Question

Any multi-core sorting implementation in .NET?

Answer 1

Here’s a multi-threaded QuickSort I put together a while back using async/await. Under a certain sort size, it “reverts” back to an elementary sort known as Double-Ended Selection Sort:

public static class SortExtensions
{
    /// <summary>
    /// Sorts the list.
    /// </summary>
    /// <typeparam name="T">
    /// The IComparable element type of the list.
    /// </typeparam>
    /// <param name="list">The list to sort.</param>
    public static async Task SortIt<T>(this IList<T> list) where T : IComparable<T> =>
        await SortIt(list, 0, list.Count - 1);

    /// <summary>
    /// Sorts the list.
    /// </summary>
    /// <typeparam name="T">The element type of the list.</typeparam>
    /// <param name="list">The list to sort.</param>
    /// <param name="lowerbound">The lower bound.</param>
    /// <param name="upperbound">The upper bound.</param>
    private static async Task SortIt<T>(
        this IList<T> list,
        int lowerbound,
        int upperbound) where T : IComparable<T>
    {
        // If list is non-existent or has zero or one element, no need to sort, so exit.
        if ((list == null) || (upperbound - lowerbound < 1))
        {
            return;
        }

        const int MinListLength = 6;

        if (upperbound - lowerbound > MinListLength)
        {
            await list.QuickSort(lowerbound, upperbound);
            return;
        }

        await list.DoubleEndedSelectionSort(lowerbound, upperbound);
    }

    /// <summary>
    /// QuickSorts the list.
    /// </summary>
    /// <typeparam name="T">The element type of the list.</typeparam>
    /// <param name="list">The list to sort.</param>
    /// <param name="lowerbound">The lower bound.</param>
    /// <param name="upperbound">The upper bound.</param>
    private static async Task QuickSort<T>(
        this IList<T> list,
        int lowerbound,
        int upperbound) where T : IComparable<T>
    {
        int left = lowerbound;
        int right = upperbound;
        int middle = (lowerbound + upperbound) >> 1;
        int pivotindex = (list[lowerbound].CompareTo(list[middle]) <= 0)
            && (list[middle].CompareTo(list[upperbound]) <= 0)
            ? middle
            : ((list[middle].CompareTo(list[lowerbound]) <= 0)
                && (list[lowerbound].CompareTo(list[upperbound]) <= 0)
                ? lowerbound
                : upperbound);
        T pivotvalue = list[pivotindex];

        do
        {
            while ((left < upperbound) && (list[left].CompareTo(pivotvalue) < 0))
            {
                left++;
            }

            while ((right > lowerbound) && (list[right].CompareTo(pivotvalue) > 0))
            {
                right--;
            }

            if (left > right)
            {
                continue;
            }

            (list[left], list[right]) = (list[right], list[left]);
            left++;
            right--;
        }
        while (right > left);

        if (lowerbound < right)
        {
            await list.SortIt(lowerbound, right);
        }

        if (left < upperbound)
        {
            await list.SortIt(left, upperbound);
        }
    }

    /// <summary>
    /// Double-ended selection sorts the list.
    /// </summary>
    /// <typeparam name="T">The element type of the list.</typeparam>
    /// <param name="list">The list to sort.</param>
    /// <param name="lowerbound">The lower bound.</param>
    /// <param name="upperbound">The upper bound.</param>
    private static async Task DoubleEndedSelectionSort<T>(
        this IList<T> list,
        int lowerbound,
        int upperbound) where T : IComparable<T>
    {
        // Initialize some working variables that will hold the sortable sublist indices.
        int left = lowerbound;
        int right = upperbound;

        // Keep sorting the list while the upper bound of the sublist is greater than the lower bound.
        while (right > left)
        {
            // Find get the indices of the smallest and largest elements of the sublist.
            (int smallest, int largest) = await list.FindSmallestAndLargest(left, right);

            if (smallest != largest)
            {
                // If they're different elements, swap the smallest with left element of the sublist.
                (list[left], list[smallest]) = (list[smallest], list[left]);
                if (largest == left)
                {
                    // If the largest element of the sublist was the left element, then now, it has to be the
                    // smallest due to the swap.
                    largest = smallest;
                }
            }

            if (largest != right)
            {
                // If the largest element of the sublist is the rightmost element, then they need to be swapped.
                (list[right], list[largest]) = (list[largest], list[right]);
            }

            // Move the sublist search indices one in from the left and the right.
            left++;
            right--;
        }
    }

    /// <summary>
    /// Finds the smallest and largest list element indices.
    /// </summary>
    /// <typeparam name="T">The element type of the list.</typeparam>
    /// <param name="list">The list to search.</param>
    /// <param name="lowerbound">The lower bound.</param>
    /// <param name="upperbound">The upper bound.</param>
    private static async Task<(int, int)> FindSmallestAndLargest<T>(
        this IList<T> list,
        int lowerbound,
        int upperbound) where T : IComparable<T>
    {
        int smallest = (list[lowerbound].CompareTo(list[upperbound]) < 0) ? lowerbound : upperbound;
        int largest = (list[lowerbound].CompareTo(list[upperbound]) < 0) ? upperbound : lowerbound;

        lowerbound++;

        int loop = upperbound;

        while (loop > lowerbound)
        {
            loop--;
            if (list[loop].CompareTo(list[smallest]) < 0)
            {
                smallest = loop;
            }

            if (list[loop].CompareTo(list[largest]) > 0)
            {
                largest = loop;
            }
        }

        return (smallest, largest);
    }
}