using System.Diagnostics;
using UnmanagedMMU.Allocators;
using UnmanagedMMU.Handles.Internal;

namespace UnmanagedMMU.Handles
{

    /// <summary>
    /// Provides a base implementation for typed unmanaged memory handles.
    /// </summary>
    /// <typeparam name="T">
    /// The unmanaged element type stored in the underlying memory block.
    /// </typeparam>
    /// <remarks>
    /// This class encapsulates the raw pointer and byte length of an unmanaged
    /// memory allocation and provides typed access to that memory.
    /// 
    /// Memory lifetime is controlled by the originating allocator; this type
    /// does not own or free the underlying memory.
    /// </remarks>
    internal unsafe abstract class MemoryHandleBase<T> : IMemoryHandle<T>, IOwnedHandle where T : unmanaged
    {

        /// <summary>
        /// The <see cref="IUnmanagedMemoryOwner"/> that owns this <see cref="MemoryHandleBase{T}"/> handle
        /// </summary>
        private readonly IUnmanagedMemoryOwner _owner;

        /// <summary>
        /// The raw pointer to the unmanaged memory block
        /// </summary>
        private readonly void* _ptr;

        /// <summary>
        /// The size of the unmanaged memory block in bytes
        /// </summary>
        private readonly nuint _bytelen;

        /// <summary>
        /// Indicates whether this <see cref="MemoryHandleBase{T}"/> has been disposed.
        /// </summary>
        private bool _disposed;

        /// <summary>
        /// Initializes a new <see cref="MemoryHandleBase{T}"/> instnace
        /// </summary>
        /// <param name="ptr">Pointer to the allocated unmanaged memory</param>
        /// <param name="byteLength">The size of the unallocated memory block in bytes</param>
        /// <param name="owner">The <see cref="IUnmanagedMemoryOwner"/> that owns the <see cref="MemoryHandleBase{T}"/> handle being created</param>
        protected MemoryHandleBase(void* ptr, nuint byteLength, IUnmanagedMemoryOwner owner)
        {
            // Defensive check
            Debug.Assert(ptr != null, message: "BUG CHECK: E_INVALID_MEMORY_HANDLE");
            _ptr = ptr;
            _bytelen = byteLength;
            _owner = owner;
        }

        /// <summary>
        /// Gets the raw pointer to the unmanaged memory block
        /// </summary>
        public virtual void* Pointer
        {
            get { return _ptr; }
        }

        /// <summary>
        /// Gets the typed pointer to the unmanged memory block
        /// </summary>
        T* IMemoryHandle<T>.Pointer
        {
            get { return (T*)_ptr; }
        }

        /// <summary>
        /// Gets the size in bytes of the unmanaged memory block
        /// </summary>
        public nuint ByteCount
        {
            get { return _bytelen; }
        }

        /// <summary>
        /// Gets the number of elements of type <typeparamref name="T"/>
        /// contained in the unmanaged memory block.
        /// </summary>
        public nuint Length
        {
            get { return _bytelen / (nuint)sizeof(T); }
        }

        /// <summary>
        /// Releases the unmanaged resources held by the <see cref="MemoryHandleBase{T}"/>.
        /// </summary>
        /// <remarks>
        /// This method is idempotent and safe to call multiple times. It ensures the underlying
        /// memory is cleaned up according to the implementation in <see cref="OnDispose()"/>.
        /// After disposal, the handle is invalid for further use.
        /// </remarks>
        public void Dispose()
        {
            if (_disposed)
            {
                return;
            }
            _disposed = true;
            OnDispose();
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Override in derived classes to define lifecycle behavior.
        /// This is an abstract method to ensure cleanup logic is always implemented.
        /// </summary>
        protected abstract void OnDispose();

        /// <summary>
        /// Returns the <see cref="IUnmanagedMemoryOwner"/> that created owns this <see cref="MemoryHandleBase{T}"/> instance
        /// </summary>
        /// <returns>The <see cref="IUnmanagedMemoryOwner"/> that created owns this <see cref="MemoryHandleBase{T}"/> instance</returns>
        public IUnmanagedMemoryOwner GetOwner()
        {
            return _owner;
        }
    }
}