Add recursive shuffling algorithm (#519)

Author: Jccqt

Algorithms.Tests/Shufflers/RecursiveShufflerTests.cs

@@ -0,0 +1,60 @@
+using Algorithms.Shufflers;
+using Algorithms.Tests.Helpers;
+using FluentAssertions;
+using NUnit.Framework;
+using System;
+
+namespace Algorithms.Tests.Shufflers
+{
+    public static class RecursiveShufflerTests
+    {
+        [Test]
+        public static void ArrayShuffled_NewArraySameSize(
+            [Random(10, 1000, 100, Distinct = true)]
+            int n)
+        {
+            // Arrange
+            var shuffler = new RecursiveShuffler<int>();
+            var (correctArray, testArray) = RandomHelper.GetArrays(n);
+
+            // Act
+            shuffler.Shuffle(testArray);
+
+            // Assert
+            testArray.Length.Should().Be(correctArray.Length);
+        }
+
+        [Test]
+        public static void ArrayShuffled_NewArraySameValues(
+            [Random(10, 1000, 100, Distinct = true)]
+            int n)
+        {
+            // Arrange
+            var shuffler = new RecursiveShuffler<int>();
+            var (correctArray, testArray) = RandomHelper.GetArrays(n);
+
+            // Act
+            shuffler.Shuffle(testArray);
+
+            // Assert
+            testArray.Should().BeEquivalentTo(correctArray);
+        }
+
+        [Test]
+        public static void ArrayShuffled_NewArraySameShuffle(
+            [Random(0, 1000, 2, Distinct = true)] int n,
+            [Random(1000, 10000, 5, Distinct = true)] int seed)
+        {
+            // Arrange
+            var shuffler = new RecursiveShuffler<int>();
+            var (correctArray, testArray) = RandomHelper.GetArrays(n);
+
+            // Act
+            shuffler.Shuffle(testArray, seed);
+            shuffler.Shuffle(correctArray, seed);
+
+            // Assert
+            correctArray.Should().BeEquivalentTo(testArray, options => options.WithStrictOrdering());
+        }
+    }
+}

Algorithms/Shufflers/RecursiveShuffler.cs

@@ -0,0 +1,46 @@
+using System;
+
+namespace Algorithms.Shufflers
+{
+    /// <summary>
+    ///     Recursive Shuffler is a recursive version of
+    ///     Fisher-Yates shuffle algorithm. This can only be used
+    ///     for educational purposes due to stack depth limits.
+    /// </summary>
+    /// <typeparam name="T">Type array input.</typeparam>
+    public class RecursiveShuffler<T> : IShuffler<T>
+    {
+        /// <summary>
+        /// This is the public overload method that calls the private overload method.
+        /// </summary>
+        /// <param name="array">Array to shuffle.</param>
+        /// <param name="seed">Random generator seed. Used to repeat the shuffle.</param>
+        public void Shuffle(T[] array, int? seed = null)
+        {
+            Shuffle(array, array.Length - 1, seed);
+        }
+
+        /// <summary>
+        /// First, it will check the length of the array on the base case.
+        /// Next, if there's still items left, it will shuffle the sub-array.
+        /// Lastly, it will randomly select index from 0 to number of items of the array
+        /// then swap the elements array[items] and array[index].
+        /// </summary>
+        /// <param name="array">Array to shuffle.</param>
+        /// <param name="items">Number of items in the array.</param>
+        /// <param name="seed">Random generator seed. Used to repeat the shuffle.</param>
+        private void Shuffle(T[] array, int items, int? seed)
+        {
+            if (items <= 0)
+            {
+                return;
+            }
+
+            Shuffle(array, items - 1, seed);
+            var random = seed is null ? new Random() : new Random(seed.Value);
+            int index = random.Next(items + 1);
+            (array[items], array[index]) = (array[index], array[items]);
+            (array[items], array[index]) = (array[index], array[items]);
+        }
+    }
+}

README.md

@@ -150,6 +150,7 @@ find more than one implementation for the same objective but using different alg
     * [Fisher-Yates Shuffler](./Algorithms/Shufflers/FisherYatesShuffler.cs)
     * [LINQ Shuffler](./Algorithms/Shufflers/LinqShuffler.cs)
     * [Naive Shuffler](./Algorithms/Shufflers/NaiveShuffler.cs)
+    * [Recursive Shuffler](./Algorithms/Shufflers/RecursiveShuffler.cs)
   * [Sequences](./Algorithms/Sequences)
     * [A000002 Kolakoski](./Algorithms/Sequences/KolakoskiSequence.cs)
     * [A000004 Zero](./Algorithms/Sequences/ZeroSequence.cs)