## C++, sort, bitonic_sort.cpp

``````// Source : https://www.geeksforgeeks.org/bitonic-sort/

/* C++ Program for Bitonic Sort. Note that this program
works only when size of input is a power of 2. */

#include <algorithm>
#include <iostream>

/*The parameter dir indicates the sorting direction, ASCENDING
or DESCENDING; if (a[i] > a[j]) agrees with the direction,
then a[i] and a[j] are interchanged.*/
void compAndSwap(int a[], int i, int j, int dir) {
if (dir == (a[i] > a[j]))
std::swap(a[i], a[j]);
}

/*It recursively sorts a bitonic sequence in ascending order,
if dir = 1, and in descending order otherwise (means dir=0).
The sequence to be sorted starts at index position low,
the parameter cnt is the number of elements to be sorted.*/
void bitonicMerge(int a[], int low, int cnt, int dir) {
if (cnt > 1) {
int k = cnt / 2;
for (int i = low; i < low + k; i++) compAndSwap(a, i, i + k, dir);
bitonicMerge(a, low, k, dir);
bitonicMerge(a, low + k, k, dir);
}
}

/* This function first produces a bitonic sequence by recursively
sorting its two halves in opposite sorting orders, and then
calls bitonicMerge to make them in the same order */
void bitonicSort(int a[], int low, int cnt, int dir) {
if (cnt > 1) {
int k = cnt / 2;

// sort in ascending order since dir here is 1
bitonicSort(a, low, k, 1);

// sort in descending order since dir here is 0
bitonicSort(a, low + k, k, 0);

// Will merge wole sequence in ascending order
// since dir=1.
bitonicMerge(a, low, cnt, dir);
}
}

/* Caller of bitonicSort for sorting the entire array of
length N in ASCENDING order */
void sort(int a[], int N, int up) { bitonicSort(a, 0, N, up); }

// Driver code
int main() {
int a[] = {3, 7, 4, 8, 6, 2, 1, 5};
int N = sizeof(a) / sizeof(a[0]);

int up = 1;  // means sort in ascending order
sort(a, N, up);

std::cout << "Sorted array: \n";
for (int i = 0; i < N; i++) std::cout << a[i] << " ";
return 0;
}
``````