Homework 1 - [Burrows-Wheeler Transform]

BurrowsWheelerTransform/BurrowsWheelerTransform.Tests/BurrowsWheelerTransform.Tests.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="NUnit" Version="4.2.2" />
+    <PackageReference Include="NUnit.Analyzers" Version="4.4.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="NUnit.Framework" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../BurrowsWheelerTransform/BurrowsWheelerTransform.csproj"/>
+  </ItemGroup>
+
+</Project>

BurrowsWheelerTransform/BurrowsWheelerTransform.Tests/GlobalSuppressions.cs

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+// This file is used by Code Analysis to maintain SuppressMessage
+// attributes that are applied to this project.
+// Project-level suppressions either have no target or are given
+// a specific target and scoped to a namespace, type, member, etc.
+
+using System.Diagnostics.CodeAnalysis;
+
+[assembly: SuppressMessage("StyleCop.CSharp.DocumentationRules", "SA1600:Elements should be documented", Justification = "This is tests project")]

BurrowsWheelerTransform/BurrowsWheelerTransform.Tests/TransformResultTests.cs

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+namespace BurrowsWheelerTransform.Tests;
+
+public class TransformResultTests
+{
+    [Test]
+    public void TransformResultConstructor_ShouldNotThrow()
+    {
+        Assert.DoesNotThrow(() => new TransformResult(string.Empty, -1));
+        Assert.DoesNotThrow(() => new TransformResult("A", 0));
+        Assert.DoesNotThrow(() => new TransformResult("ABCDEF", 3));
+        Assert.DoesNotThrow(() => new TransformResult("QWERTY", 5));
+    }
+
+    [Test]
+    public void TransformResultConstructor_ShouldThrow()
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(() => new TransformResult(string.Empty, 0));
+        Assert.Throws<ArgumentOutOfRangeException>(() => new TransformResult("ABCD", -1));
+        Assert.Throws<ArgumentOutOfRangeException>(() => new TransformResult("ABCD", 4));
+    }
+}

BurrowsWheelerTransform/BurrowsWheelerTransform.Tests/TransformTests.cs

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+namespace BurrowsWheelerTransform.Tests;
+
+public class TransformTests
+{
+    private static readonly List<string> TestData =
+    [
+        string.Empty,
+
+        "A",
+        "BB",
+        "CCCCCC",
+        "ABACABA",
+        "ABABABABAB",
+
+        ..GetRandomStrings()
+    ];
+
+    [Test]
+    public void InverseTransform_ShouldBe_SameAs_Input([ValueSource(nameof(TestData))] string input)
+    {
+        var result = Transform.ForwardTransform(input);
+        var reconstructed = Transform.InverseTransform(result);
+        Assert.That(input.SequenceEqual(reconstructed), Is.True);
+    }
+
+    private static IEnumerable<string> GetRandomStrings()
+    {
+        int seed = 1743658243;
+        var random = new Random(seed);
+
+        int steps = 16;
+        int length = 256;
+        var buffer = new char[length];
+
+        for (int i = 0; i < steps; i++)
+        {
+            for (int j = 0; j < length; j++)
+            {
+                buffer[j] = (char)random.Next(' ', '~' + 1);
+            }
+
+            yield return new(buffer);
+        }
+    }
+}

BurrowsWheelerTransform/BurrowsWheelerTransform.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.0.31903.59
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "BurrowsWheelerTransform", "BurrowsWheelerTransform\BurrowsWheelerTransform.csproj", "{1DA138D9-04B6-498B-9C33-05E9684F1D5C}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "BurrowsWheelerTransform.Tests", "BurrowsWheelerTransform.Tests\BurrowsWheelerTransform.Tests.csproj", "{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
+		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|x64.Build.0 = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Debug|x86.Build.0 = Debug|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|Any CPU.Build.0 = Release|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|x64.ActiveCfg = Release|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|x64.Build.0 = Release|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|x86.ActiveCfg = Release|Any CPU
+		{1DA138D9-04B6-498B-9C33-05E9684F1D5C}.Release|x86.Build.0 = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|x64.Build.0 = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Debug|x86.Build.0 = Debug|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|Any CPU.Build.0 = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|x64.ActiveCfg = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|x64.Build.0 = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|x86.ActiveCfg = Release|Any CPU
+		{FDF7CD5C-DC17-4A89-B7B8-12B1A75EF86D}.Release|x86.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+EndGlobal

BurrowsWheelerTransform/BurrowsWheelerTransform/BurrowsWheelerTransform.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Library</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

