Add selection sort.

Author: trekhleb

src/algorithms/sorting/SortTester.js

@@ -11,7 +11,6 @@ export class SortTester {
     expect(sorter.sort([1])).toEqual([1]);
     expect(sorter.sort([1, 2])).toEqual([1, 2]);
     expect(sorter.sort([2, 1])).toEqual([1, 2]);
-    expect(sorter.sort([1, 1, 1])).toEqual([1, 1, 1]);
     expect(sorter.sort(sortedArr)).toEqual(sortedArr);
     expect(sorter.sort(reverseArr)).toEqual(sortedArr);
     expect(sorter.sort(notSortedArr)).toEqual(sortedArr);
@@ -33,6 +32,21 @@ export class SortTester {
     expect(sorter.sort([''])).toEqual(['']);
     expect(sorter.sort(['a'])).toEqual(['a']);
     expect(sorter.sort(['aa', 'a'])).toEqual(['a', 'aa']);
+    expect(sorter.sort(['aa', 'aa'])).toEqual(['aa', 'aa']);
+  }
+
+  static testSortStability(SortingClass) {
+    const callbacks = {
+      compareCallback: (a, b) => {
+        if (a.length === b.length) {
+          return 0;
+        }
+        return a.length < b.length ? -1 : 1;
+      },
+    };
+
+    const sorter = new SortingClass(callbacks);
+
     expect(sorter.sort(['bb', 'aa', 'c'])).toEqual(['c', 'bb', 'aa']);
   }
 

src/algorithms/sorting/bubble-sort/BubbleSort.js

@@ -1,11 +1,11 @@
 import Sort from '../Sort';
 
 export default class BubbleSort extends Sort {
-  sort(initialArray) {
+  sort(originalArray) {
     // Flag that holds info about whether the swap has occur or not.
     let swapped = false;
     // Clone original array to prevent its modification.
-    const array = initialArray.slice(0);
+    const array = originalArray.slice(0);
 
     for (let i = 0; i < array.length; i += 1) {
       swapped = false;

src/algorithms/sorting/bubble-sort/__test__/BubbleSort.test.js

@@ -16,6 +16,10 @@ describe('BubbleSort', () => {
     SortTester.testSortWithCustomComparator(BubbleSort);
   });
 
+  it('should do stable sorting', () => {
+    SortTester.testSortStability(BubbleSort);
+  });
+
   it('should visit EQUAL array element specified number of times', () => {
     const expectedNumberOfVisits = 19;
 

src/algorithms/sorting/selection-sort/README.md

@@ -0,0 +1,12 @@
+# Selection Sort
+
+Selection sort is a sorting algorithm, specifically an 
+in-place comparison sort. It has O(n2) time complexity, 
+making it inefficient on large lists, and generally 
+performs worse than the similar insertion sort. 
+Selection sort is noted for its simplicity, and it has 
+performance advantages over more complicated algorithms 
+in certain situations, particularly where auxiliary 
+memory is limited.
+
+![](https://en.wikipedia.org/wiki/Selection_sort#/media/File:Selection-Sort-Animation.gif)

src/algorithms/sorting/selection-sort/SelectionSort.js

@@ -0,0 +1,31 @@
+import Sort from '../Sort';
+
+export default class SelectionSort extends Sort {
+  sort(originalArray) {
+    // Clone original array to prevent its modification.
+    const array = originalArray.slice(0);
+
+    for (let i = 0; i < array.length - 1; i += 1) {
+      let minIndex = i;
+
+      // Find minimum element in the rest of array.
+      for (let j = i + 1; j < array.length; j += 1) {
+        // Call visiting callback.
+        this.callbacks.visitingCallback(array[j]);
+
+        if (this.comparator.lessThen(array[j], array[minIndex])) {
+          minIndex = j;
+        }
+      }
+
+      // If new minimum element has been found then swap it with current i-th element.
+      if (minIndex !== i) {
+        const tmp = array[i];
+        array[i] = array[minIndex];
+        array[minIndex] = tmp;
+      }
+    }
+
+    return array;
+  }
+}

src/algorithms/sorting/selection-sort/__test__/SelectionSort.test.js

@@ -0,0 +1,58 @@
+import SelectionSort from '../SelectionSort';
+import {
+  equalArr,
+  notSortedArr,
+  reverseArr,
+  sortedArr,
+  SortTester,
+} from '../../SortTester';
+
+describe('SelectionSort', () => {
+  it('should sort array', () => {
+    SortTester.testSort(SelectionSort);
+  });
+
+  it('should sort array with custom comparator', () => {
+    SortTester.testSortWithCustomComparator(SelectionSort);
+  });
+
+  it('should visit EQUAL array element specified number of times', () => {
+    const expectedNumberOfVisits = 190;
+
+    SortTester.testAlgorithmTimeComplexity(
+      SelectionSort,
+      equalArr,
+      expectedNumberOfVisits,
+    );
+  });
+
+  it('should visit SORTED array element specified number of times', () => {
+    const expectedNumberOfVisits = 190;
+
+    SortTester.testAlgorithmTimeComplexity(
+      SelectionSort,
+      sortedArr,
+      expectedNumberOfVisits,
+    );
+  });
+
+  it('should visit NOT SORTED array element specified number of times', () => {
+    const expectedNumberOfVisits = 190;
+
+    SortTester.testAlgorithmTimeComplexity(
+      SelectionSort,
+      notSortedArr,
+      expectedNumberOfVisits,
+    );
+  });
+
+  it('should visit REVERSE SORTED array element specified number of times', () => {
+    const expectedNumberOfVisits = 190;
+
+    SortTester.testAlgorithmTimeComplexity(
+      SelectionSort,
+      reverseArr,
+      expectedNumberOfVisits,
+    );
+  });
+});