First version added.

Huffman.Tree.Tests/Huffman.Tree.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+	<TargetFramework>net6.0</TargetFramework>
+	<TargetFrameworks>net6.0;net7.0</TargetFrameworks>
+	<ImplicitUsings>enable</ImplicitUsings>
+	<Nullable>enable</Nullable>
+
+	<IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+	<PackageReference Include="AutoFixture" Version="4.17.0" />
+	<PackageReference Include="BenchmarkDotNet" Version="0.13.3" />
+	<PackageReference Include="FluentAssertions" Version="6.8.0" />
+	<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
+	<PackageReference Include="MSTest.TestAdapter" Version="3.0.2" />
+	<PackageReference Include="MSTest.TestFramework" Version="3.0.2" />
+	<PackageReference Include="coverlet.collector" Version="3.2.0">
+	  <PrivateAssets>all</PrivateAssets>
+	  <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+	</PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+	<ProjectReference Include="..\Huffman.Tree\Huffman.Tree.csproj" />
+  </ItemGroup>
+
+</Project>

Huffman.Tree.Tests/HuffmanTreeTests.cs

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+using AutoFixture;
+using BenchmarkDotNet.Attributes;
+using FluentAssertions;
+using MoreLinq;
+
+namespace Huffman.Tree.Tests
+{
+	[TestClass]
+	[MemoryDiagnoser]
+	public class HuffmanTreeTests
+	{
+		public readonly record struct TestCase
+		{
+			public (string token, long frequency)[] Tokens { get; init; }
+			public (string token, int[] bits)[] Expected { get; init; }
+		}
+
+		private static IEnumerable<TestCase> TestCases => EnumerateTestCases().SelectMany(x => x.OfType<TestCase>());
+
+		private IEnumerable<HuffmanToken<string>>? _codes;
+		private (string token, long frequency)[] _tokens;
+
+		public HuffmanTreeTests()
+		{
+			_tokens = TestCases.First().Tokens;
+		}
+
+		[TestMethod]
+		[DynamicData(nameof(EnumerateTestCases), DynamicDataSourceType.Method)]
+		public void BuildTreeShouldSuccess(TestCase testCase)
+		{
+			// This is to overcome the requirement for BuildTree() to be argument- & deferred-result-free. It's required by BenchmarkDotNet.
+			_tokens = testCase.Tokens;
+			BuildTree();
+			PrintCodesToOutput();
+			ValidateCodes(testCase.Expected);
+		}
+
+		private void ValidateCodes((string token, int[] bits)[] expected)
+		{
+			var expectedBits = expected.Select(x => (x.token, x.bits.Select(bit => Convert.ToBoolean(bit))));
+			_ = _codes.Should().NotBeNull();
+			_ = _codes!.Select(x => (x.Token, x.Code))
+				 .Should()
+				 .NotBeNull().And
+				 .BeEquivalentTo(expectedBits);
+		}
+
+		private void PrintCodesToOutput()
+		{
+			foreach (var node in _codes!)
+			{
+				string bits = node.Code!.AsString();
+				Console.WriteLine($"{node.Token} => {bits}");
+			}
+		}
+
+		public static IEnumerable<object[]> EnumerateTestCases()
+		{
+			yield return new object[]
+			{
+				new TestCase
+				{
+					// test case
+					Tokens = new[] { ("a", 10L), ("b", 5L), ("c", 2L), ("d", 14L), ("e", 15L), (";", long.MaxValue) },
+					// expected result
+					Expected = new[] { (";", new[] { 1 }), ("a", new[] { 0, 0, 1 }), ("b", new[] { 0, 0, 0, 1 }), ("c", new[] { 0, 0, 0, 0 }), ("d", new[] { 0, 1, 0 }), ("e", new[] { 0, 1, 1 }) }
+				}
+			};
+		}
+
+		[Benchmark]
+		public void BuildTree()
+		{
+			_codes = _tokens?.AsHuffmanTree().Nodes.OfType<HuffmanToken<string>>();
+			// We keep this assertions here because we want to build all the codes. For this we need to access each of them.
+			_ = _codes.Should().NotBeNull().And.AllSatisfy(node => node.Code!.AsString().Should().NotBeNull().And.NotBe(string.Empty));
+		}
+
+		[Benchmark]
+		[TestMethod]
+		public void BuildTree100()
+		{
+			var fixture = new Fixture();
+			var collection = fixture.CreateMany<(string token, long frequency)>(100);
+			_tokens = collection.ToArray();
+			_codes = _tokens?.AsHuffmanTree().Nodes.OfType<HuffmanToken<string>>();
+			_codes.ForEach(x => Console.WriteLine($"{x.Token} => {x.Code.AsString()}"));
+			// We keep this assertions here because we want to build all the codes. For this we need to access each of them.
+			_ = _codes.Should().NotBeNull().And.AllSatisfy(node => node.Code!.AsString().Should().NotBeNull().And.NotBe(string.Empty));
+		}
+	}
+}

Huffman.Tree.Tests/Usings.cs

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+global using Microsoft.VisualStudio.TestTools.UnitTesting;