using Misaki.HighPerformance.LowLevel.Buffer;
using Misaki.HighPerformance.LowLevel.Collections.Contracts;
using Misaki.HighPerformance.LowLevel.Utilities;
using System.Collections;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace Misaki.HighPerformance.LowLevel.Collections;

internal class UnsafeListDebugView<T>
    where T : unmanaged
{
    private readonly UnsafeList<T> _list;
    public UnsafeListDebugView(UnsafeList<T> list)
    {
        _list = list;
    }

    [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
    public T[] Items
    {
        get
        {
            var array = new T[_list.Count];
            for (var i = 0; i < _list.Count; i++)
            {
                array[i] = _list[i];
            }
            return array;
        }
    }
}

/// <summary>
/// A collection that allows for unsafe operations on a list of unmanaged types.
/// </summary>
/// <typeparam name="T">Represents a type that can be stored in the collection, constrained to unmanaged types for performance and safety.</typeparam>
[DebuggerTypeProxy(typeof(UnsafeListDebugView<>))]
public unsafe struct UnsafeList<T> : IUnsafeCollection<T>
    where T : unmanaged
{
    public struct Enumerator : IEnumerator<T>
    {
        private readonly UnsafeList<T>* _collection;
        private int _index;

        public readonly ref T Current => ref _collection->_array[_index];
        readonly T IEnumerator<T>.Current => Current;
        readonly object IEnumerator.Current => Current;

        public Enumerator(UnsafeList<T>* collection)
        {
            _collection = collection;
            _index = -1;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool MoveNext()
        {
            _index++;
            return _index < _collection->_count;
        }

        public void Reset()
        {
            _index = -1;
        }

        public readonly void Dispose()
        {
        }
    }

    /// <summary>
    /// A Parallel reader for an UnsafeList.
    /// </summary>
    /// <remarks>
    /// Use <see cref="AsParallelReader"/> to create a parallel reader for a list.
    /// The list must live at least as long as the parallel reader, and the parallel reader must not be used after the list is disposed.
    /// </remarks>
    public readonly unsafe struct ParallelReader
    {
        public readonly UnsafeList<T>* listData;
        public readonly int Count => listData->_count;

        public ref readonly T this[int index]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => ref listData->_array[index];
        }

        public ref readonly T this[uint index]
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => ref listData->_array[index];
        }

        internal ParallelReader(UnsafeList<T>* list)
        {
            listData = list;
        }

        public readonly Enumerator GetEnumerator()
        {
            return new Enumerator(listData);
        }

        public readonly ReadOnlySpan<T> AsSpan()
        {
            return new ReadOnlySpan<T>(listData->_array.GetUnsafePtr(), listData->_count);
        }
    }

    /// <summary>
    /// A parallel writer for an UnsafeList.
    /// </summary>
    /// <remarks>
    /// Use <see cref="AsParallelWriter"/> to create a parallel writer for a list.
    /// The list must live at least as long as the parallel writer, and the parallel writer must not be used after the list is disposed.
    /// </remarks>
    public readonly struct ParallelWriter
    {
        public readonly UnsafeList<T>* listData;

        internal ParallelWriter(UnsafeList<T>* list)
        {
            listData = list;
        }

        /// <summary>
        /// Adds a value to a collection without resizing it, ensuring capacity is checked before insertion.
        /// </summary>
        /// <param name="value">The value to be added to the collection.</param>
        public void AddNoResize(T value)
        {
            var idx = Interlocked.Increment(ref listData->_count) - 1;
            listData->CheckNoResizeCapacity(idx, 1);
            UnsafeUtility.WriteArrayElement(listData->_array.GetUnsafePtr(), idx, value);
        }

