在自底向上合并排序中合并数组时中止陷阱 6

Question

I was trying to implement Merge Sort (Bottom-up approach) until a mysterious Abort trap occurs:

void botup_mergesort(int *arr, int begin, int end)
{
    for (int merge_sz = 1; merge_sz < (end - begin); merge_sz *= 2)
    {
        for (int par = begin; par < end; par += merge_sz*2)
        {
            const int &sub_begin = par;
            const int &sub_end = std::min(par + merge_sz*2, end);

            if (sub_end - sub_begin <= 1)
            {
                continue;
            }

            const int& m = par + merge_sz;

            const int &merge_size = sub_end - sub_begin;
            int *aux = new int[merge_size];
            int cnt = -1;

            int p1 = sub_begin;
            int p2 = m;
            while (p2 < end && p1 < m && p2 < sub_end)
            {
                if (arr[p1] <= arr[p2])
                    aux[++cnt] = arr[p1++];
                else if (arr[p1] > arr[p2])
                    aux[++cnt] = arr[p2++];
            }

            while (p1 < m)
            {
                aux[++cnt] = arr[p1++];
            }

            while (p2 < sub_end && p2 < end)
            {
                aux[++cnt] = arr[p2++];
            }

            for (int i = sub_begin, m_cnt = 0; i < sub_end; ++i)
            {
                arr[i] = aux[m_cnt++];
            }
            delete[] aux;
        }
    }
}

void sort(int *arr, int begin, int end) { botup_mergesort(arr, begin, end); }

int main()
{
    int arr[] = {2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 5, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 5};
    // int arr2[] = {2, 2, 2, 2, 2, 8, 5, 3, 9, 4, 6, 2, 1, 5, 1, 5, 7};
    int n = sizeof(arr) / sizeof(int);
    cout << "Size: " << n << endl;

    sort(arr, 0, n);

    print(arr, n);
    return 0;
}

The code works fine for small arrays, ie int arr2[] , but as the array gets bigger, Abort trap comes back.

I tried using the debugger and it returned SIGABRT at delete[] aux; . I did a little search on the internet and decided to use std::vector instead of a dynamic array, and it worked. However, I really desired to know what is wrong behind this code.

My compiler is Apple clang version 11.0.3 on Visual Studio Code.

Thank you for your help! Sorry I've forgetten something, I am a inexperienced college student.

Answer 1

The following change will fix the error:

        const int& m = std::min(par + merge_sz, end);

The code could be a bit faster if a one time allocation of a full sized second array was done, and if the direction of merge was changed on each pass, instead of doing a copy back after each merge. The code would need to detect if the number of passes is odd, where the last merge would put data into the second array, where it would need to copy the data back to the first array one time.