namespace UnmanagedMMU
{
    using System;
    using System.Collections.Generic;
    using System.Runtime.CompilerServices;
    using System.Threading;
    using UnmanagedMMU.Allocators;
    using UnmanagedMMU.Diagnostics;
    using UnmanagedMMU.Handles;
    using UnmanagedMMU.Handles.Internal;


    /// <summary>
    /// Represents configurable alignment requirements for memory segments and allocations.
    /// Values are powers of 2 and reflect common hardware requirements (SIMD, cache lines, native pointer size).
    /// </summary>
    public enum SegmentAlignment
    {
        /// <summary>
        /// 8-byte alignment. Minimum for 64-bit pointers and primitives (long, double).
        /// </summary>
        Aligned8 = 8,

        /// <summary>
        /// 16-byte alignment. Required for Vector128 (SSE/NEON). 
        /// Common default for general-purpose SIMD workloads.
        /// </summary>
        Aligned16 = 16,

        /// <summary>
        /// 32-byte alignment. Required for Vector256 (AVX). 
        /// Recommended default for SIMD-heavy applications.
        /// </summary>
        Aligned32 = 32,

        /// <summary>
        /// 64-byte alignment. Matches standard CPU cache-line size.
        /// Ensures segment bases align to cache line boundaries, minimizing cache-line splits.
        /// </summary>
        Aligned64 = 64,

        /// <summary>
        /// 128-byte alignment.
        /// Advanced optimization for specific cache-aware algorithms or AVX-512 contexts.
        /// </summary>
        Aligned128 = 128
    }


    /// <summary>
    /// Implementation of segmented Bump-Allocator.
    /// 
    /// This implementation manages fixed-sized unmanaged memory segments.
    /// This ensures allocations are fast and contiguous within a segment.
    /// Once a segment is full, a new one is automatically allocated
    /// </summary>
    public unsafe sealed class SegmentedPool : IDisposable, IUnmanagedMemoryOwner
    {
        /// <summary>
        /// The default size for a <see cref="Segment"/>
        /// </summary>
        private const nuint _defaultSegmentSize = 4194304; // 4 MiB

        /// <summary>
        /// The size that a new <see cref="Segment"/> should be
        /// </summary>
        private nuint _currentSegmentSize;

        /// <summary>
        /// The alignment that each new <see cref="Segment"/> should be on
        /// </summary>
        private readonly nuint _segmentAlignment;

        /// <summary>
        /// Queue of free segments
        /// </summary>
        private readonly Stack<IntPtr> _freeSegments = new();

        /// <summary>
        /// List of active memory segments
        /// </summary>
        private readonly List<IntPtr> _activeSegments = [];

        /// <summary>
        /// Tracks the total bytes of memory reserved from the provided <see cref="IUnmanagedAllocator"/>
        /// </summary>
        private nuint _totalReserved = 0;

        /// <summary>
        /// Tracks the total amount of allocated bytes currently in use
        /// </summary>
        private nuint _totalUsed = 0;

        /// <summary>
        /// Internal lock, ensures thread safety while maintaining a simple interface
        /// </summary>
        private readonly Lock _lock = new();

        /// <summary>
        /// Indicates whether this <see cref="SegmentedPool"/> has been disposed.
        /// </summary>
        private volatile bool _disposed;

        /// <summary>
        /// Pointer to the currently in use <see cref="Segment"/>
        /// </summary>
        private Segment* _current;

        /// <summary>
        /// Allocator interface used for all underlying unmanaged memory operations.
        /// </summary>
        private readonly IUnmanagedAllocator _allocator;

        /// <summary>
        /// Indicates if allocations from the pool should be zeroed.
        /// </summary>
        private readonly bool _zeroMemory;

        /// <summary>
        /// Represents a memory segment in the <see cref="SegmentedPool"/>.
        /// </summary>
        /// <remarks>
        /// <list type="bullet">
        ///   <item><description><see cref="Ptr"/> Pointer to the start of the unmanaged memory block</description></item>
        ///   <item><description><see cref="Offset"/> indicates the current allocation position within the segment. Each new allocation advances this offset by the number of bytes allocated.</description></item>
        ///   <item><description><see cref="Size"/> is the total size, in bytes, of the segment.</description></item>
        ///   <item><description>Segments are managed internally by the <see cref="SegmentedPool"/> and should not be modified directly outside of the pool.</description></item>
        ///   <item><description>A <see cref="Segment"/> is a contiguous block of unmanaged memory from which allocations are served sequentially via bump allocation.</description></item>
        /// </list>
        /// </remarks>
        private struct Segment
        {
            /// <summary>
            /// Pointer to the start of the unmanaged memory block.
            /// </summary>
            public byte* Ptr;

