PancakeSorter.cs

using System.Collections.Generic;

namespace Algorithms.Sorters.Comparison
{
    /// <summary>
    ///     Class that implements pancake sort algorithm.
    /// </summary>
    /// <typeparam name="T">Type of array element.</typeparam>
    public class PancakeSorter<T> : IComparisonSorter<T>
    {
        /// <summary>
        ///     Sorts array using specified comparer,
        ///     internal, in-place, stable,
        ///     time complexity: O(n^2),
        ///     space complexity: O(1),
        ///     where n - array length.
        /// </summary>
        /// <param name="array">Array to sort.</param>
        /// <param name="comparer">Compares elements.</param>
        public void Sort(T[] array, IComparer<T> comparer)
        {
            var n = array.Length;

            // Start from the complete array and one by one
            // reduce current size by one
            for (var currSize = n; currSize > 1; --currSize)
            {
                // Find index of the maximum element in
                // array[0..curr_size-1]
                var mi = FindMax(array, currSize, comparer);

                // Move the maximum element to end of current array
                // if it's not already at  the end
                if (mi != currSize - 1)
                {
                    // To move to the end, first move maximum
                    // number to beginning
                    Flip(array, mi);

                    // Now move the maximum number to end by
                    // reversing current array
                    Flip(array, currSize - 1);
                }
            }
        }

        // Reverses array[0..i]
        private void Flip(T[] array, int i)
        {
            T temp;
            var start = 0;
            while (start < i)
            {
                temp = array[start];
                array[start] = array[i];
                array[i] = temp;
                start++;
                i--;
            }
        }

        // Returns index of the maximum element
        // in array[0..n-1]
        private int FindMax(T[] array, int n, IComparer<T> comparer)
        {
            var mi = 0;
            for (var i = 0; i < n; i++)
            {
                if (comparer.Compare(array[i], array[mi]) == 1)
                {
                    mi = i;
                }
            }

            return mi;
        }
    }
}