MedianOfThreeQuickSorter.cs

using System.Collections.Generic;

namespace Algorithms.Sorters.Comparison
{
    /// <summary>
    ///     Sorts arrays using quicksort (selecting median of three as a pivot).
    /// </summary>
    /// <typeparam name="T">Type of array element.</typeparam>
    public sealed class MedianOfThreeQuickSorter<T> : QuickSorter<T>
    {
        protected override T SelectPivot(T[] array, IComparer<T> comparer, int left, int right)
        {
            var leftPoint = array[left];
            var middlePoint = array[left + (right - left) / 2];
            var rightPoint = array[right];

            return FindMedian(comparer, leftPoint, middlePoint, rightPoint);
        }

        private static T FindMedian(IComparer<T> comparer, T a, T b, T c)
        {
            if (comparer.Compare(a, b) <= 0)
            {
                // a <= b <= c
                if (comparer.Compare(b, c) <= 0)
                {
                    return b;
                }

                // a <= c < b
                if (comparer.Compare(a, c) <= 0)
                {
                    return c;
                }

                // c < a <= b
                return a;
            }

            // a > b >= c
            if (comparer.Compare(b, c) >= 0)
            {
                return b;
            }

            // a >= c > b
            if (comparer.Compare(a, c) >= 0)
            {
                return c;
            }

            // c > a > b
            return a;
        }
    }
}