        /// <summary>
        /// Adds a specified number of elements from a pointer to a buffer without resizing the underlying storage.
        /// </summary>
        /// <param name="ptr">Points to the source data to be copied into the buffer.</param>
        /// <param name="count">Indicates the number of elements to be added from the source data.</param>
        public void AddRangeNoResize(ReadOnlySpan<T> collection, int count)
        {
            var index = Interlocked.Add(ref listData->_count, count) - count;
            listData->CheckNoResizeCapacity(index, count);

            fixed (T* pCollection = collection)
            {
                MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(listData->_array.GetUnsafePtr(), index), pCollection, (uint)(count * sizeof(T)));
            }
        }
    }

    private UnsafeArray<T> _array;

    private int _count;

    public readonly int Count => _count;
    public readonly int Capacity => _array.Count;
    public readonly bool IsCreated => _array.IsCreated;

    public readonly ref T this[int index]
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get => ref _array[index];
    }

    public readonly ref T this[uint index]
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get => ref _array[index];
    }

    /// <summary>
    /// Invalid constructor, use <see cref="UnsafeList(int, Allocator, AllocationOption)"/> or <see cref="UnsafeList(int, AllocationHandle, AllocationOption)"/> instead.
    /// </summary>
    public UnsafeList()
        : this(0, Allocator.Invalid)
    {
    }

    /// <summary>
    /// Initializes a new instance of UnsafeList with a specified number of initial capacity and an allocation handle.
    /// </summary>
    /// <param name="capacity">Specifies the number of initial capacity to allocate in the list, which must be greater than zero.</param>
    /// <param name="handle">A reference to an AllocationHandle that manages the memory allocation for the array.</param>
    /// <param name="allocationOption">Specifies how the memory should be allocated.</param>
    public UnsafeList(int capacity, AllocationHandle handle, AllocationOption allocationOption = AllocationOption.None)
    {
        _array = new UnsafeArray<T>(capacity, handle, allocationOption);
        _count = 0;
    }

    /// <summary>
    /// Initializes a new instance of UnsafeList with a specified number of initial capacity and an allocation type.
    /// </summary>
    /// <param name="capacity">Specifies the number of initial capacity to allocate in the list, which must be greater than zero.</param>
    /// <param name="allocator">Specifies the allocator to use for memory allocation, which determines the memory management strategy.</param>
    /// <param name="allocationOption">Determines how the memory should be allocated.</param>
    public UnsafeList(int capacity, Allocator allocator, AllocationOption allocationOption = AllocationOption.None)
        : this(capacity, AllocationManager.GetAllocationHandle(allocator), allocationOption)
    {
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    [Conditional("MHP_ENABLE_SAFETY_CHECKS")]
    private readonly void CheckNoResizeCapacity(int count)
    {
        CheckNoResizeCapacity(count, Count);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    [Conditional("MHP_ENABLE_SAFETY_CHECKS")]
    private readonly void CheckNoResizeCapacity(int index, int count)
    {
        if (index + count > Capacity)
        {
            throw new Exception($"AddNoResize assumes that list capacity is sufficient (Capacity {Capacity}, Size {Count}), requested count {count}!");
        }
    }

    private readonly void CheckIndexCount(int index, int count)
    {
        if (count < 0)
        {
            throw new ArgumentOutOfRangeException($"Value for count {count} must be positive.");
        }

        if (index < 0)
        {
            throw new ArgumentOutOfRangeException($"Value for index {index} must be positive.");
        }

        if (index > Count)
        {
            throw new ArgumentOutOfRangeException($"Value for index {index} is out of bounds.");
        }

        if (index + count > Count)
        {
            throw new ArgumentOutOfRangeException($"Value for count {count} is out of bounds.");
        }
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public Enumerator GetEnumerator()
    {
        return new((UnsafeList<T>*)UnsafeUtility.AddressOf(ref this));
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    IEnumerator<T> IEnumerable<T>.GetEnumerator()
    {
        return GetEnumerator();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    /// <summary>
    /// Provides a parallel reader for the current list, enabling thread-safe read operations.
    /// </summary>
    /// <remarks>
    /// The list must live at least as long as the parallel reader, and the parallel reader must not be used after the list is disposed.
    /// For example, if you need to access the list in job system and wait that job in another stack frame, please always allocate the list struct itself on heap.
    /// Otherwise the parallel reader will be invalid after the stack frame that creates the list is popped, even if the list's internal array is still valid.
    /// </remarks>
    /// <returns>A <see cref="ParallelReader"/> instance that can be used to read items from the list in a thread-safe manner.</returns>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ParallelReader AsParallelReader()
    {
        return new((UnsafeList<T>*)UnsafeUtility.AddressOf(ref this));
    }

