using System;
using System.Runtime.InteropServices;

namespace Susurrus.XAudio2
{
    /// <summary>
    /// Calls the methods of a native object through its vtable, as XAudio2's voices aren't COM objects.
    /// </summary>
    public static class Vtable
    {
        /// <summary>
        /// Returns the delegate for the method at <paramref name="slot"/> in the vtable of the object at <paramref
        /// name="instance"/>.  The delegate takes the instance pointer as its first argument.
        /// </summary>
        public static DelegateType Bind<DelegateType>(IntPtr instance, int slot)
        {
            if (instance == IntPtr.Zero)
            {
                throw new ArgumentException("A null object has no vtable.", nameof(instance));
            }

            if (slot < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(slot), slot, "A vtable slot can't be negative.");
            }

            IntPtr table = Marshal.ReadIntPtr(instance);
            IntPtr method = Marshal.ReadIntPtr(table, slot * IntPtr.Size);
            return Marshal.GetDelegateForFunctionPointer<DelegateType>(method);
        }
    }
}