using System;
using System.Collections;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace SpeechLib {
	/// <summary>Marshals the COM <see langword="IEnumVARIANT" /> interface to the .NET Framework <see cref="T:System.Collections.IEnumerator" /> interface, and vice versa.</summary>
	public class EnumeratorToEnumVariantMarshaler : ICustomMarshaler {
		[ComImport]
		[Guid("00020404-0000-0000-C000-000000000046")]
		[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
		private interface IEnumVARIANT {
			[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
			void Next(int celt, [MarshalAs(UnmanagedType.Struct)] out object rgvar, out uint pceltFetched);

			[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
			void Skip(uint celt);

			[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
			void Reset();

			[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
			[return: MarshalAs(UnmanagedType.Interface)]
			IEnumVARIANT Clone();
		}

		private class VARIANTEnumerator : IEnumerator {
			private IEnumVARIANT com_enum;

			private object current;

			public object Current => current;

			public VARIANTEnumerator(IEnumVARIANT com_enum) {
				this.com_enum = com_enum;
			}

			public bool MoveNext() {
				uint pceltFetched = 0u;
				com_enum.Next(1, out var rgvar, out pceltFetched);
				if (pceltFetched == 0) {
					return false;
				}
				current = rgvar;
				return true;
			}

			public void Reset() {
				com_enum.Reset();
			}
		}

		private static EnumeratorToEnumVariantMarshaler instance;

		/// <summary>Performs necessary cleanup of the managed data when it is no longer needed.</summary>
		/// <param name="pManagedObj">The managed object to be destroyed.</param>
		public void CleanUpManagedData(object pManagedObj) {
			throw new NotImplementedException();
		}

		/// <summary>Performs necessary cleanup of the unmanaged data when it is no longer needed.</summary>
		/// <param name="pNativeData">A pointer to the unmanaged data to be destroyed.</param>
		public void CleanUpNativeData(IntPtr pNativeData) {
			Marshal.Release(pNativeData);
		}

		/// <summary>Returns an instance of the custom marshaler.</summary>
		/// <param name="pstrCookie">String "cookie" parameter that can be used by the custom marshaler.</param>
		/// <returns>An instance of the custom marshaler.</returns>
		public static ICustomMarshaler GetInstance(string pstrCookie) {
			if (instance == null) {
				instance = new EnumeratorToEnumVariantMarshaler();
			}
			return instance;
		}

		/// <summary>Returns the size in bytes of the unmanaged data to be marshaled.</summary>
		/// <returns>-1 to indicate the type this marshaler handles is not a value type.</returns>
		public int GetNativeDataSize() {
			throw new NotImplementedException();
		}

		/// <summary>Marshals an object from managed code to unmanaged code.</summary>
		/// <param name="pManagedObj">The managed object to be converted.</param>
		/// <returns>A pointer to the unmanaged object.</returns>
		/// <exception cref="T:System.ArgumentNullException">
		///   <paramref name="pManagedObj" /> is <see langword="null" />.</exception>
		public IntPtr MarshalManagedToNative(object pManagedObj) {
			throw new NotImplementedException();
		}

		/// <summary>Marshals an object from unmanaged code to managed code.</summary>
		/// <param name="pNativeData">A pointer to the unmanaged object to be converted.</param>
		/// <returns>A managed object.</returns>
		/// <exception cref="T:System.ArgumentNullException">
		///   <paramref name="pNativeData" /> is <see langword="null" />.</exception>
		/// <exception cref="T:System.InvalidCastException">The unmanaged object that <paramref name="pNativeData" /> points to could not be converted.</exception>
		public object MarshalNativeToManaged(IntPtr pNativeData) {
			return new VARIANTEnumerator((IEnumVARIANT)Marshal.GetObjectForIUnknown(pNativeData));
		}
	}
}