// Copyright (c) 2026 Duplicati Inc. All rights reserved.

using System;
using System.Buffers.Binary;
using System.Collections.Generic;
using Duplicati.Proprietary.DiskImage.Filesystem.Fat32;
using NUnit.Framework;
using NUnit.Framework.Legacy;
using Assert = NUnit.Framework.Legacy.ClassicAssert;

#nullable enable

namespace Duplicati.UnitTest.DiskImage.Fat32;

/// <summary>
/// Unit tests for the <see cref="Fat32Table"/> FAT table reader.
/// </summary>
[TestFixture]
[Category("DiskImageUnit")]
public class Fat32TableTests
{
    /// <summary>
    /// Creates a valid FAT32 boot sector with known values for testing.
    /// </summary>
    private static byte[] CreateValidBootSector()
    {
        var bootSector = new byte[512];

        // Bytes per sector at offset 0x0B (512 = 0x0200)
        BinaryPrimitives.WriteUInt16LittleEndian(bootSector.AsSpan(0x0B, 2), 512);

        // Sectors per cluster at offset 0x0D (8 sectors)
        bootSector[0x0D] = 8;

        // Reserved sector count at offset 0x0E (32 sectors)
        BinaryPrimitives.WriteUInt16LittleEndian(bootSector.AsSpan(0x0E, 2), 32);

        // Number of FATs at offset 0x10 (2 FATs)
        bootSector[0x10] = 2;

        // Total sectors 32 at offset 0x20 (1,000,000 sectors = ~512MB)
        BinaryPrimitives.WriteUInt32LittleEndian(bootSector.AsSpan(0x20, 4), 1_000_000);

        // FAT size 32 at offset 0x24 (244 sectors per FAT)
        BinaryPrimitives.WriteUInt32LittleEndian(bootSector.AsSpan(0x24, 4), 244);

        // Root cluster at offset 0x2C (cluster 2)
        BinaryPrimitives.WriteUInt32LittleEndian(bootSector.AsSpan(0x2C, 4), 2);

        // FSInfo sector at offset 0x30 (sector 1)
        BinaryPrimitives.WriteUInt16LittleEndian(bootSector.AsSpan(0x30, 2), 1);

        // Filesystem type string at offset 0x52 (8 bytes: "FAT32   ")
        var fsType = System.Text.Encoding.ASCII.GetBytes("FAT32   ");
        fsType.CopyTo(bootSector, 0x52);

        // Boot sector signature at offset 510: bytes 0x55, 0xAA on disk (0xAA55 as little-endian uint16)
        BinaryPrimitives.WriteUInt16LittleEndian(bootSector.AsSpan(510, 2), 0xAA55);

        return bootSector;
    }

    /// <summary>
    /// Creates FAT data with specified entries.
    /// </summary>
    private static byte[] CreateFatData(params uint[] entries)
    {
        var fatData = new byte[entries.Length * 4];
        for (int i = 0; i < entries.Length; i++)
        {
            BinaryPrimitives.WriteUInt32LittleEndian(fatData.AsSpan(i * 4, 4), entries[i]);
        }
        return fatData;
    }

    [Test]
    public void Test_Fat32Table_FreeCluster_NotAllocated()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 2 as free
        var fatData = CreateFatData(0, 0, Fat32Table.FREE_CLUSTER);
        var fatTable = new Fat32Table(bootSector, fatData);

        Assert.IsFalse(fatTable.IsClusterAllocated(2), "Free cluster should not be allocated");
    }

    [Test]
    public void Test_Fat32Table_AllocatedCluster_IsAllocated()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 2 pointing to cluster 3 (allocated)
        var fatData = CreateFatData(0, 0, 3, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        Assert.IsTrue(fatTable.IsClusterAllocated(2), "Cluster with valid next-cluster value should be allocated");
        Assert.IsTrue(fatTable.IsClusterAllocated(3), "Cluster with EOC should be allocated");
    }

    [Test]
    public void Test_Fat32Table_EndOfChain_IsAllocated()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Test all EOC values (0x0FFFFFF8 - 0x0FFFFFFF)
        for (uint eocValue = Fat32Table.EOC_MIN; eocValue <= Fat32Table.EOC_MAX; eocValue++)
        {
            var fatData = CreateFatData(0, 0, eocValue);
            var fatTable = new Fat32Table(bootSector, fatData);

