//-----------------------------------------------------------------------
// -----------------------------------------------------------------------
// file="SlabAllocator.Chunked.cs"
// Project: StgSharp
// AuthorGroup: Nitload Space
// Copyright (c) Nitload Space. All rights reserved.
//     
// Permission is hereby granted, free of charge, to any person 
// obtaining a copy of this software and associated documentation 
// files (the “Software”), to deal in the Software without restriction, 
// including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, 
// and to permit persons to whom the Software is furnished to do so, 
// subject to the following conditions:
//     
// The above copyright notice and 
// this permission notice shall be included in all copies 
// or substantial portions of the Software.
//     
// THE SOFTWARE IS PROVIDED “AS IS”, 
// WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, 
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, 
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, 
// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, 
// ARISING FROM, OUT OF OR IN CONNECTION WITH 
// THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
//     
// -----------------------------------------------------------------------
// -----------------------------------------------------------------------
using StgSharp.Threading;

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

namespace StgSharp.HighPerformance.Memory
{
    internal sealed unsafe class ChunkedSlabAllocator<T> : SlabAllocator<T> where T: unmanaged
    {

        private readonly BufferExpansionLock _lock = new();
        private BufferStack<nuint> _buffers;
        private BufferStack<nuint> _recycle;
        private readonly int _elementSize = Unsafe.SizeOf<T>();

        private nuint _currentBuffer;
        private nuint _currentCapacity, _currentIndex;

        public ChunkedSlabAllocator(nuint count)
        {
            _currentBuffer = (nuint)NativeMemory.Alloc(count * (nuint)_elementSize);
            _currentCapacity = count;
            int initCount = count switch
            {
                <= 4 => 4,
                > 64 => 64,
                _ => (int)count
            };
            Span<nuint> span = stackalloc nuint[initCount];
            for (int i = 0; i < initCount; i++) {
                span[i] = _currentBuffer + (nuint)(i * _elementSize);
            }
            _buffers = new(4);
            _recycle = BufferStackBuilder.Create(span);
            _currentCapacity = count;
            _currentIndex = (nuint)initCount;
        }

        public override nuint Allocate()
        {
            if (_recycle.TryPop(out nuint handle)) {
                return handle;
            }
            while (true)
            {
                nuint bufferHandle = Volatile.Read(ref _currentBuffer);
                nuint curIndex = Volatile.Read(ref _currentIndex);
                _lock.EnterMetaDataRead();
                try
                {
                    if (curIndex >= _currentCapacity)
                    {
                        _lock.ExitMetaDataRead();
                        _lock.EnterExpansionProcess();
                        try
                        {
                            nuint cap = Volatile.Read(ref _currentCapacity);
                            nuint newBuffer = (nuint)NativeMemory.Alloc(cap * (nuint)_elementSize);
                            _buffers.Push(bufferHandle);
                            _ = Interlocked.Exchange(ref _currentBuffer, newBuffer);
                            _ = Interlocked.Exchange(ref _currentIndex, 0);
                        }
                        finally
                        {
                            _lock.ExitExpansionProcess();
                        }
                    }
                    curIndex = Volatile.Read(ref _currentIndex);
                    _ = Interlocked.Add(
                        ref Unsafe.As<nuint, ulong>(ref _currentIndex), (ulong)_elementSize);
                    return _currentBuffer + ((nuint)_elementSize * curIndex);
                }
                finally
                {
                    if (_lock.IsThreadReadingMetaData) {
                        _lock.ExitMetaDataRead();
                    }
                }
            }
        }

        public override void Dispose()
        {
            NativeMemory.Free((void*)_currentBuffer);
            while (_buffers.TryPop(out nuint buffer)) {
                NativeMemory.Free((void*)buffer);
            }
            _lock.Dispose();
            _buffers.Dispose();
            _recycle.Dispose();
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override void EnterBufferReading() { }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override void ExitBufferReading() { }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override void Free(nuint index)
        {
            _recycle.Push(index);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override SlabAllocationHandle<T> ReadAllocation()
        {
            return new SlabAllocationHandle<T>(this, false);
        }

        ~ChunkedSlabAllocator()
        {
            Dispose();
        }

    }
}