// Copyright (c) StellaOps. All rights reserved.
// Licensed under BUSL-1.1. See LICENSE in the project root.

using FluentAssertions;
using StellaOps.Scanner.ChangeTrace.ByteDiff;

namespace StellaOps.Scanner.ChangeTrace.Tests.ByteDiff;

/// <summary>
/// Tests for ByteLevelDiffer.
/// </summary>
[Trait("Category", "Unit")]
public sealed class ByteLevelDifferTests
{
    private readonly ByteLevelDiffer _differ = new();

    [Fact]
    public async Task CompareAsync_IdenticalStreams_ReturnsNoDeltas()
    {
        // Arrange
        var data = CreateTestData(4096);
        using var fromStream = new MemoryStream(data);
        using var toStream = new MemoryStream(data);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().BeEmpty();
    }

    [Fact]
    public async Task CompareAsync_DifferentStreams_ReturnsDeltas()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99); // Different seed = different data
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().NotBeEmpty();
    }

    [Fact]
    public async Task CompareAsync_PartialChange_DetectsChangedWindow()
    {
        // Arrange
        var fromData = CreateTestData(8192);
        var toData = (byte[])fromData.Clone();
        // Modify only the second window (bytes 2048-4095)
        for (var i = 2048; i < 4096; i++)
        {
            toData[i] = (byte)(toData[i] ^ 0xFF);
        }

        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().HaveCount(1);
        deltas[0].Offset.Should().Be(2048);
        deltas[0].Size.Should().Be(2048);
    }

    [Fact]
    public async Task CompareAsync_TruncatedToFile_ReportsDeltas()
    {
        // Arrange
        var fromData = CreateTestData(8192);
        var toData = fromData[..4096]; // Truncated to half
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().NotBeEmpty();
        deltas.Should().Contain(d => d.ToHash == "truncated");
    }

    [Fact]
    public async Task CompareAsync_WithOptions_UsesWindowSize()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = (byte[])fromData.Clone();
        // Modify only first 512 bytes
        for (var i = 0; i < 512; i++)
        {
            toData[i] = (byte)(toData[i] ^ 0xFF);
        }

        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        var options = new ByteDiffOptions
        {
            WindowSize = 1024,
            StepSize = 1024
        };

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream, options);

        // Assert
        deltas.Should().HaveCount(1);
        deltas[0].Size.Should().Be(1024); // Reports full window even though only 512 changed
    }

    [Fact]
    public async Task CompareAsync_WithContext_IncludesContextDescription()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99);
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        var options = new ByteDiffOptions
        {
            IncludeContext = true
        };

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream, options);

        // Assert
        // Context is included when there are multiple consecutive changes
        deltas.Should().NotBeEmpty();
    }

    [Fact]
    public async Task CompareAsync_LargeFiles_UsesSampling()
    {
        // Arrange
        var size = 11 * 1024 * 1024; // 11MB - over default 10MB limit
        var fromData = new byte[size];
        var toData = new byte[size];
        new Random(42).NextBytes(fromData);
        new Random(43).NextBytes(toData);

        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        var options = new ByteDiffOptions
        {
            MaxFileSize = 10 * 1024 * 1024
        };

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream, options);

        // Assert - Should complete without memory issues and return sampled deltas
        deltas.Should().NotBeEmpty();
    }

    [Fact]
    public async Task CompareAsync_DisableSectionAnalysis_UsesFullBinaryComparison()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99);
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        var options = new ByteDiffOptions
        {
            AnalyzeBySections = false
        };

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream, options);

        // Assert
        deltas.Should().NotBeEmpty();
        deltas.Should().AllSatisfy(d => d.Section.Should().BeNull());
    }

    [Fact]
    public async Task CompareAsync_CancellationToken_Respected()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99);
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);
        var cts = new CancellationTokenSource();
        cts.Cancel();

        // Act & Assert
        var action = async () => await _differ.CompareAsync(fromStream, toStream, ct: cts.Token);
        await action.Should().ThrowAsync<OperationCanceledException>();
    }

