Odd Even Transposition Sort / Brick Sort using pthreads

Difficulty Level : Hard
Last Updated : 06 May, 2019

Odd-Even Transposition Sort is a parallel sorting algorithm. It is based on the Bubble Sort technique, which compares every 2 consecutive numbers in the array and swap them if first is greater than the second to get an ascending order array. It consists of 2 phases – the odd phase and even phase:

Odd phase: Every odd indexed element is compared with the next even indexed element(considering 1-based indexing).
Even phase: Every even indexed element is compared with the next odd indexed element.

This article uses the concept of multi-threading, specifically pthread. In each iteration, every pair of 2 consecutive elements is compared using individual threads executing in parallel as illustrated below.

Examples:

Input: { 2, 1, 4, 9, 5, 3, 6, 10 }
Output: 1, 2, 3, 4, 5, 6, 9, 10

Input: { 11, 19, 4, 20, 1, 22, 25, 8}
Output: 1, 4, 8, 11, 19, 20, 22, 25

Note: Compile the program using following command on your Linux based system.

g++ program_name.cpp -pthread

Below is the implementation of the above topic:

// CPP Program for Odd-Even Transpostion sort
// using pthreads
  
#include <bits/stdc++.h>
#include <pthread.h>
  
using namespace std;
  
// size of array
#define n 8
  
// maximum number of threads
int max_threads = (n + 1) / 2;
  
int a[] = { 2, 1, 4, 9, 5, 3, 6, 10 };
int tmp;
  
// Function to compare and exchange
// the consecutive elements of the array
void* compare(void* arg)
{
  
    // Each thread compares
    // two consecutive elements of the array
    int index = tmp;
    tmp = tmp + 2;
  
    if ((a[index] > a[index + 1]) && (index + 1 < n)) {
        swap(a[index], a[index + 1]);
    }
}
  
void oddEven(pthread_t threads[])
{
    int i, j;
  
    for (i = 1; i <= n; i++) {
        // Odd step
        if (i % 2 == 1) {
            tmp = 0;
  
            // Creating threads
            for (j = 0; j < max_threads; j++)
                pthread_create(&threads[j], NULL, compare, NULL);
  
            // joining threads i.e. waiting
            // for all the threads to complete
            for (j = 0; j < max_threads; j++)
                pthread_join(threads[j], NULL);
        }
  
        // Even step
        else {
            tmp = 1;
  
            // Creating threads
            for (j = 0; j < max_threads - 1; j++)
                pthread_create(&threads[j], NULL, compare, NULL);
  
            // joining threads i.e. waiting
            // for all the threads to complete
            for (j = 0; j < max_threads - 1; j++)
                pthread_join(threads[j], NULL);
        }
    }
}
  
// Function to print an array
void printArray()
{
    int i;
    for (i = 0; i < n; i++)
        cout << a[i] << " ";
    cout << endl;
}
  
// Driver Code
int main()
{
  
    pthread_t threads[max_threads];
  
    cout << "Given array is: ";
    printArray();
  
    oddEven(threads);
  
    cout << "\nSorted array is: ";
    printArray();
  
    return 0;
}

Output:

Given array is:  2 1 4 9 5 3 6 10
Sorted array is: 1 2 3 4 5 6 9 10

Time complexity: The time complexity is reduced to O(N) due to parallel computation using threads.
Work complexity: The work complexity of this program is O(N) as N/2 number of threads(resources) are being used to sort the array. So, the work-time complexity of the program is O(N^2).

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