    /// <summary>
    /// Provides a parallel writer for the current list, enabling thread-safe additions to the list.
    /// </summary>
    /// <remarks>
    /// The list must live at least as long as the parallel writer, and the parallel writer must not be used after the list is disposed.
    /// For example, if you need to access the list in job system and wait that job in another stack frame, please always allocate the list struct itself on heap.
    /// Otherwise the parallel writer will be invalid after the stack frame that creates the list is popped, even if the list's internal array is still valid.
    /// </remarks>
    /// <returns>A <see cref="ParallelWriter"/> instance that can be used to add items to the list in a thread-safe manner.</returns>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public ParallelWriter AsParallelWriter()
    {
        return new((UnsafeList<T>*)UnsafeUtility.AddressOf(ref this));
    }

    /// <summary>
    /// Converts the current list to an UnsafeArray representation.
    /// </summary>
    /// <remarks>
    /// The returned <see cref="UnsafeArray{T}"/> shares the same underlying data as the list and does not own the memory.
    /// </remarks>
    /// <returns>A new <see cref="UnsafeArray{T}"/> instance.</returns>
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly UnsafeArray<T> AsUnsafeArray()
    {
        return new UnsafeArray<T>((T*)_array.GetUnsafePtr(), _count);
    }

    /// <summary>
    /// Converts the current list to a read-only collection that provides unsafe access to its elements.
    /// </summary>
    /// <returns>A new <see cref="ReadOnlyUnsafeCollection{T}"/> instance that allows for read-only access to the list's elements without copying.</returns>
    public readonly ReadOnlyUnsafeCollection<T> AsReadOnly()
    {
        return new ReadOnlyUnsafeCollection<T>((T*)_array.GetUnsafePtr(), _count);
    }

    /// <summary>
    /// Adds a new element to the end of the list, resizing the internal array if necessary.
    /// </summary>
    /// <param name="value">The element to be added to the list.</param>
    public void Add(T value)
    {
        if (_count >= Capacity)
        {
            Resize(Math.Max(1, Capacity * 2));
        }

        UnsafeUtility.WriteArrayElement(_array.GetUnsafePtr(), _count, value);
        _count++;
    }

    /// <summary>
    /// Adds the specified value to the collection without resizing the underlying storage.
    /// </summary>
    /// <param name="value">The value to add to the collection.</param>
    public void AddNoResize(T value)
    {
        CheckNoResizeCapacity(1);

        UnsafeUtility.WriteArrayElement(_array.GetUnsafePtr(), _count, value);
        _count++;
    }

    /// <summary>
    /// Adds a range of elements to the collection.
    /// </summary>
    /// <param name="values">A span containing the elements to add.</param>
    public void AddRange(Span<T> values)
    {
        var newSize = _count + values.Length;
        if (newSize > Capacity)
        {
            Resize(Capacity + values.Length);
        }

        fixed (T* ptr = values)
        {
            MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), _count), ptr, (uint)(values.Length * sizeof(T)));
        }

        _count += values.Length;
    }

    /// <summary>
    /// Adds a range of elements from a pointer to the collection.
    /// </summary>
    /// <param name="ptr">Points to the source data to be copied into the collection.</param>
    /// <param name="count">Indicates the number of elements to be added from the source data.</param>
    public void AddRange(T* ptr, int count)
    {
        var newSize = _count + count;
        if (newSize > Capacity)
        {
            Resize(Capacity + count);
        }

        MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), _count), ptr, (uint)(count * sizeof(T)));
        _count += count;
    }