            /// <summary>
            /// The current offset into the <see cref="Segment"/> where the next allocation will occur.
            /// </summary>
            public nuint Offset;

            /// <summary>
            /// Total size of the  <see cref="Segment"/> in bytes.
            /// </summary>
            public nuint Size;
        }


        /// <summary>
        /// Readonly snapshot of the current pool state for diagnostics.
        /// </summary>
        public readonly struct PoolState
        {
            /// <summary>
            /// The configured segment alignment setting for this pool instance.
            /// This is the MINIMUM alignment requirement for the segment base.
            /// </summary>
            public nuint SegmentAlignment { get; init; }

            /// <summary>
            /// The size, in bytes, of each segment allocated by the pool.
            /// </summary>
            public nuint SegmentSize { get; init; }

            /// <summary>
            /// Total bytes currently reserved from the system.
            /// </summary>
            public nuint TotalReserved { get; init; }

            /// <summary>
            /// Total bytes currently used by allocations.
            /// </summary>
            public nuint TotalUsed { get; init; }

            /// <summary>
            /// Current active segment base address.
            /// </summary>
            public nuint CurrentBase { get; init; }

            /// <summary>
            /// Current offset within the segment.
            /// </summary>
            public nuint CurrentOffset { get; init; }

            /// <summary>
            /// Checks if the current segment base is aligned to the configured <see cref="SegmentAlignment"/> setting.
            /// </summary>
            public bool BaseAligned { get; init; }

            /// <summary>
            /// Number of active segments currently being used.
            /// </summary>
            public int ActiveSegmentCount { get; init; }

            /// <summary>
            /// Number of recycled segments available for reuse.
            /// </summary>
            public int FreeSegmentCount { get; init; }

            /// <summary>
            /// Total number of segments in the pool (Active + Free).
            /// </summary>
            public int TotalSegmentCount { get; init; }

            /// <summary>
            /// Bytes lost to alignment padding vs actual data in current segment.
            /// Computed as: CurrentOffset - TotalUsedBytes.
            /// </summary>
            public nuint PaddingBytes { get; init; }

            /// <summary>
            /// Memory that could be freed if Trim() is called with default args (minFreeSegments: 16).
            /// </summary>
            public nuint PotentialSavings { get; init; }

            public string Suggestion { get; init; }
        }

        /// <summary>
        /// Readonly snapshot of a specific segment.
        /// </summary>
        public readonly struct SegmentInfo
        {
            /// <summary>
            /// Segment index within the active list.
            /// </summary>
            public int Index { get; init; }

            /// <summary>
            /// Base address of the segment (actual unmanaged memory pointer).
            /// </summary>
            public nuint BaseAddress { get; init; }

            /// <summary>
            /// Current offset usage (bytes used since segment reset).
            /// </summary>
            public nuint UsedBytes { get; init; }

            /// <summary>
            /// Total capacity of the segment.
            /// </summary>
            public nuint Size { get; init; }

            /// <summary>
            /// Indicates if this is the currently active segment.
            /// </summary>
            public bool IsActive { get; init; }

            /// <summary>
            /// The alignment requirement for the segment base itself.
            /// </summary>
            public nuint AlignmentRequirement { get; init; }

            /// <summary>
            /// True if <see cref="BaseAddress"/> is a multiple of <see cref="AlignmentRequirement"/>.
            /// </summary>
            public bool IsAligned { get; init; }

            public override string ToString()
            {
                bool aligned = IsAligned;
                string status = IsActive ? "CURRENT" : "INACTIVE";
                double usagePercent = Size > 0 ? ((double)UsedBytes / Size) * 100 : 0;
                string alignStatus = aligned ? "Aligned" : "Misaligned";

                return $"  Segment #{Index} [{status}]\n" +
                       $"    Base Address:      0x{(nuint)BaseAddress:x}\n" +
                       $"    Size:              {FormatBytes(Size)}\n" +
                       $"    Used:              {FormatBytes(UsedBytes)} ({usagePercent:F2}%)\n" +
                       $"    Base Alignment:    {AlignmentRequirement} bytes - {alignStatus}";
            }

            private static string FormatBytes(nuint bytes)
            {
                if (bytes >= 1073741824) return $"{bytes / 1073741824} GiB";
                if (bytes >= 1048576) return $"{bytes / 1048576} MiB";
                if (bytes >= 1024) return $"{bytes / 1024} KiB";
                return $"{bytes} B";
            }
        }