            Assert.IsTrue(fatTable.IsClusterAllocated(2), $"Cluster with EOC value 0x{eocValue:X8} should be allocated");
        }
    }

    [Test]
    public void Test_Fat32Table_BadCluster_NotAllocated()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 2 as bad
        var fatData = CreateFatData(0, 0, Fat32Table.BAD_CLUSTER);
        var fatTable = new Fat32Table(bootSector, fatData);

        Assert.IsFalse(fatTable.IsClusterAllocated(2), "Bad cluster should not be allocated");
    }

    [Test]
    public void Test_Fat32Table_GetClusterChain_SingleCluster()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 2 having EOC immediately
        var fatData = CreateFatData(0, 0, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var chain = fatTable.GetClusterChain(2);

        Assert.AreEqual(1, chain.Count, "Single cluster chain should have 1 entry");
        Assert.AreEqual(2u, chain[0], "Chain should contain the start cluster");
    }

    [Test]
    public void Test_Fat32Table_GetClusterChain_MultipleClusters()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with chain: 2 -> 3 -> 4 -> EOC
        var fatData = CreateFatData(0, 0, 3, 4, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var chain = fatTable.GetClusterChain(2);

        Assert.AreEqual(3, chain.Count, "Chain should have 3 entries");
        Assert.AreEqual(2u, chain[0], "First entry should be cluster 2");
        Assert.AreEqual(3u, chain[1], "Second entry should be cluster 3");
        Assert.AreEqual(4u, chain[2], "Third entry should be cluster 4");
    }

    [Test]
    public void Test_Fat32Table_GetClusterChain_CircularDetection()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with circular chain: 2 -> 3 -> 2 (circular)
        var fatData = CreateFatData(0, 0, 3, 2);
        var fatTable = new Fat32Table(bootSector, fatData);

        var ex = Assert.Throws<InvalidOperationException>(() => fatTable.GetClusterChain(2));
        StringAssert.Contains("Circular", ex!.Message);
    }

    [Test]
    public void Test_Fat32Table_AllocationBitmap_MatchesFatEntries()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with mixed entries:
        // 0: reserved (typically EOC or 0)
        // 1: reserved (typically EOC or 0)
        // 2: free
        // 3: allocated (points to 4)
        // 4: EOC (end of chain)
        // 5: bad cluster
        var fatData = CreateFatData(
            Fat32Table.FREE_CLUSTER,  // cluster 0
            Fat32Table.FREE_CLUSTER,  // cluster 1
            Fat32Table.FREE_CLUSTER,  // cluster 2 (free)
            4,                        // cluster 3 (allocated, points to 4)
            Fat32Table.EOC_MIN,       // cluster 4 (end of chain)
            Fat32Table.BAD_CLUSTER    // cluster 5 (bad)
        );
        var fatTable = new Fat32Table(bootSector, fatData);

        // Reserved clusters 0 and 1 - treated as free (not allocated)
        Assert.IsFalse(fatTable.IsClusterAllocated(0), "Cluster 0 should not be allocated");
        Assert.IsFalse(fatTable.IsClusterAllocated(1), "Cluster 1 should not be allocated");

        // Cluster 2 is free
        Assert.IsFalse(fatTable.IsClusterAllocated(2), "Cluster 2 (free) should not be allocated");

        // Cluster 3 is allocated (points to next cluster)
        Assert.IsTrue(fatTable.IsClusterAllocated(3), "Cluster 3 (allocated) should be allocated");

        // Cluster 4 is EOC
        Assert.IsTrue(fatTable.IsClusterAllocated(4), "Cluster 4 (EOC) should be allocated");

        // Cluster 5 is bad
        Assert.IsFalse(fatTable.IsClusterAllocated(5), "Cluster 5 (bad) should not be allocated");

        // Verify counts
        Assert.AreEqual(6u, fatTable.TotalClusters, "Total clusters should be 6");
        Assert.AreEqual(2u, fatTable.AllocatedClusters, "Allocated clusters should be 2 (clusters 3 and 4)");
        Assert.AreEqual(4u, fatTable.FreeClusters, "Free clusters should be 4 (clusters 0, 1, 2, and 5 is counted as free since it's bad)");
    }