    /// <summary>
    /// Adds the elements of the specified collection to the current list without resizing the underlying storage.
    /// </summary>
    /// <param name="collection">A read-only span containing the elements to add. The span must not exceed the available capacity.</param>
    public void AddRangeNoResize(ReadOnlySpan<T> collection)
    {
        CheckNoResizeCapacity(collection.Length);

        fixed (T* pCollection = collection)
        {
            MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), _count), pCollection, (uint)(collection.Length * sizeof(T)));
        }

        _count += collection.Length;
    }

    /// <summary>
    /// Adds a range of elements from a pointer to the collection without resizing the underlying storage.
    /// </summary>
    /// <param name="ptr">Points to the source data to be copied into the collection.</param>
    /// <param name="count">Indicates the number of elements to be added from the source data.</param>
    public void AddRangeNoResize(T* ptr, int count)
    {
        CheckNoResizeCapacity(count);

        MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), _count), ptr, (uint)(count * sizeof(T)));
        _count += count;
    }

    /// <summary>
    /// Removes a range of elements from the list starting at the specified index.
    /// </summary>
    /// <param name="start">The zero-based index at which to start removing elements.</param>
    /// <param name="length">The number of elements to remove.</param>
    public void RemoveRange(int start, int length)
    {
        CheckIndexCount(start, length);

        if (length <= 0)
        {
            return;
        }

        var copyFrom = Math.Min(start + length, _count);
        MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), start),
            UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), copyFrom),
            (uint)((_count - copyFrom) * sizeof(T))
        );
        _count -= length;
    }

    /// <summary>
    /// Removes the element at the specified index from the collection.
    /// </summary>
    /// <param name="index">The zero-based index of the element to remove.</param>
    public void RemoveAt(int index)
    {
        RemoveRange(index, 1);
    }

    /// <summary>
    /// Removes a range of elements from the list starting at the specified index by swapping them with the last elements.
    /// </summary>
    /// <param name="start">The zero-based index at which to start removing elements.</param>
    /// <param name="length">The number of elements to remove.</param>
    public void RemoveRangeSwapBack(int start, int length)
    {
        CheckIndexCount(start, length);

        if (length <= 0)
        {
            return;
        }

        var copyFrom = Math.Min(_count - length, start + length);
        MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), start),
            UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), copyFrom),
            (uint)((_count - copyFrom) * sizeof(T))
        );
        _count -= length;
    }

    /// <summary>
    /// Removes the element at the specified index by swapping it with the last element and reducing the collection
    /// size.
    /// </summary>
    /// <param name="index">The zero-based index of the element to remove. Must be within the bounds of the collection.</param>
    public void RemoveAtSwapBack(int index)
    {
        RemoveRangeSwapBack(index, 1);
    }

    public void Resize(int newSize, AllocationOption option = AllocationOption.None)
    {
        _array.Resize(newSize, option);

        if (_count > newSize)
        {
            _count = newSize;
        }
    }

    /// <summary>
    /// Sets the count of the collection to a new value without modifying the underlying storage.
    /// </summary>
    /// <remarks>
    /// This method will not initialize new elements, so it should be used with caution. The new count must be between 0 and the current capacity of the collection.
    /// </remarks>
    /// <param name="newCount">The new count value to set for the collection.</param>
    /// <exception cref="ArgumentOutOfRangeException">Thrown when the new count is outside the valid range.</exception>
    public void UnsafeSetCount(int newCount)
    {
        if (newCount < 0 || newCount > Capacity)
        {
            throw new ArgumentOutOfRangeException(nameof(newCount), $"Value for newCount {newCount} must be between 0 and Capacity {Capacity}.");
        }

        _count = newCount;
    }

    public void Clear()
    {
        _count = 0;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly void* GetUnsafePtr()
    {
        return _array.GetUnsafePtr();
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly Span<T> AsSpan()
    {
        return _array.AsSpan(0, _count);
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public readonly Span<T> AsSpan(int start, int length)
    {
        CheckIndexCount(start, length);
        return _array.AsSpan(start, length);
    }