        /// <summary>
        /// Initializes a new <see cref="SegmentedPool"/> with the specified default <paramref name="segmentSize"/> and <paramref name="initialSegments"/> count
        /// </summary>
        /// <param name="segmentSize">Size of each segment in bytes (default 4 MiB)</param>
        /// <param name="segmentAlignment">Alignment requirement for each allocated segment and . Must be a power of 2 (Default 32)</param>
        /// <param name="initialSegments">Number of segments to pre-allocate to the pool</param>
        /// <param name="zeroMemory">When true, memory returned from the pool is zero-initialized.
        /// When false, memory may contain previously used data and it is the caller's responsibility to clear it if required.</param>
        /// <exception cref="ArgumentException">
        /// Thrown if <paramref name="segmentSize"/> is zero, or if <paramref name="initialSegments"/> is less than 1.
        /// </exception>
        public SegmentedPool(nuint segmentSize = _defaultSegmentSize, SegmentAlignment segmentAlignment = SegmentAlignment.Aligned16, int initialSegments = 4, bool zeroMemory = false)
            : this(segmentSize, segmentAlignment, initialSegments, zeroMemory, new DefaultUnmanagedAllocator())
        {
        }

        /// <summary>
        /// Initializes a new <see cref="SegmentedPool"/> with the specified default <paramref name="segmentSize"/> and <paramref name="initialSegments"/> count
        /// </summary>
        /// <param name="segmentSize">Size of each segment in bytes (default 4 MiB)</param>
        /// <param name="segmentAlignment">Alignment requirement for each allocated segment. Must be a power of 2 (Default 32)</param>
        /// <param name="initialSegments">Number of segments to pre-allocate to the pool</param>
        /// <param name="zeroMemory">When true, memory returned from the pool is zero-initialized.
        /// When false, memory may contain previously used data and it is the caller's responsibility to clear it if required.</param>
        /// <param name="allocator">IUnmanagedAllocator instance that implements the allocator</param>
        /// <exception cref="ArgumentException">
        /// Thrown if <paramref name="segmentSize"/> is zero, or if <paramref name="initialSegments"/> is less than 1.
        /// </exception>
        internal SegmentedPool(nuint segmentSize, SegmentAlignment segmentAlignment, int initialSegments, bool zeroMemory, IUnmanagedAllocator allocator)
        {
            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(segmentSize);
            ArgumentOutOfRangeException.ThrowIfLessThan(initialSegments, 1);
            _allocator = allocator;
            _currentSegmentSize = segmentSize;
            _segmentAlignment = (nuint)segmentAlignment;
            _zeroMemory = zeroMemory;

            Segment* seg;
            // Pre-allocate segments
            for (int i = 0; i < initialSegments; i++)
            {
                seg = AllocateNewSegment(_currentSegmentSize);
                ZeroSegment(seg);
                _freeSegments.Push((IntPtr)seg);
            }
                

            _current = (Segment*)_freeSegments.Pop();
            _activeSegments.Add((IntPtr)_current);
        }

        /// <summary>
        /// Gets the number of free segments  available for use in the pool.
        /// </summary>
        /// <returns>The number of free segments.</returns>
        public int FreeSegmentCount
        {
            get
            {
                lock (_lock)
                {
                    return _freeSegments.Count;
                }
            }
        }

        /// <summary>
        /// Gets the number of segments currently in use in the pool.
        /// </summary>
        /// <returns>The number of currently active Segments.</returns>
        public int ActiveSegmentCount
        {
            get
            {
                lock (_lock)
                {
                    return _activeSegments.Count;
                }
            }
        }

        /// <summary>
        /// Returns the total number of bytes that have currently been allocated.
        /// </summary>
        /// <returns>The total number of bytes that have been allocated.</returns>
        public nuint TotalAllocatedBytes
        {
            get
            {
                lock (_lock)
                {
                    return _totalReserved;
                }
            }
        }

