index.js

/**
 * Creates a HeapNode object for use in the MinHeap class.
 * The purpose of HeapNode is to establish some consistency across different
 * potential data types you'd use with MinHeap.
 */
class HeapNode {
  /**
   * Instantiates a HeapNode with a value property equal to the value passed.
   * @param {*} value
   */
  constructor(value) {
    this.value = value;
  }
}

/**
 * Creates a min heap data structure
 * Public methods are insert(), pull(), and peek().
 */
class MinHeap {
  #list = [];

  /**
   * Compares two nodes. Can be overridden with a custom function passed to the
   * class constructor.
   *
   * @param {HeapNode} n1 - a node to compare
   * @param {HeapNode} n2 - a node to compare
   * @returns Returns true if n2's value is greater than n1's; false otherwise.
   */
  #compare = (n1, n2) => {
    if (!n1 instanceof HeapNode || !n2 instanceof HeapNode) {
      throw "Comparison argument must be an instance of HeapNode.";
    }
    return n2.value > n1.value;
  };

  /**
   * Instantiates a MinHeap.
   *
   * @param {Function} compareFunction - optional custom comparison function.
   * See the #compare member for implementation details.
   */
  constructor(compareFunction) {
    if (compareFunction && typeof compareFunction === "function") {
      this.#compare = compareFunction;
    }
  }

  /**
   * Gets the parent node's index.
   *
   * @param {Number} i index of the node whose parent you want.
   * @returns the index of the parent node.
   */
  #parent(i) {
    return Math.floor((i - 1) * 0.5);
  }

  /**
   * Gets the index of the node's left child.
   *
   * @param {Number} i index of the node whose left child you want.
   * @returns the index of the node's left child.
   */
  #leftChild(i) {
    const childIndex = i * 2 + 1;
    return this.#list[childIndex] && childIndex;
  }

  /**
   * Gets the index of the node's right child.
   *
   * @param {Number} i index of the node whose right child you want.
   * @returns the index of the node's right child.
   */
  #rightChild(i) {
    const childIndex = i * 2 + 2;
    return this.#list[childIndex] && childIndex;
  }

  /**
   * Bubbles a node up the heap to its proper position, to maintain the min heap
   * strutcure.
   *
   * @param {Number} i index of the node to bubble up.
   */
  #heapifyUp(i) {
    if (this.#list.length < 2) return;

    while (this.#parent(i) >= 0) {
      const iParent = this.#parent(i);

      // Get references to the nodes themselves
      let node = this.#list[i];
      let parentNode = this.#list[iParent];

      // Exit early if the node at i is greater than the parent node
      if (this.#compare(parentNode, node)) break;

      // Swap the nodes and set them:
      [parentNode, node] = [node, parentNode];
      this.#list[iParent] = parentNode;
      this.#list[i] = node;

      i = iParent;
    }
  }

  /**
   * Moves a node down the heap to its proper position, to maintain the min heap
   * strutcure.
   *
   * @param {Number} i index of the node to move down.
   */
  #heapifyDown(i) {
    if (this.#list.length < 2) return;

    while (this.#leftChild(i)) {
      // Get the child indeces, and find the smaller of the two nodes
      const iChildL = this.#leftChild(i);
      const iChildR = this.#rightChild(i);
      let iSmallestChild =
        iChildR && this.#compare(this.#list[iChildR], this.#list[iChildL])
          ? iChildR
          : iChildL;

      // Get references to the nodes themselves
      let node = this.#list[i];
      let childNode = this.#list[iSmallestChild];

      // Exit early if the child is greater than the node at i
      if (this.#compare(node, childNode)) break;

      // Swap the nodes and set them
      [node, childNode] = [childNode, node];
      this.#list[i] = node;
      this.#list[iSmallestChild] = childNode;

      i = iSmallestChild;
    }
  }

  /**
   * Peek the root node.
   *
   * @returns the root node's value property.
   */
  peek() {
    return this.#list[0].value;
  }

  /**
   * Removes and returns the root node. Readjusts the heap to maintain its
   * required structure.
   *
   * @returns the root node's value property.
   */
  pull() {
    if (this.#list.length === 0) return;

    const firstNode = this.#list[0];
    const poppedNode = this.#list.pop();

    if (this.#list.length !== 0) {
      this.#list[0] = poppedNode;
      this.#heapifyDown(0);
    }

    return firstNode.value;
  }

  /**
   * Inserts a HeapNode instance.
   *
   * @param {*} value - The value of the node to insert.
   * @returns the value passed.
   */
  insert(value) {
    const length = this.#list.push(new HeapNode(value));
    this.#heapifyUp(length - 1);
    return value;
  }

  /**
   * Gets the heap array. Useful for debugging/logging.
   *
   * @returns the heap array.
   */
  getList() {
    return this.#list;
  }

  /**
   * Checks if the heap is empty.
   *
   * @returns true if empty, false otherwise.
   */
  isEmpty() {
    return this.#list.length === 0;
  }
}

export default MinHeap;