using System;
using System.Runtime.InteropServices;
using Vanara.Extensions;

namespace Vanara.InteropServices
{
	/// <summary>Marshals strings that are allocated by native code and must be freed after use.</summary>
	/// <remarks>
	/// The supplied cookie must be a string representation of a <see cref="CharSet"/> value. If no cookie is supplied, the value
	/// <see cref="CharSet.Unicode"/> is used.
	/// </remarks>
	/// <typeparam name="TMem">The type of the memory allocator.</typeparam>
	/// <seealso cref="Vanara.InteropServices.GenericStringMarshalerBase{TMem}"/>
	/// <seealso cref="System.Runtime.InteropServices.ICustomMarshaler"/>
	public class GenericStringMarshaler<TMem> : GenericStringMarshalerBase<TMem> where TMem : ISimpleMemoryMethods, new()
	{
		private GenericStringMarshaler(CharSet charSet) : base(charSet) { }

		/// <summary>Gets the instance.</summary>
		/// <param name="cookie">The cookie.</param>
		/// <returns>A new instance of this class.</returns>
		public static ICustomMarshaler GetInstance(string cookie) => new GenericStringMarshaler<TMem>(CharSetFromString(cookie));
	}

	/// <summary>Base abstract class for marshaling strings that are allocated by native code and must be freed after use.</summary>
	/// <typeparam name="TMem">The type of the memory allocator.</typeparam>
	/// <remarks>
	/// The supplied cookie must be a string representation of a <see cref="CharSet"/> value. If no cookie is supplied, the value
	/// <see cref="CharSet.Unicode"/> is used.
	/// </remarks>
	/// <seealso cref="System.Runtime.InteropServices.ICustomMarshaler"/>
	public abstract class GenericStringMarshalerBase<TMem> : ICustomMarshaler where TMem : ISimpleMemoryMethods, new()
	{
		private static readonly TMem mem = new();
		private readonly CharSet charSet;

		/// <summary>Initializes a new instance of the <see cref="GenericStringMarshalerBase{TMem}"/> class.</summary>
		/// <param name="charSet">The character set.</param>
		protected GenericStringMarshalerBase(CharSet charSet) => this.charSet = charSet;

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

		/// <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 virtual void CleanUpNativeData(IntPtr pNativeData) => mem.FreeMem(pNativeData);

		/// <summary>Returns the size of the native data to be marshaled.</summary>
		/// <returns>The size in bytes of the native data.</returns>
		public virtual int GetNativeDataSize() => IntPtr.Size;

		/// <summary>Converts the managed data to unmanaged data.</summary>
		/// <param name="ManagedObj">The managed object to be converted.</param>
		/// <returns>Returns the COM view of the managed object.</returns>
		public virtual IntPtr MarshalManagedToNative(object ManagedObj) => StringHelper.AllocString(ManagedObj as string, charSet, mem.AllocMem);

		/// <summary>Converts the unmanaged data to managed data.</summary>
		/// <param name="pNativeData">A pointer to the unmanaged data to be wrapped.</param>
		/// <returns>Returns the managed view of the COM data.</returns>
		public virtual object MarshalNativeToManaged(IntPtr pNativeData) => StringHelper.GetString(pNativeData, charSet);

		/// <summary>Gets the CharSet from a string.</summary>
		/// <param name="value">The string value.</param>
		/// <param name="defaultValue">The default value if <paramref name="value"/> is not a valid CharSet.</param>
		/// <returns>A CharSet value.</returns>
		protected static CharSet CharSetFromString(string value, CharSet defaultValue = CharSet.Unicode)
		{
			if (!string.IsNullOrEmpty(value))
				try { return (CharSet)Enum.Parse(typeof(CharSet), value, true); } catch { }
			return defaultValue;
		}
	}
}