    [Test]
    public void Test_Fat32Table_EntryMask_IgnoresUpperBits()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 2 having upper 4 bits set (should be masked)
        // 0xF0000003 with mask 0x0FFFFFFF = 0x00000003 (points to cluster 3)
        uint entryWithUpperBits = 0xF0000003;
        var fatData = CreateFatData(0, 0, entryWithUpperBits, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var nextCluster = fatTable.GetNextCluster(2);

        Assert.AreEqual(3u, nextCluster, "Upper 4 bits should be masked, leaving value 3");
        Assert.IsTrue(fatTable.IsClusterAllocated(2), "Cluster should be allocated even with upper bits set");
    }

    [Test]
    public void Test_Fat32Table_GetNextCluster_ReturnsMaskedValue()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT where cluster 2 points to cluster 5 (0x00000005)
        var fatData = CreateFatData(0, 0, 5, Fat32Table.EOC_MIN, Fat32Table.EOC_MIN, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var nextCluster = fatTable.GetNextCluster(2);

        Assert.AreEqual(5u, nextCluster, "GetNextCluster should return the next cluster number");
    }

    [Test]
    public void Test_Fat32Table_ClusterOutOfRange_Throws()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        var fatData = CreateFatData(0, 0, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        // Try to access cluster beyond the FAT size
        var ex = Assert.Throws<ArgumentOutOfRangeException>(() => fatTable.IsClusterAllocated(100));
        StringAssert.Contains("out of range", ex!.Message);
    }

    [Test]
    public void Test_Fat32Table_GetClusterChain_StartClusterOutOfRange_Throws()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        var fatData = CreateFatData(0, 0, Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var ex = Assert.Throws<ArgumentOutOfRangeException>(() => fatTable.GetClusterChain(100));
        StringAssert.Contains("out of range", ex!.Message);
    }

    [Test]
    public void Test_Fat32Table_GetClusterChain_InvalidClusterInChain_Throws()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT where cluster 2 points to cluster 3 which is free (invalid in a chain)
        var fatData = CreateFatData(0, 0, 3, Fat32Table.FREE_CLUSTER);
        var fatTable = new Fat32Table(bootSector, fatData);

        var ex = Assert.Throws<InvalidOperationException>(() => fatTable.GetClusterChain(2));
        StringAssert.Contains("Invalid cluster", ex!.Message);
    }

    [Test]
    public void Test_Fat32Table_Constructor_NullFatData_Throws()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        var ex = Assert.Throws<ArgumentNullException>(() => new Fat32Table(bootSector, null!));
        Assert.AreEqual("fatData", ex!.ParamName);
    }

    [Test]
    public void Test_Fat32Table_EmptyClusterChain()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create FAT with cluster 0 having EOC
        var fatData = CreateFatData(Fat32Table.EOC_MIN);
        var fatTable = new Fat32Table(bootSector, fatData);

        var chain = fatTable.GetClusterChain(0);

        Assert.AreEqual(1, chain.Count, "Single cluster chain should have 1 entry");
        Assert.AreEqual(0u, chain[0], "Chain should contain cluster 0");
    }

    [Test]
    public void Test_Fat32Table_LongClusterChain()
    {
        var bootSectorData = CreateValidBootSector();
        var bootSector = new Fat32BootSector(bootSectorData);

        // Create a chain of 10 clusters: 2 -> 3 -> 4 -> ... -> 11 -> EOC
        var entries = new List<uint> { 0, 0 }; // clusters 0 and 1
        for (uint i = 2; i <= 10; i++)
        {
            entries.Add(i + 1); // each cluster points to the next
        }
        entries.Add(Fat32Table.EOC_MIN); // cluster 11 is EOC

        var fatData = CreateFatData(entries.ToArray());
        var fatTable = new Fat32Table(bootSector, fatData);

        var chain = fatTable.GetClusterChain(2);

        Assert.AreEqual(10, chain.Count, "Chain should have 10 entries");
        for (int i = 0; i < 10; i++)
        {
            Assert.AreEqual((uint)(i + 2), chain[i], $"Chain entry {i} should be cluster {i + 2}");
        }
    }
}