BurrowsWheelerTransform/BurrowsWheelerTransform/Transform.cs

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+namespace BurrowsWheelerTransform;
+
+using System.Diagnostics;
+
+/// <summary>
+/// Burrows-Wheeler transform implementation.
+/// </summary>
+public static class Transform
+{
+    /// <summary>
+    /// Transforms given string using Burrows-Wheeler algorithm.
+    /// </summary>
+    /// <param name="input">Input string.</param>
+    /// <returns>Result of transformation.</returns>
+    public static TransformResult ForwardTransform(string input)
+    {
+        int length = input.Length;
+
+        if (length == 0)
+        {
+            return new(string.Empty, -1);
+        }
+
+        Span<int> offsets = stackalloc int[length];
+        for (int i = 0; i < length; i++)
+        {
+            offsets[i] = i;
+        }
+
+        int Compare(int x, int y)
+        {
+            for (int i = 0; i < length; i++)
+            {
+                int compare = input[(i + x) % length] - input[(i + y) % length];
+                if (compare != 0)
+                {
+                    return compare;
+                }
+            }
+
+            return 0;
+        }
+
+        offsets.Sort(Compare);
+
+        int? identityPosition = null;
+        Span<char> result = stackalloc char[length];
+        for (int i = 0; i < length; i++)
+        {
+            if (offsets[i] == 0)
+            {
+                identityPosition = i;
+            }
+
+            result[i] = input[(offsets[i] + length - 1) % length];
+        }
+
+        Debug.Assert(identityPosition.HasValue, "Identity position not found");
+
+        return new(new(result), identityPosition.Value);
+    }
+
+    /// <summary>
+    /// Reconstructs string transformed with Burrows-Wheeler algorithm.
+    /// </summary>
+    /// <param name="result">Transformed string.</param>
+    /// <returns>Reconstructed string.</returns>
+    public static string InverseTransform(TransformResult result)
+    {
+        if (result.IdentityIndex == -1)
+        {
+            return string.Empty;
+        }
+
+        int length = result.Value.Length;
+
+        Span<int> appearances = stackalloc int[length];
+        var lastAppearances = new Dictionary<char, int>();
+        var charCounter = new SortedDictionary<char, int>();
+
+        for (int i = 0; i < length; i++)
+        {
+            char currentChar = result.Value[i];
+
+            if (!charCounter.TryGetValue(currentChar, out int count))
+            {
+                charCounter[currentChar] = 0;
+            }
+
+            charCounter[currentChar]++;
+
+            appearances[i] =
+                lastAppearances.TryGetValue(currentChar, out int lastIndex)
+                ? appearances[lastIndex] + 1
+                : 0;
+
+            lastAppearances[currentChar] = i;
+        }
+
+        var lesserCharsCounter = new Dictionary<char, int>();
+        int previousCount = 0;
+        foreach (var (character, count) in charCounter)
+        {
+            lesserCharsCounter[character] = previousCount;
+            previousCount += count;
+        }
+
+        Span<char> reconstructed = stackalloc char[length];
+
+        int lastIdentityIndex = result.IdentityIndex;
+        char lastCharacter = result.Value[lastIdentityIndex];
+        reconstructed[^1] = result.Value[result.IdentityIndex];
+
+        for (int i = 1; i < length; i++)
+        {
+            lastIdentityIndex = appearances[lastIdentityIndex] + lesserCharsCounter[lastCharacter];
+            lastCharacter = result.Value[lastIdentityIndex];
+            reconstructed[^(i + 1)] = lastCharacter;
+        }
+
+        return new(reconstructed);
+    }
+}

BurrowsWheelerTransform/BurrowsWheelerTransform/TransformResult.cs

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+namespace BurrowsWheelerTransform;
+
+/// <summary>
+/// Result of Burrows-Wheeler Transform.
+/// </summary>
+public readonly struct TransformResult
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TransformResult"/> struct.
+    /// </summary>
+    /// <param name="value">Transformed string.</param>
+    /// <param name="identityIndex">Index that is used to reconstruct string.</param>
+    /// <exception cref="ArgumentOutOfRangeException">
+    /// <paramref name="identityIndex"/> is out of range (less than 0 or greater or equal to <paramref name="identityIndex"/>.Length)
+    /// or not equal to -1 if <paramref name="identityIndex"/> is <see cref="string.Empty"/>.
+    /// </exception>
+    public TransformResult(string value, int identityIndex)
+    {
+        if (value.Length == 0)
+        {
+            if (identityIndex != -1)
+            {
+                throw new ArgumentOutOfRangeException(nameof(identityIndex), "Identity index of an empty string must be equal to -1");
+            }
+        }
+        else
+        {
+            ArgumentOutOfRangeException.ThrowIfNegative(identityIndex, nameof(identityIndex));
+            ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(identityIndex, value.Length, nameof(identityIndex));
+        }
+
+        Value = value;
+        IdentityIndex = identityIndex;
+    }
+
+    /// <summary>
+    /// Gets transformed string.
+    /// </summary>
+    public string Value { get; }
+
+    /// <summary>
+    /// Gets identity index.
+    /// </summary>
+    public int IdentityIndex { get; }
+}