    [Fact]
    public async Task CompareAsync_NullFromStream_ThrowsArgumentNullException()
    {
        // Arrange
        using var toStream = new MemoryStream();

        // Act & Assert
        var action = async () => await _differ.CompareAsync(null!, toStream);
        await action.Should().ThrowAsync<ArgumentNullException>();
    }

    [Fact]
    public async Task CompareAsync_NullToStream_ThrowsArgumentNullException()
    {
        // Arrange
        using var fromStream = new MemoryStream();

        // Act & Assert
        var action = async () => await _differ.CompareAsync(fromStream, null!);
        await action.Should().ThrowAsync<ArgumentNullException>();
    }

    [Fact]
    public async Task CompareAsync_DeltaHasCorrectScope()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99);
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().AllSatisfy(d => d.Scope.Should().Be("byte"));
    }

    [Fact]
    public async Task CompareAsync_DeltasAreDeterministic()
    {
        // Arrange
        var fromData = CreateTestData(8192);
        var toData = CreateTestData(8192, seed: 99);

        // Act - Compare twice
        using var fromStream1 = new MemoryStream(fromData);
        using var toStream1 = new MemoryStream(toData);
        var deltas1 = await _differ.CompareAsync(fromStream1, toStream1);

        using var fromStream2 = new MemoryStream(fromData);
        using var toStream2 = new MemoryStream(toData);
        var deltas2 = await _differ.CompareAsync(fromStream2, toStream2);

        // Assert - Results should be identical
        deltas1.Should().HaveCount(deltas2.Count);
        for (var i = 0; i < deltas1.Count; i++)
        {
            deltas1[i].Offset.Should().Be(deltas2[i].Offset);
            deltas1[i].Size.Should().Be(deltas2[i].Size);
            deltas1[i].FromHash.Should().Be(deltas2[i].FromHash);
            deltas1[i].ToHash.Should().Be(deltas2[i].ToHash);
        }
    }

    [Fact]
    public async Task CompareAsync_HashesAreValidSha256Hex()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = CreateTestData(4096, seed: 99);
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().NotBeEmpty();
        foreach (var delta in deltas)
        {
            if (delta.FromHash != "absent" && delta.FromHash != "truncated" && delta.FromHash != "removed")
            {
                delta.FromHash.Should().MatchRegex("^[a-f0-9]{64}$");
            }
            if (delta.ToHash != "absent" && delta.ToHash != "truncated" && delta.ToHash != "removed")
            {
                delta.ToHash.Should().MatchRegex("^[a-f0-9]{64}$");
            }
        }
    }

    [Fact]
    public async Task CompareAsync_MinConsecutiveChanges_MergesDeltas()
    {
        // Arrange
        var fromData = CreateTestData(8192);
        var toData = CreateTestData(8192, seed: 99); // Completely different
        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        var options = new ByteDiffOptions
        {
            WindowSize = 1024,
            StepSize = 1024,
            MinConsecutiveChanges = 2
        };

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream, options);

        // Assert - Should have merged consecutive changes
        deltas.Should().NotBeEmpty();
        // With 8192 bytes and 1024 window, there are 8 windows
        // All windows are different, so they should be merged into fewer deltas
        deltas.Count.Should().BeLessThan(8);
    }

    [Fact]
    public async Task CompareAsync_AddedBytes_DetectedAsAbsent()
    {
        // Arrange
        var fromData = CreateTestData(4096);
        var toData = new byte[8192];
        Array.Copy(fromData, toData, fromData.Length);
        new Random(99).NextBytes(toData.AsSpan(4096)); // Add random data at end

        using var fromStream = new MemoryStream(fromData);
        using var toStream = new MemoryStream(toData);

        // Act
        var deltas = await _differ.CompareAsync(fromStream, toStream);

        // Assert
        deltas.Should().Contain(d => d.FromHash == "absent");
    }

    // Helper methods

    private static byte[] CreateTestData(int size, int seed = 42)
    {
        var data = new byte[size];
        new Random(seed).NextBytes(data);
        return data;
    }
}