MERGE SORT

void merge(int a[], int m, int b[], int n)
{
        int len = m+n-1;
        m = m-1;
        n = n-1;
        int * buffer = calloc(len+1, sizeof(int));
        int buffer_length = len+1;
        for(int i = 0; i < m+1;i++)
                buffer[i] = a[i];
        while(m>=0 && n>=0){
                if(a[m] < b[n])
                        buffer[len--] = b[n--];
                else
                        buffer[len--] = a[m--];

        }

        while(n >= 0){
                buffer[len--] = b[n--];
        }

        for(int i = 0; i < buffer_length;i++)
                a[i] = buffer[i];

        free(buffer);
}

void merge_sort(int a[], int len)
{
        if(len == 0 || len ==1)
                return;

        int mid = (len-1)/2;

        merge_sort(a, mid+1);
        merge_sort(a+mid+1,len-mid-1);

        merge(a, mid+1, a+mid+1, len-mid-1);

}