a documented merge example in c

merge.c

void merge(int arr[], int startIndex, int endIndex)
{
    // 1. calculate the middle index
    int midIndex = startIndex + ((endIndex - startIndex) / 2);
    // 2. determine left array size
    int leftArrSize = midIndex - startIndex + 1;
    // 3. determine right array size
    int rightArrSize = endIndex - midIndex;
    // 4. initialize left and right arrays
    int leftArr[leftArrSize];
    int rightArr[rightArrSize];

    // 5. copy values from the original array into the right and left arrays
    for (int i = 0; i < leftArrSize; i++)
    {
        leftArr[i] = arr[startIndex + i];
    }
    for (int i = 0; i < rightArrSize; i++)
    {
        rightArr[i] = arr[midIndex + 1 + i];
    }

    // 6. create our crawlers
    int leftCrawler = 0;
    int rightCrawler = 0;
    int resultCrawler = startIndex;

    // 7. crawls over these arrays mutating the original array with the smaller value
    // until either the left or right array is empty.
    while (leftCrawler < leftArrSize && rightCrawler < rightArrSize)
    {
        if (leftArr[leftCrawler] < rightArr[rightCrawler])
        {
            arr[resultCrawler] = leftArr[leftCrawler];
            leftCrawler++;
        }
        else
        {
            arr[resultCrawler] = rightArr[rightCrawler];
            rightCrawler++;
        }
        resultCrawler++;
    }

    // 8. insert the remaining elements of the left or right array into the result.
    while (leftCrawler < leftArrSize)
    {
        arr[resultCrawler] = leftArr[leftCrawler];
        resultCrawler++;
        leftCrawler++;
    }
    while (rightCrawler < rightArrSize)
    {
        arr[resultCrawler] = rightArr[rightCrawler];
        resultCrawler++;
        rightCrawler++;
    }
}