        /// <summary>
        /// Gets the total number of bytes currently in use across all active segments.
        /// </summary>
        /// <returns>The total number of bytes that are in use.</returns>
        public nuint TotalUsedBytes
        {
            get
            {
                lock (_lock)
                {
                    return _totalUsed;
                }
            }
        }

        /// <summary>
        /// Gets the size, in bytes, used when allocating new <see cref="Segment"/> instances.
        /// </summary>
        /// <remarks>
        /// This reflects the most recently configured size and affects only future <see cref="Segment"/> allocations.
        /// </remarks>
        public nuint CurrentSegmentSize
        {
            get { return _currentSegmentSize; }
        }

        /// <summary>
        /// Gets a value indicating whether this <see cref="SegmentedPool"/> instance has been disposed.
        /// </summary>
        /// <returns>True, if the current <see cref="SegmentedPool"/> instance has been disposed of, false otherwise.</returns>
        /// <remarks>
        /// Once disposed, further calls to allocation or reset methods will throw <see cref="ObjectDisposedException"/>.
        /// </remarks>
        public bool IsDisposed
        {
            get { return _disposed; }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static nuint AlignUp(nuint value, nuint alignment)
        {
            return (value + alignment - 1) & ~(alignment - 1);
        }

        /// <summary>
        /// Zeroes the memory <see cref="Segment"></see> if the <see cref="SegmentedPool"/> is configured to do so.
        /// Called whenever a segment becomes active for use (new or reused).
        /// </summary>
        /// <param name="segment">Pointer to the Segment struct to initialize.</param>

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void ZeroSegment(Segment* segment)
        {
            if (_zeroMemory)
            {
                Unsafe.InitBlockUnaligned(segment->Ptr, 0, (uint)segment->Size);
            }
        }

        /// <summary>
        /// Allocates a block of unmanaged memory of size <paramref name="count"/> for elements of type <typeparamref name="T"/> and returns a pointer to the allocated memory.
        /// </summary>
        /// <typeparam name="T">The unmanaged value type to store in the allocated memory. Must be a struct or primitive type.</typeparam>
        /// <param name="count">The number of elements of type <typeparamref name="T"/> to allocate.</param>
        /// <returns>A <typeparamref name="T"/> pointer to the first element of the allocated unmanaged memory block. The memory is valid until the <see cref="SegmentedPool"/> is reset or disposed.</returns>
        /// <remarks>
        /// <list type="bullet">
        ///   <item><description>This allocation is performed in unmanaged memory and bypasses the .NET garbage collector.</description></item>
        ///   <item><description>Accessing the memory after <see cref="Reset"/> or <see cref="Dispose"/> has been called is undefined behavior and may lead to crashes.</description></item>
        /// </list>
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if <paramref name="count"/> is less than or equal to zero.
        /// </exception>
        /// <exception cref="OverflowException">
        /// Thrown if the total allocation size (count * sizeof(T)) exceeds the maximum allowable size.
        /// </exception>
        private T* Alloc<T>(int count) where T : unmanaged
        {
            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(count);
            if ((nuint)count > nuint.MaxValue / (nuint)(sizeof(T)))
            {
                throw new OverflowException($"Requested allocation of {count} elements of type {typeof(T)} exceeds allowable maximum memory size.");
            }
            nuint bytes = (nuint)(count * sizeof(T));
            nuint alignment = _segmentAlignment > (nuint)sizeof(T) ? _segmentAlignment : (nuint)sizeof(T);

            lock (_lock)
            {
                ThrowIfDisposed();
                nuint currentPtr = (nuint)_current->Ptr + _current->Offset;
                nuint alignedPtr = AlignUp(currentPtr, alignment);
                nuint alignedOffset = alignedPtr - (nuint)_current->Ptr;
                // Check space INCLUDING padding
                if (alignedOffset + bytes > _current->Size)
                {
                    SwitchSegment(bytes);

                    // Recalcuate from new the base of the new segment
                    currentPtr = (nuint)_current->Ptr;
                    alignedPtr = AlignUp(currentPtr, alignment);
                    alignedOffset = alignedPtr - (nuint)_current->Ptr;
                }

