// -----------------------------------------------------------------------------
// file="BufferStack"
// Project: StgSharp
// Copyright (c) Nitload.
// SPDX-License-Identifier: MIT
// -----------------------------------------------------------------------------

using System;
using System.Buffers;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Nitload.Common.HighPerformance.Memory
{
    [CollectionBuilder(typeof(BufferStackBuilder), nameof(BufferStackBuilder.Create))]
    public unsafe class BufferStack<T> : IDisposable where T : unmanaged
    {

        private byte* _buffer;
        private ulong _count;

        public BufferStack(
               int capacity
        )
        {
            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(capacity);

            AllocateBuffer(capacity);
        }

        public int Count => (int)_count;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Clear()
        {
            ((Header*)_buffer)->Top = 0;
            _count = 0;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Dispose()
        {
            Marshal.FreeHGlobal((nint)_buffer);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public T Pop()
        {
            Header* header = (Header*)_buffer;
            if (header->Top == 0) {
                throw new InvalidOperationException("Stack underflow");
            }

            header->Top--;
            _count--;
            T* dataStart = (T*)(_buffer + sizeof(Header));
            return dataStart[header->Top];
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public void Push(
                    T item
        )
        {
            Header* header = (Header*)_buffer;
            if (header->Top >= header->Capacity)
            {
                Expand();
                header = (Header*)_buffer;
            }

            T* dataStart = (T*)(_buffer + sizeof(Header));
            dataStart[header->Top] = item;
            header->Top++;
            _count++;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public bool TryPop(
                    out T value
        )
        {
            Header* header = (Header*)_buffer;
            if (header->Top == 0)
            {
                value = default;
                return false;
            }
            header->Top--;
            _count--;
            T* dataStart = (T*)(_buffer + sizeof(Header));
            value = dataStart[header->Top];
            return true;
        }

        /// <summary>
        ///   Fills the stack in place via a caller-supplied callback, mirroring <c>
        ///   CapacityFixedStack&lt;T&gt;.FillRange </c>.
        /// </summary>
        /// <remarks>
        ///   Preferred over building a temporary span and handing it to <see
        ///   cref="BufferStackBuilder.Create{T}(ReadOnlySpan{T})" />: <list type="bullet"> ///
        ///   <item> The caller no longer needs a scratch buffer at all — the /// callback writes
        ///   straight into this stack's native storage, so there /// is no <c> stackalloc </c> to
        ///   size and no copy afterwards. </item> /// <item><c> Span&lt;T&gt; </c> does not
        ///   implicitly convert to /// <c> ReadOnlySpan&lt;T&gt; </c> in a way that participates in
        ///   generic /// type inference before C# 14, so <c> Create(span) </c> fails with ///
        ///   CS0411 on older compilers. Passing the element type through the /// enclosing instance
        ///   sidesteps inference entirely. </item> ///</list>
        /// </remarks>
        internal unsafe void FillRange(
                             SpanAction<T, nuint> filling,
                             nuint buffer
        )
        {
            if (filling is null) {
                return;
            }

            Header* header = (Header*)_buffer;
            T* dataStart = (T*)(_buffer + sizeof(Header));

            // Hand out only the unused tail, matching CapacityFixedStack:
            // filling is an append, not a rewrite of live entries.
            int start = (int)header->Top;
            int room = (int)header->Capacity - start;
            if (room <= 0) {
                return;
            }

            filling(new Span<T>(dataStart + start, room), buffer);

            header->Top = header->Capacity;
            _count = header->Capacity;
        }

        // Internal method for efficient bulk initialization
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal unsafe void InitializeFromSpan(
                             ReadOnlySpan<T> span
        )
        {
            if (span.IsEmpty) {
                return;
            }

            Header* header = (Header*)_buffer;
            T* dataStart = (T*)(_buffer + sizeof(Header));

            fixed (T* sourcePtr = span) {
                Buffer.MemoryCopy(sourcePtr, dataStart, header->Capacity * (nuint)sizeof(T),
                                  (nuint)span.Length * (nuint)sizeof(T));
            }

            header->Top = (nuint)span.Length;
            _count = (ulong)span.Length;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void AllocateBuffer(
                     int capacity
        )
        {
            nuint size = (nuint)sizeof(Header) + ((nuint)capacity * (nuint)sizeof(T));
            _buffer = (byte*)NativeMemory.Alloc(size);
            Header* header = (Header*)_buffer;
            header->Top = 0;
            header->Capacity = (nuint)capacity;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void Expand()
        {
            Header* header = (Header*)_buffer;
            nuint newCap = header->Capacity * 2;
            nuint newSize = (nuint)sizeof(Header) + (newCap * (nuint)sizeof(T));
            _buffer = (byte*)NativeMemory.Realloc(_buffer, newSize);
            header = (Header*)_buffer;
            header->Capacity = newCap;
        }

        private struct Header
        {

            public nuint Capacity;
            public nuint Top;

        }

    }
}