using System.Collections.Generic;
using System.Linq;

namespace Vanara.PInvoke;

/// <summary>
/// Interface that identifies a structure containing only a 4-byte size field followed by a pointer to an array of <typeparamref name="T"/>.
/// </summary>
/// <typeparam name="T">A marshalable structure.</typeparam>
public interface IArrayStruct<T> where T : struct { }

/// <summary>Extension methods for <see cref="IArrayStruct{T}"/>.</summary>
public static class IArrayStructExtensions
{
	/// <summary>Gets the array from an <see cref="IArrayStruct{T}"/> instance.</summary>
	/// <typeparam name="T">The type of the array.</typeparam>
	/// <param name="ias">The <see cref="IArrayStruct{T}"/> instance.</param>
	/// <returns>The array contained in the instance.</returns>
	public static T[] GetArray<T>(this IArrayStruct<T> ias) where T : struct
	{
		using var pin = new PinnedObject(ias);
		return ((IntPtr)pin).ToArray<T>((int)((IntPtr)pin).ToStructure<uint>(), sizeof(uint))!;
	}
}

/// <summary>Allows marshaling of arrays in place of a structure supporting <see cref="IArrayStruct{T}"/>.</summary>
/// <typeparam name="T">The type of the array element.</typeparam>
/// <seealso cref="IVanaraMarshaler"/>
public class IArrayStructMarshaler<T> : IVanaraMarshaler where T : struct
{
	/// <summary>The header size.</summary>
	public static readonly int hSz = sizeof(uint);

	/// <inheritdoc/>
	public SizeT GetNativeSize() => hSz + IntPtr.Size;

	/// <inheritdoc/>
	public SafeAllocatedMemoryHandle MarshalManagedToNative(object? managedObject)
	{
		if (managedObject is IEnumerable<T> ias)
		{
			var cnt = ias.Count();
			var mem = new SafeHGlobalHandle(hSz + InteropExtensions.SizeOf<T>() * cnt);
			mem.Write(cnt);
			mem.Write(ias, true, hSz);
			return mem;
		}
		throw new ArgumentException("Unexpected type. Value must be of IEnumerable<T>.");
	}

	/// <inheritdoc/>
	public object? MarshalNativeToManaged(IntPtr pNativeData, SizeT allocatedBytes)
	{
		if (pNativeData == default) return new T[0];
		var cnt = pNativeData.ToStructure<uint>();
		return pNativeData.ToArray<T>((int)cnt, hSz, allocatedBytes);
	}
}