using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace Capnp
{
    /// <summary>
    /// SerializerState specialization for unmanaged primitive types (including enum).
    /// </summary>
    /// <typeparam name="T">List element type, must be primitive. Static constructor will throw if the type does not work.</typeparam>
    public class ListOfPrimitivesSerializer<T> :
        SerializerState,
        IReadOnlyList<T>
        where T : unmanaged
    {
        static readonly int ElementSize;

        static ListOfPrimitivesSerializer()
        {
            if (typeof(T).IsEnum)
            {
                ElementSize = Marshal.SizeOf(Enum.GetUnderlyingType(typeof(T)));
            }
            else
            {
                ElementSize = Marshal.SizeOf<T>();
            }
        }

        /// <summary>
        /// The list's data
        /// </summary>
        public Span<T> Data => MemoryMarshal.Cast<ulong, T>(RawData).Slice(0, Count);

        /// <summary>
        /// Gets or sets the value at given index.
        /// </summary>
        /// <param name="index">Element index</param>
        /// <returns>Element value</returns>
        public T this[int index]
        {
            get
            {
                ListSerializerHelper.EnsureAllocated(this);
                return Data[index];
            }
            set
            {
                ListSerializerHelper.EnsureAllocated(this);
                Data[index] = value;
            }
        }

        /// <summary>
        /// This list's element count.
        /// </summary>
        public int Count => ListElementCount;

        /// <summary>
        /// Initializes this list with a specific size. The list can be initialized only once.
        /// </summary>
        /// <param name="count">List element count</param>
        /// <exception cref="InvalidOperationException">The list was already initialized</exception>
        /// <exception cref="ArgumentOutOfRangeException"><paramref name="count"/> is negative or greater than 2^29-1</exception>
        public void Init(int count)
        {
            if (IsAllocated)
                throw new InvalidOperationException("Already initialized");

            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count));

            SetListOfValues((byte)(8 * ElementSize), count);
        }

        /// <summary>
        /// Initializes the list with given content.
        /// </summary>
        /// <param name="items">List content. Can be null in which case the list is simply not initialized.</param>
        /// <exception cref="InvalidOperationException">The list was already initialized</exception>
        /// <exception cref="ArgumentOutOfRangeException">More than 2^29-1 items.</exception>
        public void Init(IReadOnlyList<T>? items)
        {
            if (items == null)
            {
                return;
            }

            Init(items.Count);

            switch (items)
            {
                case T[] array:
                    array.CopyTo(Data);
                    break;

                case ArraySegment<T> segment:
                    segment.AsSpan().CopyTo(Data);
                    break;

                case ListOfPrimitivesDeserializer<T> deser:
                    deser.Span.CopyTo(Data);
                    break;

                case ListOfPrimitivesSerializer<T> ser:
                    ser.Data.CopyTo(Data);
                    break;

                default:
                    for (int i = 0; i < items.Count; i++)
                    {
                        this[i] = items[i];
                    }
                    break;
            }
        }

        IEnumerable<T> Enumerate()
        {
            for (int i = 0; i < Count; i++)
                yield return Data[i];
        }

        /// <summary>
        /// Implements <see cref="IEnumerable{T}"/>.
        /// </summary>
        /// <returns></returns>
        public IEnumerator<T> GetEnumerator() => Enumerate().GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }
}