using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Text;
using WhiteMagic;
using WhiteMagic.Native;

namespace WhiteMagicTest;

/// <summary>
/// Regression tests for the edge-case defects found in the second review pass:
/// char sizing, reference-containing structs, unchecked counts, the ReadString
/// partial-chunk skip, and ExternalReader construction against awkward targets.
/// Each test fails against the pre-fix code.
/// </summary>
public class MemoryHardeningTests
{
    private static ExternalReader OpenSelf()
    {
        return new ExternalReader(
            Process.GetCurrentProcess(),
            ProcessAccess.VmRead | ProcessAccess.VmWrite | ProcessAccess.VmOperation | ProcessAccess.QueryInformation);
    }

    // ── char sizing (MarshalCache / MemoryBase.Read<char>) ──────────────────

    [Fact]
    public void MarshalCache_char_size_is_two_bytes()
    {
        Assert.Equal(2, MarshalCache<char>.Size);
    }

    [Fact]
    public void Read_char_writes_and_reads_back()
    {
        using var reader = OpenSelf();
        char slot = '\0';
        GCHandle pin = GCHandle.Alloc(slot, GCHandleType.Pinned);
        try
        {
            IntPtr addr = pin.AddrOfPinnedObject();
            Assert.True(reader.Write(addr, 'Z'));
            Assert.Equal('Z', reader.Read<char>(addr));
        }
        finally
        {
            pin.Free();
        }
    }

    [Fact]
    public void Read_char_array_writes_and_reads_back()
    {
        using var reader = OpenSelf();
        char[] slot = new char[4];
        GCHandle pin = GCHandle.Alloc(slot, GCHandleType.Pinned);
        try
        {
            IntPtr addr = pin.AddrOfPinnedObject();
            char[] expected = ['w', 'o', 'w', '!'];
            Assert.True(reader.Write(addr, expected));
            Assert.Equal(expected, reader.Read<char>(addr, 4));
        }
        finally
        {
            pin.Free();
        }
    }

    // ── reference-containing structs route to the marshal path ──────────────

    [Fact]
    public void MarshalCache_flags_struct_with_reference_field_as_marshal_required()
    {
        // Has a string field but no [MarshalAs]; the blittable path (MemoryMarshal.Read)
        // throws for a reference-containing T, so the cache must route it to the marshal
        // path. A struct with a managed reference cannot be pinned, so the routing flag —
        // not a live round-trip — is the regression guard here.
        Assert.True(MarshalCache<StructWithReference>.TypeRequiresMarshal);
    }

    // ── unchecked count guards ──────────────────────────────────────────────

    [Fact]
    public void Read_array_with_negative_count_throws_argument_out_of_range()
    {
        using var reader = OpenSelf();
        int dummy = 0;
        GCHandle pin = GCHandle.Alloc(dummy, GCHandleType.Pinned);
        try
        {
            Assert.Throws<ArgumentOutOfRangeException>(() => reader.Read<int>(pin.AddrOfPinnedObject(), -1));
        }
        finally
        {
            pin.Free();
        }
    }

    [Fact]
    public void Read_array_with_zero_count_returns_empty()
    {
        using var reader = OpenSelf();
        int dummy = 0;
        GCHandle pin = GCHandle.Alloc(dummy, GCHandleType.Pinned);
        try
        {
            Assert.Empty(reader.Read<int>(pin.AddrOfPinnedObject(), 0));
        }
        finally
        {
            pin.Free();
        }
    }

    // ── ReadString partial-chunk advance ────────────────────────────────────

    [Fact]
    public void ReadString_advances_by_actual_bytes_when_reads_are_partial()
    {
        // The reader serves at most 3 bytes per call. The string is longer than one
        // chunk with the terminator well past it. If ReadString advanced by the
        // requested count instead of the bytes actually returned, it would skip
        // data and truncate the result.
        byte[] data = Encoding.ASCII.GetBytes("ABCDEFGHIJ\0");
        var reader = new PartialReader(data, maxChunk: 3);

        string result = reader.ReadString(IntPtr.Zero, Encoding.ASCII, maxLength: 64);

        Assert.Equal("ABCDEFGHIJ", result);
    }

    [Fact]
    public void ReadString_stops_at_null_across_partial_chunks()
    {
        byte[] data = Encoding.ASCII.GetBytes("hi\0garbage");
        var reader = new PartialReader(data, maxChunk: 1);

        string result = reader.ReadString(IntPtr.Zero, Encoding.ASCII, maxLength: 64);

        Assert.Equal("hi", result);
    }

    // ── ExternalReader construction ─────────────────────────────────────────

    [Fact]
    public void ExternalReader_opens_self_with_default_access()
    {
        // The default access set must be small enough to open a normal process.
        using var reader = new ExternalReader(Process.GetCurrentProcess());
        Assert.False(reader.Handle.IsInvalid);
    }

    /// <summary>
    /// A <see cref="MemoryBase"/> that serves bytes from an in-memory buffer and
    /// caps every read to <c>maxChunk</c> bytes, to exercise partial-read handling.
    /// The address is treated as a zero-based index into the buffer.
    /// </summary>
    private sealed class PartialReader(byte[] data, int maxChunk) : MemoryBase
    {
        public override IntPtr ImageBase => IntPtr.Zero;
        public override SafeMemoryHandle Handle => null!;
        public override bool Is64Bit => Environment.Is64BitProcess;
        public override int ProcessId => Environment.ProcessId;

        public override byte[] ReadBytes(IntPtr address, int count, bool isRelative = false)
        {
            int start = (int)address;
            if (start < 0 || start >= data.Length || count <= 0)
                return [];
            int n = Math.Min(Math.Min(count, maxChunk), data.Length - start);
            return data[start..(start + n)];
        }

        public override int WriteBytes(IntPtr address, ReadOnlySpan<byte> bytes, bool isRelative = false)
            => throw new NotSupportedException();

        public override void Dispose() { }
    }
}

/// <summary>
/// A struct that carries a managed reference. <see cref="RuntimeHelpers.IsReferenceOrContainsReferences{T}"/>
/// reports <see langword="true"/>, so it cannot travel the blittable read path.
/// </summary>
[StructLayout(LayoutKind.Sequential)]
public struct StructWithReference
{
    public int Id;
    public string Name;
}