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

namespace WhiteMagic;

/// <summary>
/// Out-of-process memory reader that accesses the target's memory through
/// <see cref="NativeMethods.ReadProcessMemory"/> and
/// <see cref="NativeMethods.WriteProcessMemory"/>.
/// </summary>
public sealed class ExternalReader : MemoryBase
{
    private readonly SafeMemoryHandle _handle;
    private readonly IntPtr _imageBase;
    private bool _disposed;

    /// <summary>
    /// The default access rights: enough to read, write, allocate, query, run a remote
    /// thread, and wait on it. This deliberately omits <see cref="ProcessAccess.AllAccess"/>,
    /// which over-requests and makes <c>OpenProcess</c> fail on protected processes where
    /// these narrower rights would succeed.
    /// </summary>
    public const ProcessAccess DefaultAccess =
        ProcessAccess.VmRead | ProcessAccess.VmWrite | ProcessAccess.VmOperation
        | ProcessAccess.QueryInformation | ProcessAccess.CreateThread | ProcessAccess.Synchronize;

    /// <summary>
    /// Opens a process for external memory access.
    /// </summary>
    /// <param name="process">The target process.</param>
    /// <param name="desiredAccess">The access rights to request. Defaults to
    /// <see cref="DefaultAccess"/>.</param>
    public ExternalReader(Process process, ProcessAccess desiredAccess = DefaultAccess)
    {
        _handle = NativeMethods.OpenProcess(desiredAccess, false, process.Id);
        if (_handle.IsInvalid)
        {
            int error = Marshal.GetLastPInvokeError();
            throw new InvalidOperationException(
                $"OpenProcess failed for PID {process.Id}: error {error}");
        }

        // Process.MainModule throws Win32Exception for a bitness-mismatched or protected
        // target; a missing image base must not sink the whole reader.
        try
        {
            _imageBase = process.MainModule?.BaseAddress ?? IntPtr.Zero;
        }
        catch (System.ComponentModel.Win32Exception)
        {
            _imageBase = 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();
        }
    }
}