                T* ptr = (T*)(_current->Ptr + alignedOffset);
                _current->Offset = alignedOffset + bytes;
                _totalUsed += bytes;
                return ptr;
            }
        }

        private T* AllocateWithAlignment<T>(int count, nuint alignment) where T : unmanaged
        {

            ArgumentOutOfRangeException.ThrowIfNegativeOrZero(count);
            if ((nuint)count > nuint.MaxValue / (nuint)(sizeof(T)))
            {
                throw new OverflowException($"Requested allocation of {count} elements of type {typeof(T)} exceeds allowable maximum memory size.");
            }

            nuint bytes = (nuint)(count * sizeof(T));

            lock (_lock)
            {
                ThrowIfDisposed();
                nuint currentPtr = (nuint)_current->Ptr + _current->Offset;
                nuint alignedPtr = AlignUp(currentPtr, alignment);
                nuint alignedOffset = alignedPtr - (nuint)_current->Ptr;

                if (alignedOffset + bytes > _current->Size)
                {
                    SwitchSegment(bytes);
                    currentPtr = (nuint)_current->Ptr;
                    alignedPtr = AlignUp(currentPtr, alignment);
                    alignedOffset = alignedPtr - (nuint)_current->Ptr;
                }

                T* ptr = (T*)(_current->Ptr + alignedOffset);
                _current->Offset = alignedOffset + bytes;
                _totalUsed += bytes;
                return ptr;
            }
        }

        /// <summary>
        /// Switches to a new <see cref="Segment"/> when the current <see cref="Segment"/> is full.
        /// </summary>
        /// <param name="requiredBytes">The number of bytes required for the upcoming allocation. If the current <see cref="Segment"/> does not have enough free space, a new <see cref="Segment"/> will be used.</param>
        private void SwitchSegment(nuint requiredBytes)
        {
            Segment* segment;

            // Allocate fresh Segment if needed
            if (_freeSegments.Count == 0 || requiredBytes > _currentSegmentSize)
            {
                segment = AllocateNewSegment(requiredBytes > _currentSegmentSize ? requiredBytes : _currentSegmentSize);
            }
            else
            {
                segment = (Segment*)_freeSegments.Pop();
                segment->Offset = 0;
            }
            ZeroSegment(segment);
            _activeSegments.Add((IntPtr)segment);
            _current = segment;
        }

        /// <summary>
        /// Allocates a new <see cref="Segment"/>
        /// </summary>
        /// <param name="size"> Size, in bytes, for the new <see cref="Segment"/>. </param>
        /// <returns>A pointer to the newly allocated <see cref="Segment"/></returns>
        private Segment* AllocateNewSegment(nuint size)
        {
            byte* ptr = (byte*)_allocator.AllocAligned(size, _segmentAlignment);
            // Allocate metadata struct with its natural alignment (8 bytes for 64-bit nuint)
            Segment* segment = (Segment*)_allocator.AllocAligned((nuint)sizeof(Segment), 8);
            segment->Ptr = ptr;
            segment->Offset = 0;
            segment->Size = size;

            _totalReserved += size;
            return segment;
        }

        /// <summary>
        /// Sets the current <see cref="Segment"/> size used for subsequent allocations.
        /// </summary>
        /// <param name="newSize">
        /// The new segment size, in bytes, that will be used when allocating future <see cref="Segment"/> instances.
        /// Must be greater than zero.
        /// </param>
        /// <remarks>
        /// This does not affect any <see cref="Segment"/> that have already been allocated or are currently active.
        /// Only new <see cref="Segment"/> created after calling this method will use the updated size.
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if <paramref name="newSize"/> is zero.
        /// </exception>
        public void SetSegmentSize(nuint newSize)
        {
            ThrowIfDisposed();
            if (newSize == 0)
            {
                throw new ArgumentOutOfRangeException(nameof(newSize), "Segment size must be greater than zero.");
            }
            _currentSegmentSize = newSize;
        }

        /// <summary>
        /// Resets the current <see cref="Segment"/> size back to the default 4 MiB
        /// </summary>
        /// <remarks>
        /// Future <see cref="Segment"/> allocations will revert to using this default size 
        /// Active and free <see cref="Segment"/> are not modified.
        /// </remarks>
        public void ResetSegmentSize()
        {
            ThrowIfDisposed();
            _currentSegmentSize = _defaultSegmentSize;
        }

