Add K-Nearest Neighbor (KNN) implementation and Test

Algorithms.Tests/MachineLearning/KNearestNeighborsTests.cs

@@ -0,0 +1,86 @@
+using NUnit.Framework;
+using Algorithms.MachineLearning;
+using System;
+
+namespace Algorithms.Tests.MachineLearning;
+
+[TestFixture]
+public class KNearestNeighborsTests
+{
+    [Test]
+    public void Constructor_InvalidK_ThrowsException()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => new KNearestNeighbors<string>(0));
+    }
+
+    [Test]
+    public void AddSample_NullFeatures_ThrowsException()
+    {
+        var knn = new KNearestNeighbors<string>(3);
+        double[]? features = null;
+        Assert.Throws<ArgumentNullException>(() => knn.AddSample(features!, "A"));
+    }
+
+    [Test]
+    public void Predict_NoTrainingData_ThrowsException()
+    {
+        var knn = new KNearestNeighbors<string>(1);
+        Assert.Throws<InvalidOperationException>(() => knn.Predict(new[] { 1.0 }));
+    }
+
+    [Test]
+    public void Predict_NullFeatures_ThrowsException()
+    {
+        var knn = new KNearestNeighbors<string>(1);
+        knn.AddSample(new[] { 1.0 }, "A");
+        double[]? features = null;
+        Assert.Throws<ArgumentNullException>(() => knn.Predict(features!));
+    }
+
+    [Test]
+    public void EuclideanDistance_DifferentLengths_ThrowsException()
+    {
+        Assert.Throws<ArgumentException>(() => KNearestNeighbors<string>.EuclideanDistance(new[] { 1.0 }, new[] { 1.0, 2.0 }));
+    }
+
+    [Test]
+    public void EuclideanDistance_CorrectResult()
+    {
+        double[] a = { 1.0, 2.0 };
+        double[] b = { 4.0, 6.0 };
+        double expected = 5.0;
+        double actual = KNearestNeighbors<string>.EuclideanDistance(a, b);
+        Assert.That(actual, Is.EqualTo(expected).Within(1e-9));
+    }
+
+    [Test]
+    public void Predict_SingleNeighbor_CorrectLabel()
+    {
+        var knn = new KNearestNeighbors<string>(1);
+        knn.AddSample(new[] { 1.0, 2.0 }, "A");
+        knn.AddSample(new[] { 3.0, 4.0 }, "B");
+        var label = knn.Predict(new[] { 1.1, 2.1 });
+        Assert.That(label, Is.EqualTo("A"));
+    }
+
+    [Test]
+    public void Predict_MajorityVote_CorrectLabel()
+    {
+        var knn = new KNearestNeighbors<string>(3);
+        knn.AddSample(new[] { 0.0, 0.0 }, "A");
+        knn.AddSample(new[] { 0.1, 0.1 }, "A");
+        knn.AddSample(new[] { 1.0, 1.0 }, "B");
+        var label = knn.Predict(new[] { 0.05, 0.05 });
+        Assert.That(label, Is.EqualTo("A"));
+    }
+
+    [Test]
+    public void Predict_TieBreaker_ReturnsConsistentLabel()
+    {
+        var knn = new KNearestNeighbors<string>(2);
+        knn.AddSample(new[] { 0.0, 0.0 }, "A");
+        knn.AddSample(new[] { 1.0, 1.0 }, "B");
+        var label = knn.Predict(new[] { 0.5, 0.5 });
+        Assert.That(label, Is.EqualTo("A"));
+    }
+}

Algorithms/MachineLearning/KNearestNeighbors.cs

@@ -0,0 +1,108 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Algorithms.MachineLearning;
+
+/// <summary>
+/// K-Nearest Neighbors (KNN) classifier implementation.
+/// This algorithm classifies data points based on the majority label of their k nearest neighbors.
+/// </summary>
+/// <typeparam name="TLabel">
+/// The type of the label used for classification. This can be any type that represents the class or category of a sample.
+/// </typeparam>
+public class KNearestNeighbors<TLabel>
+{
+    private readonly List<(double[] Features, TLabel Label)> trainingData = new();
+    private readonly int k;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="KNearestNeighbors{TLabel}"/> classifier.
+    /// </summary>
+    /// <param name="k">Number of neighbors to consider for classification.</param>
+    /// <exception cref="ArgumentOutOfRangeException">Thrown if k is less than 1.</exception>
+    public KNearestNeighbors(int k)
+    {
+        if (k < 1)
+        {
+            throw new ArgumentOutOfRangeException(nameof(k), "k must be at least 1.");
+        }
+
+        this.k = k;
+    }
+
+    /// <summary>
+    /// Calculates the Euclidean distance between two feature vectors.
+    /// </summary>
+    /// <param name="a">First feature vector.</param>
+    /// <param name="b">Second feature vector.</param>
+    /// <returns>Euclidean distance.</returns>
+    /// <exception cref="ArgumentException">Thrown if vectors are of different lengths.</exception>
+    public static double EuclideanDistance(double[] a, double[] b)
+    {
+        if (a.Length != b.Length)
+        {
+            throw new ArgumentException("Feature vectors must be of the same length.");
+        }
+
+        double sum = 0;
+        for (int i = 0; i < a.Length; i++)
+        {
+            double diff = a[i] - b[i];
+            sum += diff * diff;
+        }
+
+        return Math.Sqrt(sum);
+    }
+
+    /// <summary>
+    /// Adds a training sample to the classifier.
+    /// </summary>
+    /// <param name="features">Feature vector of the sample.</param>
+    /// <param name="label">Label of the sample.</param>
+    public void AddSample(double[] features, TLabel label)
+    {
+        if (features == null)
+        {
+            throw new ArgumentNullException(nameof(features));
+        }
+
+        trainingData.Add((features, label));
+    }
+
+    /// <summary>
+    /// Predicts the label for a given feature vector using the KNN algorithm.
+    /// </summary>
+    /// <param name="features">Feature vector to classify.</param>
+    /// <returns>Predicted label.</returns>
+    /// <exception cref="InvalidOperationException">Thrown if there is no training data.</exception>
+    public TLabel Predict(double[] features)
+    {
+        if (trainingData.Count == 0)
+        {
+            throw new InvalidOperationException("No training data available.");
+        }
+
+        if (features == null)
+        {
+            throw new ArgumentNullException(nameof(features));
+        }
+
+        // Compute distances to all training samples
+        var distances = trainingData
+            .Select(td => (Label: td.Label, Distance: EuclideanDistance(features, td.Features)))
+            .OrderBy(x => x.Distance)
+            .Take(k)
+            .ToList();
+
+        // Majority vote
+        var labelCounts = distances
+            .GroupBy(x => x.Label)
+            .Select(g => new { Label = g.Key, Count = g.Count() })
+            .OrderByDescending(x => x.Count)
+            .ThenBy(x => x.Label?.GetHashCode() ?? 0)
+            .ToList();
+
+        return labelCounts.First().Label;
+    }
+}

README.md

@@ -108,6 +108,7 @@ find more than one implementation for the same objective but using different alg
     * [CollaborativeFiltering](./Algorithms/RecommenderSystem/CollaborativeFiltering)
   * [Machine Learning](./Algorithms/MachineLearning)
     * [Linear Regression](./Algorithms/MachineLearning/LinearRegression.cs)
+    * [K-Nearest Neighbors](./Algorithms/MachineLearning/KNearestNeighbors.cs)
     * [Logistic Regression](./Algorithms/MachineLearning/LogisticRegression.cs)
   * [Searches](./Algorithms/Search)
     * [A-Star](./Algorithms/Search/AStar/)