javascript-algorithms-and-data-structures/src/algorithms/sorting/radix-sort/RadixSort.js

Algorithms and data-structures implemented on JavaScript

import Sort from '../Sort';

// Using charCode (a = 97, b = 98, etc), we can map characters to buckets from 0 - 25
const BASE_CHAR_CODE = 97;
const NUMBER_OF_POSSIBLE_DIGITS = 10;
const ENGLISH_ALPHABET_LENGTH = 26;

export default class RadixSort extends Sort {
  /**
   * @param {*[]} originalArray
   * @return {*[]}
   */
  sort(originalArray) {
    // Assumes all elements of array are of the same type
    const isArrayOfNumbers = this.isArrayOfNumbers(originalArray);

    let sortedArray = [...originalArray];
    const numPasses = this.determineNumPasses(sortedArray);

    for (let currentIndex = 0; currentIndex < numPasses; currentIndex += 1) {
      const buckets = isArrayOfNumbers ?
        this.placeElementsInNumberBuckets(sortedArray, currentIndex) :
        this.placeElementsInCharacterBuckets(sortedArray, currentIndex, numPasses);

      // Flatten buckets into sortedArray, and repeat at next index
      sortedArray = buckets.reduce((acc, val) => {
        return [...acc, ...val];
      }, []);
    }

    return sortedArray;
  }

  /**
   * @param {*[]} array
   * @param {number} index
   * @return {*[]}
   */
  placeElementsInNumberBuckets(array, index) {
    // See below. These are used to determine which digit to use for bucket allocation
    const modded = 10 ** (index + 1);
    const divided = 10 ** index;
    const buckets = this.createBuckets(NUMBER_OF_POSSIBLE_DIGITS);

    array.forEach((element) => {
      this.callbacks.visitingCallback(element);
      if (element < divided) {
        buckets[0].push(element);
      } else {
        /**
         * Say we have element of 1,052 and are currently on index 1 (starting from 0). This means
         * we want to use '5' as the bucket. `modded` would be 10 ** (1 + 1), which
         * is 100. So we take 1,052 % 100 (52) and divide it by 10 (5.2) and floor it (5).
         */
        const currentDigit = Math.floor((element % modded) / divided);
        buckets[currentDigit].push(element);
      }
    });

    return buckets;
  }

  /**
   * @param {*[]} array
   * @param {number} index
   * @param {number} numPasses
   * @return {*[]}
   */
  placeElementsInCharacterBuckets(array, index, numPasses) {
    const buckets = this.createBuckets(ENGLISH_ALPHABET_LENGTH);

    array.forEach((element) => {
      this.callbacks.visitingCallback(element);
      const currentBucket = this.getCharCodeOfElementAtIndex(element, index, numPasses);
      buckets[currentBucket].push(element);
    });

    return buckets;
  }

  /**
   * @param {string} element
   * @param {number} index
   * @param {number} numPasses
   * @return {number}
   */
  getCharCodeOfElementAtIndex(element, index, numPasses) {
    // Place element in last bucket if not ready to organize
    if ((numPasses - index) > element.length) {
      return ENGLISH_ALPHABET_LENGTH - 1;
    }

    /**
     * If each character has been organized, use first character to determine bucket,
     * otherwise iterate backwards through element
     */
    const charPos = index > element.length - 1 ? 0 : element.length - index - 1;

    return element.toLowerCase().charCodeAt(charPos) - BASE_CHAR_CODE;
  }

  /**
   * Number of passes is determined by the length of the longest element in the array.
   * For integers, this log10(num), and for strings, this would be the length of the string.
   */
  determineNumPasses(array) {
    return this.getLengthOfLongestElement(array);
  }

  /**
   * @param {*[]} array
   * @return {number}
   */
  getLengthOfLongestElement(array) {
    if (this.isArrayOfNumbers(array)) {
      return Math.floor(Math.log10(Math.max(...array))) + 1;
    }

    return array.reduce((acc, val) => {
      return val.length > acc ? val.length : acc;
    }, -Infinity);
  }

  /**
   * @param {*[]} array
   * @return {boolean}
   */
  isArrayOfNumbers(array) {
    // Assumes all elements of array are of the same type
    return this.isNumber(array[0]);
  }

  /**
   * @param {number} numBuckets
   * @return {*[]}
   */
  createBuckets(numBuckets) {
    /**
     * Mapping buckets to an array instead of filling them with
     * an array prevents each bucket from containing a reference to the same array
     */
    return new Array(numBuckets).fill(null).map(() => []);
  }

  /**
   * @param {*} element
   * @return {boolean}
   */
  isNumber(element) {
    return Number.isInteger(element);
  }
}