using System;
using System.Runtime.InteropServices;

namespace VisualStudioMock
{
  delegate int GetInterfaceDelegate(IntPtr @this, [MarshalAs(UnmanagedType.LPStruct)] Guid iid, out IntPtr ppv);
  delegate int AddRefDelegate(IntPtr @this);
  delegate int ReleaseDelegate(IntPtr @this);

  sealed class ComProxy
  {
    private const int S_OK = 0;
    private const int E_NOINTERFACE = -2147467262;

    private static readonly Guid IID_IUnknown = new Guid("00000000-0000-0000-c000-000000000046");
    private static readonly Guid IID_IManagedObject = new Guid("C3FCC19E-A970-11D2-8B5A-00A0C9B7C9C4");

    private readonly object _realInstance;
    private readonly IntPtr _realInstanceN;
    private readonly IntPtr _proxyInstanceN;
    private readonly GetInterfaceDelegate _getInterface;
    private readonly IntPtr _getInterfaceN;
    private readonly AddRefDelegate _addRef;
    private readonly ReleaseDelegate _release;

    internal ComProxy(object instance)
    {
      this._realInstance = instance;
      this._realInstanceN = Marshal.GetIUnknownForObject(instance);
      this._proxyInstanceN = Marshal.AllocHGlobal(IntPtr.Size * 4);

      IntPtr vtbl = _proxyInstanceN + IntPtr.Size;
      Marshal.WriteIntPtr(_proxyInstanceN, vtbl);
      Marshal.WriteIntPtr(vtbl, this._getInterfaceN = Marshal.GetFunctionPointerForDelegate((Delegate)(this._getInterface = new GetInterfaceDelegate(GetInterface))));
      Marshal.WriteIntPtr(vtbl, IntPtr.Size * 1, Marshal.GetFunctionPointerForDelegate((Delegate)(this._addRef = new AddRefDelegate(AddRef))));
      Marshal.WriteIntPtr(vtbl, IntPtr.Size * 2, Marshal.GetFunctionPointerForDelegate((Delegate)(this._release = new ReleaseDelegate(Release))));
    }

    internal object GetIUnknown()
    {
      return Marshal.GetObjectForIUnknown(_proxyInstanceN);
    }

    private int GetInterface(IntPtr @this, [MarshalAs(UnmanagedType.LPStruct)] Guid iid, out IntPtr ppv)
    {
      if (iid == IID_IUnknown)
      {
        ppv = _proxyInstanceN;
        Ext.LogCall(_realInstance.GetType().Name, nameof(GetInterface), "IUnknown OK");
        return S_OK;
      }

      if (iid == IID_IManagedObject)
      {
        ppv = IntPtr.Zero;
        Ext.LogCall(_realInstance.GetType().Name, nameof(GetInterface), "IManagedObject Fail");
        return E_NOINTERFACE;
      }

      int result = Marshal.QueryInterface(_realInstanceN, ref iid, out ppv);
      if (result == S_OK)
      {
        // Patch the vtable with our own thunks
        IntPtr origVtbl = Marshal.ReadIntPtr(ppv);

        Type iType = null;
        foreach (Type i in _realInstance.GetType().GetInterfaces())
          if (i.GUID == iid)
          {
            iType = i;
            break;
          }
        if (iType != null)
        {
          // TODO generate proper thunks in machine code
          int slotCount = Marshal.GetEndComSlot(iType) + 1;
          int slotsSizeBytes = IntPtr.Size * slotCount;
          IntPtr newVtbl = Marshal.AllocHGlobal(slotsSizeBytes);
          ...
        }

        Ext.LogCall(_realInstance.GetType().Name, nameof(GetInterface), (iType == null ? iid.ToString() : iType.Name) + " OK");
      }
      else
      {
        Ext.LogCall(_realInstance.GetType().Name, nameof(GetInterface), iid.ToString() + " Fail");
      }
      return result;
    }

    private int AddRef(IntPtr @this)
    {
      return 1;
    }

    private int Release(IntPtr @this)
    {
      return 1;
    }
  }
}