Add counting sort.

README.md

@@ -80,6 +80,7 @@ a set of rules that precisely defines a sequence of operations.
   * [Merge Sort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/merge-sort)
   * [Quicksort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/quick-sort) - in-place and non-in-place implementations
   * [Shellsort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/shell-sort)
+  * [Counting Sort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/counting-sort)
 * **Tree**  
   * [Depth-First Search](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/tree/depth-first-search) (DFS)
   * [Breadth-First Search](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/tree/breadth-first-search) (BFS)
@@ -225,3 +226,4 @@ Below is the list of some of the most used Big O notations and their performance
 | **Merge sort**        | n log(n)  | n log(n)  | n log(n)      | n         | Yes       |
 | **Quick sort**        | n log(n)  | n log(n)  | n^2           | log(n)    | No        |
 | **Shell sort**        | n log(n)  | depends on gap sequence   | n (log(n))^2  | 1         | No        |
+| **Counting sort**     | n + r     | n + r     | n + r         | n + r     | Yes       |

src/algorithms/sorting/SortTester.js

@@ -11,6 +11,7 @@ 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([3, 4, 2, 1, 0, 0, 4, 3, 4, 2])).toEqual([0, 0, 1, 2, 2, 3, 3, 4, 4, 4]);
     expect(sorter.sort(sortedArr)).toEqual(sortedArr);
     expect(sorter.sort(reverseArr)).toEqual(sortedArr);
     expect(sorter.sort(notSortedArr)).toEqual(sortedArr);

src/algorithms/sorting/counting-sort/CountingSort.js

@@ -0,0 +1,69 @@
+import Sort from '../Sort';
+
+export default class CountingSort extends Sort {
+  /**
+   * @param {number[]} originalArray
+   * @param {number} [biggestElement]
+   */
+  sort(originalArray, biggestElement = 0) {
+    // Detect biggest element in array in order to build in order to build
+    // number bucket array later.
+    let detectedBiggestElement = biggestElement;
+    if (!detectedBiggestElement) {
+      originalArray.forEach((element) => {
+        // Visit element.
+        this.callbacks.visitingCallback(element);
+
+        if (this.comparator.greaterThan(element, detectedBiggestElement)) {
+          detectedBiggestElement = element;
+        }
+      });
+    }
+
+    // Init buckets array.
+    // This array will hold frequency of each number from originalArray.
+    const buckets = Array(detectedBiggestElement + 1).fill(0);
+    originalArray.forEach((element) => {
+      // Visit element.
+      this.callbacks.visitingCallback(element);
+
+      buckets[element] += 1;
+    });
+
+    // Add previous frequencies to the current one for each number in bucket
+    // to detect how many numbers less then current one should be standing to
+    // the left of current one.
+    for (let bucketIndex = 1; bucketIndex < buckets.length; bucketIndex += 1) {
+      buckets[bucketIndex] += buckets[bucketIndex - 1];
+    }
+
+    // Now let's shift frequencies to the right so that they show correct numbers.
+    // I.e. if we won't shift right than the value of buckets[5] will display how many
+    // elements less than 5 should be placed to the left of 5 in sorted array
+    // INCLUDING 5th. After shifting though this number will not include 5th anymore.
+    buckets.pop();
+    buckets.unshift(0);
+
+    // Now let's assemble sorted array.
+    const sortedArray = Array(originalArray.length).fill(null);
+    for (let elementIndex = 0; elementIndex < originalArray.length; elementIndex += 1) {
+      // Get the element that we want to put into correct sorted position.
+      const element = originalArray[elementIndex];
+
+      // Visit element.
+      this.callbacks.visitingCallback(element);
+
+      // Get correct position of this element in sorted array.
+      const elementSortedPosition = buckets[element];
+
+      // Put element into correct position in sorted array.
+      sortedArray[elementSortedPosition] = element;
+
+      // Increase position of current element in the bucket for future correct placements.
+      buckets[element] += 1;
+    }
+
+    // Return sorted array.
+    return sortedArray;
+  }
+}

