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

namespace WhiteMagic;

/// <summary>
/// In-process memory reader that accesses the owning process's memory through
/// <see cref="NativeMethods.ReadProcessMemory"/> and
/// <see cref="NativeMethods.WriteProcessMemory"/> on a handle to the current
/// process. Unlike the unsafe-deref approach, this fails softly (returns
/// empty / zero bytes) on invalid or protected addresses instead of crashing
/// the host process with an <see cref="AccessViolationException"/>.
/// </summary>
public sealed class InProcessReader : MemoryBase
{
    private readonly SafeMemoryHandle _handle;
    private readonly IntPtr _imageBase;
    private bool _disposed;

    /// <summary>
    /// Creates an in-process reader for the current process.
    /// </summary>
    public InProcessReader()
    {
        Process current = Process.GetCurrentProcess();
        _handle = NativeMethods.OpenProcess(
            ProcessAccess.VmRead | ProcessAccess.VmWrite | ProcessAccess.VmOperation | ProcessAccess.QueryInformation,
            false,
            current.Id);
        if (_handle.IsInvalid)
        {
            int error = Marshal.GetLastPInvokeError();
            throw new InvalidOperationException(
                $"OpenProcess failed for PID {current.Id}: error {error}");
        }

        _imageBase = current.MainModule?.BaseAddress ?? IntPtr.Zero;
    }

    /// <inheritdoc />
    public override IntPtr ImageBase => _imageBase;

    /// <inheritdoc />
    public override SafeMemoryHandle Handle => _handle;

    /// <inheritdoc />
    public override byte[] ReadBytes(IntPtr address, int count, bool isRelative = false)
    {
        if (isRelative)
            address = GetAbsolute(address);

        byte[] buffer = new byte[count];
        if (!NativeMethods.ReadProcessMemory(_handle, address, buffer, count, out nint bytesRead))
        {
            return [];
        }

        if ((int)bytesRead != count)
        {
            Array.Resize(ref buffer, (int)bytesRead);
        }

        return buffer;
    }

    /// <inheritdoc />
    public override int WriteBytes(IntPtr address, ReadOnlySpan<byte> bytes, bool isRelative = false)
    {
        if (isRelative)
            address = GetAbsolute(address);

        if (!NativeMethods.WriteProcessMemory(_handle, address, bytes, bytes.Length, out nint written))
        {
            return 0;
        }

        return (int)written;
    }

    /// <inheritdoc />
    public override void Dispose()
    {
        if (!_disposed)
        {
            _disposed = true;
            _handle.Dispose();
        }
    }
}