//-----------------------------------------------------------------------
// -----------------------------------------------------------------------
// file="SlabAllocator"
// Project: StgSharp
// AuthorGroup: Nitload
// Copyright (c) Nitload. All rights reserved.
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using StgSharp.HighPerformance.Memory;
using System;

namespace StgSharp.Mathematics.Memory
{
    public static class SlabAllocator
    {

        /// <summary>
        ///   Creates a new SLAB allocator instance with the specified configuration.
        /// </summary>
        /// <param name="count">
        ///   Initial number of objects that can be allocated.
        /// </param>
        /// <param name="layout">
        ///   Memory layout strategy for the SLAB allocator.
        /// </param>
        /// <param name="concurrentSupport">
        ///   Whether to enable concurrent access support.
        /// </param>
        /// <returns>
        ///   A new SLAB allocator instance.
        /// </returns>
        /// <exception cref="ArgumentOutOfRangeException">
        ///   Thrown when an invalid layout is specified.
        /// </exception>
        /// <exception cref="NotSupportedException">
        ///   Thrown when requesting unsupported configurations.
        /// </exception>
        /// <remarks>
        ///   <para><strong> Important Threading Considerations: </strong></para> <para>While SLAB
        ///   ///   allocators themselves may be designed for single-threaded use, allocated objects
        ///   are ///   frequently shared across multiple threads in real-world applications. Due to
        ///   the ///   special implementation of expandable concurrent sequential SLAB allocators,
        ///   they rely ///   on internal  expansion locks to ensure that memory buffer expansions
        ///   (which involve ///   copying the entire  buffer to a new location) do not occur while
        ///   other threads are ///   reading from allocated regions. </para> <para><strong>
        ///   Sequential Layout Limitations: ///</strong><br /> Sequential layout SLAB allocators
        ///   cannot provide a single-threaded ///   version because:<list
        ///   type="bullet"><item><description>They use contiguous memory ///   layout that requires
        ///   buffer expansion when capacity is exceeded 
        ///   ///</description></item><item><description>Buffer expansion involves copying all  /// 
        ///   existing data to a new memory location</description></item><item><description> ///  
        ///   Without proper synchronization, concurrent access to allocated objects during  ///  
        ///   expansion would cause memory corruption</description></item><item><description>The 
        ///   ///   expansion lock mechanism is essential even if allocation itself is single- ///  
        ///   threaded </description></item></list></para> <para><strong>Chunked Layout:
        ///   ///</strong><br /> Chunked layout allocators can optionally provide single-threaded
        ///   ///   versions since they don't  require buffer expansion and copying. Each chunk is
        ///   ///   allocated independently, avoiding the  memory relocation issues present in
        ///   sequential ///   layouts. </para>
        /// </remarks>
        public static SlabAllocator<T> Create<T>(
                                       nuint count,
                                       SlabBufferLayout layout,
                                       bool concurrentSupport = true
        ) where T : unmanaged
        {
            if (concurrentSupport) {
                return layout switch
                {
                    SlabBufferLayout.Sequential => new ConcurrentSequentialSlabAllocator<T>(count),
                    SlabBufferLayout.Chunked => new ConcurrentChunkedSlabAllocator<T>(count),
                    _ => throw new ArgumentOutOfRangeException(nameof(layout), layout, null)
                };
            }
            return layout switch
            {
                SlabBufferLayout.Sequential => throw new NotSupportedException("Non-concurrent version of sequential layout SLAB is not supported due to expansion lock requirements for memory safety during buffer expansion."),
                SlabBufferLayout.Chunked => new ChunkedSlabAllocator<T>(count),
                _ => throw new ArgumentOutOfRangeException(nameof(layout), layout, null)
            };
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static SlabAllocator<T> FromBuffer<T>(
                                       nuint buffer,
                                       nuint count,
                                       Action<nuint> freeHandle
        ) where T : unmanaged
        {
            return new FixedSlabAllocator<T>(buffer, count, freeHandle);
        }

    }

    public abstract class SlabAllocator<T> : IDisposable where T : unmanaged
    {

        public abstract nuint Allocate();

        public abstract void Dispose();

        public abstract void EnterBufferReading();

        public abstract void ExitBufferReading();

        public abstract void Free(
                             nuint index
        );

        public abstract SlabAllocationHandle<T> ReadAllocation();

    }

    public enum SlabBufferLayout
    {

        Sequential,
        Chunked

    }
}