    /// <summary>
    /// Copies elements from a source UnsafeCollection to a destination Span, ensuring both have the same size.
    /// </summary>
    /// <param name="destination">Represents the target span where elements are copied to.</param>
    public readonly void CopyTo(Span<T> destination)
    {
        var size = Math.Min(destination.Length, Count);
        fixed (T* pDest = destination)
        {
            MemCpy(pDest, _array.GetUnsafePtr(), (uint)(size * sizeof(T)));
        }
    }

    /// <summary>
    /// Copies a range of elements from a source collection to a destination span, ensuring both are adequately sized.
    /// </summary>
    /// <param name="destination">The span where the elements will be copied to.</param>
    /// <param name="sourceIndex">The starting index in the source collection for the copy operation.</param>
    /// <param name="destinationIndex">The starting index in the destination span where the elements will be placed.</param>
    /// <param name="length">The number of elements to copy from the source to the destination.</param>
    /// <exception cref="ArgumentOutOfRangeException">Thrown when the specified range exceeds the bounds of the source collection or destination span.</exception>
    public readonly void CopyTo(Span<T> destination, int sourceIndex, int destinationIndex, int length)
    {
        if (sourceIndex + length > _count || destinationIndex + length > destination.Length)
        {
            throw new ArgumentOutOfRangeException(nameof(length), "Source collection or destination span is too small for the specified range.");
        }

        fixed (T* pDest = destination)
        {
            MemCpy(pDest + destinationIndex, (byte*)_array.GetUnsafePtr() + sourceIndex * sizeof(T), (nuint)(length * sizeof(T)));
        }
    }

    /// <summary>
    /// Copies elements from a source span to a destination unsafe collection, ensuring both have the same size.
    /// </summary>
    /// <param name="source">Represents the span containing the elements to be copied to the unsafe collection.</param>
    public void CopyFrom(ReadOnlySpan<T> source)
    {
        if (_count < source.Length)
        {
            Resize(source.Length);
        }

        fixed (T* pSrc = source)
        {
            MemCpy(_array.GetUnsafePtr(), pSrc, (nuint)(source.Length * sizeof(T)));
        }
    }

    /// <summary>
    /// Copies a specified range of elements from a source span to a destination collection.
    /// </summary>
    /// <param name="source">The span containing the elements to be copied.</param>
    /// <param name="sourceIndex">The starting index in the source span from which to begin copying.</param>
    /// <param name="destinationIndex">The starting index in the destination collection where the elements will be placed.</param>
    /// <param name="length">The number of elements to copy from the source span to the destination collection.</param>
    /// <exception cref="ArgumentOutOfRangeException">Thrown when the specified range exceeds the bounds of the source span or destination collection.</exception>
    public void CopyFrom(ReadOnlySpan<T> source, int sourceIndex, int destinationIndex, int length)
    {
        if (sourceIndex + length > source.Length)
        {
            throw new ArgumentOutOfRangeException(nameof(length), "Source span or destination collection is too small for the specified range.");
        }

        if (destinationIndex + length > _count)
        {
            Resize(destinationIndex + length);
        }

        fixed (T* pSrc = source)
        {
            MemCpy((byte*)_array.GetUnsafePtr() + destinationIndex * sizeof(T), pSrc + sourceIndex, (nuint)(length * sizeof(T)));
        }
    }

    /// <summary>
    /// Creates a new <see cref="List{T}"/> containing the elements.
    /// </summary>
    /// <returns>A <see cref="List{T}"/> containing all elements.</returns>
    public readonly List<T> ToList()
    {
        var list = new List<T>(_count);
        var span = new Span<T>(_array.GetUnsafePtr(), _count);
        list.AddRange(span);
        return list;
    }

    public void Dispose()
    {
        _array.Dispose();
        _count = 0;
    }

    public static implicit operator ReadOnlyUnsafeCollection<T>(UnsafeList<T> list)
    {
        return list.AsReadOnly();
    }

    public static implicit operator Span<T>(UnsafeList<T> list)
    {
        return list.AsSpan();
    }
}