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

namespace WhiteMagic;

/// <summary>
/// Shared ReadProcessMemory / WriteProcessMemory wrappers used by both
/// <see cref="ExternalReader"/> and <see cref="InProcessReader"/>. Kept in a single
/// location to keep the two readers byte-for-byte consistent on partial-read handling,
/// write-return semantics, and failure modes.
/// </summary>
internal static class RpmHelper
{
    /// <summary>
    /// Reads up to <paramref name="count"/> bytes from <paramref name="address"/> in
    /// the process identified by <paramref name="handle"/>. Returns:
    /// <list type="bullet">
    /// <item>An empty array if <see cref="NativeMethods.ReadProcessMemory"/> fails and
    /// reports zero bytes read.</item>
    /// <item>A truncated array of exactly <c>bytesRead</c> bytes when the call returns
    /// <see langword="false"/> but the OS has placed a partial copy in the buffer
    /// (for example, <c>ERROR_PARTIAL_COPY</c>).</item>
    /// <item>The full buffer on success.</item>
    /// </list>
    /// </summary>
    public static byte[] ReadBytes(SafeMemoryHandle handle, IntPtr address, int count)
    {
        byte[] buffer = new byte[count];
        bool ok = NativeMethods.ReadProcessMemory(handle, address, buffer, count, out nint bytesRead);

        if (!ok && bytesRead == 0)
        {
            return [];
        }

        if ((int)bytesRead < count)
        {
            // Either a successful short read, or a failed-but-partial RPM. In both
            // cases honor the bytes the OS actually produced rather than padding.
            byte[] partial = new byte[(int)bytesRead];
            Buffer.BlockCopy(buffer, 0, partial, 0, (int)bytesRead);
            return partial;
        }

        return buffer;
    }

    /// <summary>
    /// Writes <paramref name="bytes"/> to <paramref name="address"/> in the process
    /// identified by <paramref name="handle"/>. Returns the number of bytes actually
    /// written, or 0 on total failure.
    /// </summary>
    public static int WriteBytes(SafeMemoryHandle handle, IntPtr address, ReadOnlySpan<byte> bytes)
    {
        if (!NativeMethods.WriteProcessMemory(handle, address, bytes, bytes.Length, out nint written))
        {
            return 0;
        }
        return (int)written;
    }
}