CountingSorter.cs

using System;
using System.Linq;

namespace Algorithms.Sorters.Integer
{
    /// <summary>
    ///     Counting sort is an algorithm for sorting a collection of objects according to keys that are small integers; that
    ///     is, it is an integer sorting algorithm. It operates by counting the number of objects that have each distinct key
    ///     value, and using arithmetic on those counts to determine the positions of each key value in the output sequence.
    ///     Its running time is linear in the number of items and the difference between the maximum and minimum key values, so
    ///     it is only suitable for direct use in situations where the variation in keys is not significantly greater than the
    ///     number of items. However, it is often used as a subroutine in another sorting algorithm, radix sort, that can
    ///     handle larger keys more efficiently.
    /// </summary>
    public class CountingSorter : IIntegerSorter
    {
        /// <summary>
        ///     <para>
        ///         Sorts input array using counting sort algorithm.
        ///     </para>
        ///     <para>
        ///         Time complexity: O(n+k), where k is the range of the non-negative key values.
        ///     </para>
        ///     <para>
        ///         Space complexity: O(n+k), where k is the range of the non-negative key values.
        ///     </para>
        /// </summary>
        /// <param name="array">Input array.</param>
        public void Sort(int[] array)
        {
            if (array.Length == 0)
            {
                return;
            }

            var max = array.Max();
            var min = array.Min();
            var count = new int[max - min + 1];
            var output = new int[array.Length];
            for (var i = 0; i < array.Length; i++)
            {
                count[array[i] - min]++;
            }

            for (var i = 1; i < count.Length; i++)
            {
                count[i] += count[i - 1];
            }

            for (var i = array.Length - 1; i >= 0; i--)
            {
                output[count[array[i] - min] - 1] = array[i];
                count[array[i] - min]--;
            }

            Array.Copy(output, array, array.Length);
        }
    }
}