From d2f4586d147ff33bf4792a11900f6f579ce06e8b Mon Sep 17 00:00:00 2001
From: Ayaan Khan <ayaankhan98@gmail.com>
Date: Sun, 21 Jun 2020 00:03:36 +0530
Subject: [PATCH] Bug Fix heap sort [Fresh Implementation]

---
 sorting/heap_sort.cpp | 119 ++++++++++++++++++++++++++++--------------
 1 file changed, 81 insertions(+), 38 deletions(-)

diff --git a/sorting/heap_sort.cpp b/sorting/heap_sort.cpp
index 9948bb821..405e4641d 100644
--- a/sorting/heap_sort.cpp
+++ b/sorting/heap_sort.cpp
@@ -1,52 +1,95 @@
-#include <algorithm>
+/**
+ * \addtogroup sorting Sorting Algorithm
+ * @{
+ * \file
+ * \brief [Heap Sort Algorithm
+ * (HEAP SORT)](https://en.wikipedia.org/wiki/Heapsort) implementation
+ *
+ * \author [Ayaan Khan](http://github.com/ayaankhan98)
+ *
+ * \details
+ *  heapsort is a comparison-based sorting algorithm. 
+ *  Heapsort can be thought of as an improved selection sort: 
+ *  like selection sort, heapsort divides its input into a sorted 
+ *  and an unsorted region, and it iteratively shrinks the unsorted 
+ *  region by extracting the largest element from it and inserting 
+ *  it into the sorted region. Unlike selection sort, 
+ *  heapsort does not waste time with a linear-time scan of the 
+ *  unsorted region; rather, heap sort maintains the unsorted region 
+ *  in a heap data structure to more quickly find the largest element 
+ *  in each step.
+ *  
+ *  Time Complexity - O(nlog(n))
+ *
+ */
 #include <iostream>
 
-void heapify(int *a, int i, int n) {
-    int largest = i;
-    const int l = 2 * i + 1;
-    const int r = 2 * i + 2;
+/**
+ * 
+ * Utility Lambda function to print the array after
+ * sorting.
+ *
+ * @param arr array to be printed
+ * @param sz size of array
+ *
+ */
+auto printArray = [] (int *arr, int sz) {
+    for (int i = 0 ; i < sz ; i++)
+        std::cout << arr[i] <<"  ";
+    std::cout << "\n";
+};
 
-    if (l < n && a[l] > a[largest])
+/**
+ *
+ * The heapify procedure can be thought of as building a heap from 
+ * the bottom up by successively sifting downward to establish the 
+ * heap property. 
+ * 
+ * @param arr array be to sorted
+ * @param 
+ */
+void(*heapify)(int *arr, int n, int i) = [] (int *arr, int n, int i) {
+    int largest = i;
+    int l = 2*i + 1;
+    int r = 2*i + 2;
+
+    if (l < n && arr[l] > arr[largest])
         largest = l;
 
-    if (r < n && a[r] > a[largest])
+    if (r < n && arr[r] > arr[largest])
         largest = r;
 
     if (largest != i) {
-        std::swap(a[i], a[largest]);
-        heapify(a, n, largest);
+        std::swap(arr[i], arr[largest]);
+        heapify(arr, n, largest);
     }
-}
+};
 
-void heapsort(int *a, int n) {
-    for (int i = n - 1; i >= 0; --i) {
-        std::swap(a[0], a[i]);
-        heapify(a, 0, i);
+/**
+ * heapSort lambda function utilizes heapify procedure to sort
+ * the array
+ *
+ * @param arr array to be sorted
+ * @param n size of array
+ *
+ */
+auto heapSort = [] (int *arr, int n) {
+    for (int i = n-1 ; i >= 0; i--)
+        heapify(arr, n, i);
+
+    for (int i = n-1 ; i >= 0; i--) {
+        std::swap(arr[0], arr[i]);
+        heapify(arr, i, 0);
     }
-}
-
-void build_maxheap(int *a, int n) {
-    for (int i = n / 2 - 1; i >= 0; --i) {
-        heapify(a, i, n);
-    }
-}
+};
 
+/** Main function */
 int main() {
-    int n;
-    std::cout << "Enter number of elements of array\n";
-    std::cin >> n;
-    int a[20];
-    for (int i = 0; i < n; ++i) {
-        std::cout << "Enter Element " << i << std::endl;
-        std::cin >> a[i];
-    }
-
-    build_maxheap(a, n);
-    heapsort(a, n);
-    std::cout << "Sorted Output\n";
-    for (int i = 0; i < n; ++i) {
-        std::cout << a[i] << std::endl;
-    }
-
-    std::getchar();
+    int arr[] = {-10, 78, -1, -6, 7, 4, 94, 5, 99, 0};
+    int sz = sizeof(arr) / sizeof(arr[0]);  // sz - size of array
+    printArray(arr, sz);  // displaying the array before sorting
+    heapSort(arr, sz);  // calling heapsort to sort the array
+    printArray(arr, sz);  // display array after sorting
+    return 0;
 }
+/** @} */