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

@@ -0,0 +1,61 @@
+# Counting Sort
+
+In computer science, **counting sort** is an algorithm for sorting 
+a collection of objects according to keys that are small integers; 
+that is, it is an integer sorting algorithm. It operates by 
+counting the number of objects that have each distinct key value, 
+and using arithmetic on those counts to determine the positions 
+of each key value in the output sequence. Its running time is 
+linear in the number of items and the difference between the 
+maximum and minimum key values, so it is only suitable for direct 
+use in situations where the variation in keys is not significantly 
+greater than the number of items. However, it is often used as a 
+subroutine in another sorting algorithm, radix sort, that can 
+handle larger keys more efficiently.
+
+Because counting sort uses key values as indexes into an array, 
+it is not a comparison sort, and the `Ω(n log n)` lower bound for 
+comparison sorting does not apply to it. Bucket sort may be used 
+for many of the same tasks as counting sort, with a similar time 
+analysis; however, compared to counting sort, bucket sort requires 
+linked lists, dynamic arrays or a large amount of preallocated 
+memory to hold the sets of items within each bucket, whereas 
+counting sort instead stores a single number (the count of items) 
+per bucket.
+
+Counting sorting works best when the range of numbers for each array
+element is very small.
+
+## Algorithm
+
+**Step I**
+
+In first step we calculate the count of all the elements of the 
+input array `A`. Then Store the result in the count array `C`.
+The way we count is depected below.
+
+![Counting Sort](https://3.bp.blogspot.com/-jJchly1BkTc/WLGqCFDdvCI/AAAAAAAAAHA/luljAlz2ptMndIZNH0KLTTuQMNsfzDeFQCLcB/s1600/CSortUpdatedStepI.gif)
+
+**Step II**
+
+In second step we calculate how many elements exist in the input 
+array `A` which are less than or equals for the given index. 
+`Ci` = numbers of elements less than or equals to `i` in input array.
+
+![Counting Sort](https://1.bp.blogspot.com/-1vFu-VIRa9Y/WLHGuZkdF3I/AAAAAAAAAHs/8jKu2dbQee4ap9xlVcNsILrclqw0UxAVACLcB/s1600/Step-II.png)
+
+**Step III**
+
+In this step we place the input array `A` element at sorted 
+position by taking help of constructed count array `C` ,i.e what 
+we constructed in step two. We used the result array `B` to store 
+the sorted elements. Here we handled the index of `B` start from
+zero.
+ 
+![Counting Sort](https://1.bp.blogspot.com/-xPqylngqASY/WLGq3p9n9vI/AAAAAAAAAHM/JHdtXAkJY8wYzDMBXxqarjmhpPhM0u8MACLcB/s1600/ResultArrayCS.gif)
+
+## References
+
+- [Wikipedia](https://en.wikipedia.org/wiki/Counting_sort)
+- [YouTube](https://www.youtube.com/watch?v=OKd534EWcdk&index=61&t=0s&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)
+- [EfficientAlgorithms](https://efficientalgorithms.blogspot.com/2016/09/lenear-sorting-counting-sort.html)

src/algorithms/sorting/counting-sort/__test__/CountingSort.test.js

@@ -0,0 +1,69 @@
+import CountingSort from '../CountingSort';
+import {
+  equalArr,
+  notSortedArr,
+  reverseArr,
+  sortedArr,
+  SortTester,
+} from '../../SortTester';
+
+// Complexity constants.
+const SORTED_ARRAY_VISITING_COUNT = 60;
+const NOT_SORTED_ARRAY_VISITING_COUNT = 60;
+const REVERSE_SORTED_ARRAY_VISITING_COUNT = 60;
+const EQUAL_ARRAY_VISITING_COUNT = 60;
+
+describe('CountingSort', () => {
+  it('should sort array', () => {
+    SortTester.testSort(CountingSort);
+  });
+
+  it('should allow to use specify maximum integer value in array to make sorting faster', () => {
+    const visitingCallback = jest.fn();
+    const sorter = new CountingSort({ visitingCallback });
+
+    // Detect biggest number in array in prior.
+    const biggestElement = notSortedArr.reduce((accumulator, element) => {
+      return element > accumulator ? element : accumulator;
+    }, 0);
+
+    const sortedArray = sorter.sort(notSortedArr, biggestElement);
+
+    expect(sortedArray).toEqual(sortedArr);
+    // Normally visitingCallback is being called 60 times but in this case
+    // it should be called only 40 times.
+    expect(visitingCallback).toHaveBeenCalledTimes(40);
+  });
+
+  it('should visit EQUAL array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      CountingSort,
+      equalArr,
+      EQUAL_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      CountingSort,
+      sortedArr,
+      SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit NOT SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      CountingSort,
+      notSortedArr,
+      NOT_SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+
+  it('should visit REVERSE SORTED array element specified number of times', () => {
+    SortTester.testAlgorithmTimeComplexity(
+      CountingSort,
+      reverseArr,
+      REVERSE_SORTED_ARRAY_VISITING_COUNT,
+    );
+  });
+});