javascript-algorithms-and-data-structures/src/algorithms/sorting/counting-sort/CountingSort.js

Algorithms and data-structures implemented on JavaScript

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;
  }
}