortInPlace', () => {
+  it('should sort array', () => {
+    SortTester.testSort(QuickSortInPlace);
+  });
+
+  it('should sort array with custom comparator', () => {
+    SortTester.testSortWithCustomComparator(QuickSortInPlace);
+  });
+
+  it('should visit EQUAL array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      QuickSortInPlace,
+      equalArr,
+      EQUAL_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      QuickSortInPlace,
+      sortedArr,
+      SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit NOT SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      QuickSortInPlace,
+      notSortedArr,
+      NOT_SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit REVERSE SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      QuickSortInPlace,
+      reverseArr,
+      REVERSE_SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+});