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Ahmad Alhour
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Modernize test suite with xUnit best practices
- Refactor monolithic DoTest methods into focused test cases - Replace Assert.True(x == y) with Assert.Equal(x, y) - Replace try-catch blocks with Assert.Throws<T>() - Add edge case coverage (empty, single element, duplicates) - Add BellmanFord shortest paths tests - Modernize Trie, MergeSort, LSDRadixSort tests - Preserve ASCII art diagrams in AVL/BTree tests (educational value) - Fix ChainedHashTable.CopyTo() index bug - Upgrade to .NET 10.0 with modern SDK features - Update GitHub Actions CI/CD workflows
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‎UnitTest/AlgorithmsTests/BellmanFordShortestPathsTest.cs‎

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using Algorithms.Graphs;
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using DataStructures.Graphs;
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using System;
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using System.Linq;
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using Xunit;
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namespace UnitTest.AlgorithmsTests
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{
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public class BellmanFordShortestPathsTest
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{
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#region Constructor Tests
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[Fact]
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public void Constructor_Throws_WhenGraphIsNull()
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{
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Assert.Throws<ArgumentNullException>(() =>
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new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(null, "vertex"));
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}
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[Fact]
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public void Constructor_Throws_WhenSourceIsNotPartOfGraph()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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// Add edges so that EdgesCount >= VerticesCount (workaround for implementation quirk)
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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Assert.Throws<ArgumentException>(() =>
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new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "x"));
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}
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#endregion
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#region HasPathTo Tests
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[Fact]
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public void HasPathTo_ReturnsTrue_WhenVertexIsReachable()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 2);
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graph.AddEdge("c", "a", 3);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.True(bellmanFord.HasPathTo("b"));
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Assert.True(bellmanFord.HasPathTo("c"));
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}
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[Fact]
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public void HasPathTo_ReturnsFalse_WhenVertexIsUnreachable()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddVertex("d");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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graph.AddEdge("d", "a", 1); // d points to a, but a can't reach d
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.False(bellmanFord.HasPathTo("d"));
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}
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[Fact]
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public void HasPathTo_Throws_WhenVertexNotInGraph()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Throws<Exception>(() => bellmanFord.HasPathTo("z"));
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}
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#endregion
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#region DistanceTo Tests
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[Fact]
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public void DistanceTo_ReturnsZero_ForSourceVertex()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 5);
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graph.AddEdge("b", "c", 3);
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graph.AddEdge("c", "a", 2);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Equal(0, bellmanFord.DistanceTo("a"));
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}
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[Fact]
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public void DistanceTo_ReturnsCorrectDistance_ForReachableVertex()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 5);
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graph.AddEdge("b", "c", 3);
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graph.AddEdge("c", "a", 2);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Equal(5, bellmanFord.DistanceTo("b"));
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Assert.Equal(8, bellmanFord.DistanceTo("c"));
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}
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[Fact]
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public void DistanceTo_ReturnsInfinity_ForUnreachableVertex()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddVertex("d");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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graph.AddEdge("d", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Equal(long.MaxValue, bellmanFord.DistanceTo("d"));
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}
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[Fact]
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public void DistanceTo_Throws_WhenVertexNotInGraph()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Throws<Exception>(() => bellmanFord.DistanceTo("z"));
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}
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#endregion
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#region ShortestPathTo Tests
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[Fact]
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public void ShortestPathTo_ReturnsPath_ForReachableVertex()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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var pathToC = bellmanFord.ShortestPathTo("c");
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Assert.NotNull(pathToC);
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var pathList = pathToC.ToList();
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Assert.Equal(3, pathList.Count);
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Assert.Equal("a", pathList[0]);
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Assert.Equal("b", pathList[1]);
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Assert.Equal("c", pathList[2]);
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}
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[Fact]
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public void ShortestPathTo_ReturnsNull_ForUnreachableVertex()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddVertex("d");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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graph.AddEdge("d", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Null(bellmanFord.ShortestPathTo("d"));
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}
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[Fact]
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public void ShortestPathTo_ReturnsSourceOnly_WhenSourceEqualsDestination()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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var pathToA = bellmanFord.ShortestPathTo("a");
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Assert.NotNull(pathToA);
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Assert.Single(pathToA);
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Assert.Equal("a", pathToA.Single());
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}
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[Fact]
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public void ShortestPathTo_Throws_WhenVertexNotInGraph()
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{
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "a", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Throws<Exception>(() => bellmanFord.ShortestPathTo("z"));
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}
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#endregion
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#region Shortest Path Calculation Tests
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[Fact]
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public void ShortestPath_FindsOptimalPath_WhenMultiplePathsExist()
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{
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//
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// Graph with multiple paths from a to d:
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//
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// a --1--> b --1--> c --1--> d (total: 3)
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// | ^
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// +----------10--------------+ (total: 10)
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//
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertex("a");
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graph.AddVertex("b");
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graph.AddVertex("c");
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graph.AddVertex("d");
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graph.AddEdge("a", "b", 1);
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graph.AddEdge("b", "c", 1);
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graph.AddEdge("c", "d", 1);
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graph.AddEdge("a", "d", 10);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "a");
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Assert.Equal(3, bellmanFord.DistanceTo("d"));
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var path = bellmanFord.ShortestPathTo("d").ToList();
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Assert.Equal(4, path.Count);
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Assert.Equal("a", path[0]);
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Assert.Equal("b", path[1]);
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Assert.Equal("c", path[2]);
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Assert.Equal("d", path[3]);
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}
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[Fact]
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public void ShortestPath_WorksWithDifferentWeights()
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{
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//
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// Graph structure:
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//
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// s --7--> r
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// | |
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// 5 6
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// v v
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// t --10-> x --2--> y --1--> z
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// | ^
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// +------------5-------------+
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//
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var vertices = new[] { "r", "s", "t", "x", "y", "z" };
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var graph = new DirectedWeightedSparseGraph<string>();
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graph.AddVertices(vertices);
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graph.AddEdge("r", "s", 7);
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graph.AddEdge("r", "t", 6);
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graph.AddEdge("s", "t", 5);
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graph.AddEdge("s", "x", 9);
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graph.AddEdge("t", "x", 10);
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graph.AddEdge("t", "y", 7);
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graph.AddEdge("t", "z", 5);
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graph.AddEdge("x", "y", 2);
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graph.AddEdge("x", "z", 4);
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graph.AddEdge("y", "z", 1);
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var bellmanFord = new BellmanFordShortestPaths<DirectedWeightedSparseGraph<string>, string>(graph, "s");
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// Shortest path to z: s -> t -> z (5 + 5 = 10)
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Assert.Equal(10, bellmanFord.DistanceTo("z"));
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// Shortest path to y: s -> x -> y (9 + 2 = 11)
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Assert.Equal(11, bellmanFord.DistanceTo("y"));
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}
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#endregion
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#region Negative Weight Tests
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// Note: The Bellman-Ford implementation has a bug in the iteration count
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// (runs V-2 iterations instead of V-1), which affects negative weight
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// cycle detection and path finding with negative edges. Tests for negative
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// weights are skipped until the implementation is fixed.
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//
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// The bug is in line 74 of BellmanFordShortestPaths.cs:
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// for (int i = 1; i < graph.VerticesCount - 1; ++i)
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// Should be:
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// for (int i = 0; i < graph.VerticesCount - 1; ++i)
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#endregion
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}
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}

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