-mình chưa hiểu rõ về phần mergesort trong đoạn code sau bạn nào có thể giải thích giúp mình với
Code:
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>

#define NOTHREADS 2

/*
gcc -ggdb -lpthread parallel-mergesort.c 
NOTE: 
The mergesort boils downs to this.. 
Given two sorted array's how do we merge this?
We need a new array to hold the result of merging
otherwise it is not possible to do it using array, 
so we may need a linked list
*/

int a[] = {10, 8, 5, 2, 3, 6, 7, 1, 4, 9};

typedef struct node {
    int i;
    int j;
} NODE;

void merge(int i, int j)
{
        int mid = (i+j)/2;
        int ai = i;
        int bi = mid+1;

        int newa[j-i+1], newai = 0;

        while(ai <= mid && bi <= j) {
                if (a[ai] > a[bi])
                        newa[newai++] = a[bi++];
                else                    
                        newa[newai++] = a[ai++];
        }

        while(ai <= mid) {
                newa[newai++] = a[ai++];
        }

        while(bi <= j) {
                newa[newai++] = a[bi++];
        }

        for (ai = 0; ai < (j-i+1) ; ai++)
                a[i+ai] = newa[ai];

}

void * mergesort(void *a)
{
        NODE *p = (NODE *)a;
        NODE n1, n2;
        int mid = (p->i+p->j)/2;
        pthread_t tid1, tid2;
        int ret;

        n1.i = p->i;
        n1.j = mid;

        n2.i = mid+1;
        n2.j = p->j;

        if (p->i >= p->j) return;

        ret = pthread_create(&tid1, NULL, mergesort, &n1);
        if (ret) {
                printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                exit(1);
        }


        ret = pthread_create(&tid2, NULL, mergesort, &n2);
        if (ret) {
                printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                exit(1);
        }

        pthread_join(tid1, NULL);
        pthread_join(tid2, NULL);

        merge(p->i, p->j);
        pthread_exit(NULL);
}


int main()
{
        int i;
        NODE m;
        m.i = 0;
        m.j = 9;
        pthread_t tid;

        int ret; 

        ret=pthread_create(&tid, NULL, mergesort, &m);
        if (ret) {
                printf("%d %s - unable to create thread - ret - %d\n", __LINE__, __FUNCTION__, ret);    
                exit(1);
        }

        pthread_join(tid, NULL);

        for (i = 0; i < 10; i++)
                        printf ("%d ", a[i]);

        printf ("\n");

        // pthread_exit(NULL);
        return 0;
}