MinHeap.js

/**
 *   Min Heap is one of the two Binary Heap types (the other is Max Heap)
 *   which maintains the smallest value of its input array on top and remaining values in loosely (but not perfectly sorted) order.
 *
 *   Min Heaps can be expressed as a 'complete' binary tree structure
 *   (in which all levels of the binary tree are filled, with the exception of the last level which must be filled left-to-right).
 *
 *   However the Min Heap class below expresses this tree structure as an array
 *   which represent the binary tree node values in an array ordered from root-to-leaf, left-to-right.
 *
 *   In the array representation, the parent node-child node relationship is such that the
 *      * parent index relative to its two children are: (parentIdx * 2) and (parent * 2 + 1)
 *      * and either child's index position relative to its parent is: Math.floor((childIdx-1)/2)
 *
 *   The parent and respective child values define much of heap behavior as we continue to sort or not sort depending on their values.
 *      * The parent value must be less than or equal to either child's value.
 *
 *   This is a condensed overview but for more information and visuals here is a nice read: https://www.geeksforgeeks.org/binary-heap/
 */

class MinHeap {
  constructor (array) {
    this.heap = this.initializeHeap(array)
  }

  /**
   *   startingParent represents the parent of the last index (=== array.length-1)
   *   and iterates towards 0 with all index values below sorted to meet heap conditions
  */
  initializeHeap (array) {
    const startingParent = Math.floor((array.length - 2) / 2)

    for (let currIdx = startingParent; currIdx >= 0; currIdx--) {
      this.sinkDown(currIdx, array.length - 1, array)
    }
    return array
  }

  /**
   *   overall functionality: heap-sort value at a starting index (currIdx) towards end of heap
   *
   *   currIdx is considered to be a starting 'parent' index of two children indices (childOneIdx, childTwoIdx).
   *   endIdx represents the last valid index in the heap.
   *
   *   first check that childOneIdx and childTwoIdx are both smaller than endIdx
   *   and check for the smaller heap value between them.
   *
   *   the child index with the smaller heap value is set to a variable called swapIdx.
   *
   *   swapIdx's value will be compared to currIdx (the 'parent' index)
   *   and if swapIdx's value is smaller than currIdx's value, swap the values in the heap,
   *   update currIdx and recalculate the new childOneIdx to check heap conditions again.
   *
   *   if there is no swap, it means the children indices and the parent index satisfy heap conditions and can exit the function.
  */
  sinkDown (currIdx, endIdx, heap) {
    let childOneIdx = currIdx * 2 + 1

    while (childOneIdx <= endIdx) {
      const childTwoIdx = childOneIdx + 1 <= endIdx ? childOneIdx + 1 : -1
      const swapIdx = childTwoIdx !== -1 && heap[childTwoIdx] < heap[childOneIdx]
        ? childTwoIdx
        : childOneIdx

      if (heap[swapIdx] < heap[currIdx]) {
        this.swap(currIdx, swapIdx, heap)
        currIdx = swapIdx
        childOneIdx = currIdx * 2 + 1
      } else {
        return
      }
    }
  }

  /**
   *   overall functionality: heap-sort value at a starting index (currIdx) towards front of heap.
   *
   *   while the currIdx's value is smaller than its parent's (parentIdx) value, swap the values in the heap
   *   update currIdx and recalculate the new parentIdx to check heap condition again.
   *
   *   iteration does not end while a valid currIdx has a value smaller than its parentIdx's value
  */
  bubbleUp (currIdx) {
    let parentIdx = Math.floor((currIdx - 1) / 2)

    while (currIdx > 0 && this.heap[currIdx] < this.heap[parentIdx]) {
      this.swap(currIdx, parentIdx, this.heap)
      currIdx = parentIdx
      parentIdx = Math.floor((currIdx - 1) / 2)
    }
  }

  peek () {
    return this.heap[0]
  }

  /**
   *   the min heap value should be the first value in the heap (=== this.heap[0])
   *
   *   firstIdx value and lastIdx value are swapped
   *   the resulting min heap value now resides at heap[heap.length-1] which is popped and later returned.
   *
   *   the remaining values in the heap are re-sorted
  */
  extractMin () {
    this.swap(0, this.heap.length - 1, this.heap)
    const min = this.heap.pop()
    this.sinkDown(0, this.heap.length - 1, this.heap)
    return min
  }

  // a new value is pushed to the end of the heap and sorted up
  insert (value) {
    this.heap.push(value)
    this.bubbleUp(this.heap.length - 1)
  }

  // index-swapping helper method
  swap (idx1, idx2, heap) {
    const temp = heap[idx1]
    heap[idx1] = heap[idx2]
    heap[idx2] = temp
  }
}

export { MinHeap }