## C++, sort, heap_sort.cpp

``````/**
* \file
* \brief [Heap Sort Algorithm
* (heap sort)](https://en.wikipedia.org/wiki/Heapsort) implementation
*
* \author [Ayaan Khan](http://github.com/ayaankhan98)
*
* \details
*  Heap-sort is a comparison-based sorting algorithm.
*  Heap-sort can be thought of as an improved selection sort:
*  like selection sort, heap sort 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,
*  heap sort 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 - \f\$O(n \log(n))\f\$
*
*/
#include <algorithm>
#include <cassert>
#include <iostream>

/**
*
* Utility function to print the array after
* sorting.
*
* @param arr array to be printed
* @param sz size of array
*
*/
template <typename T>
void printArray(T *arr, int sz) {
for (int i = 0; i < sz; i++) std::cout << arr[i] << "  ";
std::cout << "\n";
}

/**
*
* @{
*
* 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 to be sorted
* @param n size of array
* @param i node position in Binary Tress or element position in
*          Array to be compared with it's childern
*
*/
template <typename T>
void heapify(T *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 && arr[r] > arr[largest])
largest = r;

if (largest != i) {
std::swap(arr[i], arr[largest]);
heapify(arr, n, largest);
}
}

/**
* Utilizes heapify procedure to sort
* the array
*
* @param arr array to be sorted
* @param n size of array
*
*/
template <typename T>
void heapSort(T *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);
}
}

/**
*
* @}
* Test cases to test the program
*
*/
void test() {
std::cout << "Test 1\n";
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
assert(std::is_sorted(arr, arr + sz));
std::cout << "Test 1 Passed\n========================\n";

std::cout << "Test 2\n";
double arr2[] = {4.5, -3.6, 7.6, 0, 12.9};
sz = sizeof(arr2) / sizeof(arr2[0]);
printArray(arr2, sz);
heapSort(arr2, sz);
printArray(arr2, sz);
assert(std::is_sorted(arr2, arr2 + sz));
std::cout << "Test 2 passed\n";
}

/** Main function */
int main() {
test();
return 0;
}
``````