        /// <summary>
        /// Allocates a block of unmanaged memory of size <paramref name="count"/> for elements of type <typeparamref name="T"/> and returns a handle representing the allocation.
        /// </summary>
        /// <typeparam name="T">The unmanaged value type to store in the allocated memory. Must be a struct or primitive type.</typeparam>
        /// <param name="count">The number of elements of type <typeparamref name="T"/> to allocate.</param>
        /// <returns>A <see cref="IMemoryHandle{T}"/> representing the allocated memory. The handle is valid until either <see cref="Reset"/> or <see cref="Dispose"/> is called on this <see cref="SegmentedPool"/>.</returns>
        /// <remarks>
        /// <list type="bullet">
        ///   <item><description>This allocation is performed in unmanaged memory and bypasses the .NET garbage collector.</description></item>
        ///   <item><description>Accessing the memory after <see cref="Reset"/> or <see cref="Dispose"/> has been called is undefined behavior.</description></item>
        /// </list>
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if <paramref name="count"/> is less than or equal to zero.
        /// </exception>
        /// <exception cref="OverflowException">
        /// Thrown if the total allocation size (count * sizeof(T)) exceeds the maximum allowable size.
        /// </exception>
        public IMemoryHandle<T> Allocate<T>(int count) where T : unmanaged
        {
            T* ptr = Alloc<T>(count);
            nuint byteLength = (nuint)count * (nuint)sizeof(T);

            nuint alignment = _segmentAlignment > (nuint)sizeof(T) ? _segmentAlignment : (nuint)sizeof(T);
            return new SegmentedMemoryHandle<T>(ptr, byteLength, alignment, this);
        }

        /// <summary>
        /// Allocates a block of unmanaged memory of size <paramref name="count"/> for elements of type <typeparamref name="T"/> with the specified <paramref name="alignment"/> and returns a handle representing the allocation.
        /// </summary>
        /// <typeparam name="T">The unmanaged value type to store in the allocated memory. Must be a struct or primitive type.</typeparam>
        /// <param name="count">The number of elements of type <typeparamref name="T"/> to allocate.</param>
        /// <param name="alignment">The alignment to aliign the allocation to inside of the currently active <see cref="Segment"/></param>
        /// <returns>A <see cref="IMemoryHandle{T}"/> representing the allocated memory. The handle is valid until either <see cref="Reset"/> or <see cref="Dispose"/> is called on this <see cref="SegmentedPool"/>.</returns>
        /// <remarks>
        /// <list type="bullet">
        ///   <item><description>This allocation is performed in unmanaged memory and bypasses the .NET garbage collector.</description></item>
        ///   <item><description>Accessing the memory after <see cref="Reset"/> or <see cref="Dispose"/> has been called is undefined behavior.</description></item>
        /// </list>
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if <paramref name="count"/> is less than or equal to zero.
        /// </exception>
        /// <exception cref="OverflowException">
        /// Thrown if the total allocation size (count * sizeof(T)) exceeds the maximum allowable size.
        /// </exception>
        public IMemoryHandle<T> AllocateAligned<T>(int count, SegmentAlignment alignment) where T : unmanaged
        {
            nuint requestedAlignment = (nuint)alignment;
            nuint typeSize = (nuint)sizeof(T);
            // Ensure alignment is at least the size of T (never under-align for types)
            nuint effectiveAlignment = requestedAlignment < typeSize ? typeSize : requestedAlignment;

            T* ptr = AllocateWithAlignment<T>(count, effectiveAlignment);
            nuint byteLength = (nuint)count * (nuint)sizeof(T);
            return new SegmentedMemoryHandle<T>(ptr, byteLength, effectiveAlignment, this);

        }

        /// <summary>
        /// Frees unused <see cref="Segment"/> in the free pool, reducing unmanaged memory usage.
        /// </summary>
        /// <param name="minFreeSegments"> Minimum number of free segments to retain for future allocations. Defaults to <c>16</c>. </param>
        /// <remarks>
        /// Segments beyond the retained count will have their unmanaged memory released back to the system.
        /// </remarks>
        /// <exception cref="ArgumentOutOfRangeException">
        /// Thrown if <paramref name="minFreeSegments"/> is negative.
        /// </exception>
        public void Trim(int minFreeSegments = 16)
        {
            ThrowIfDisposed();
            ArgumentOutOfRangeException.ThrowIfNegative(minFreeSegments, nameof(minFreeSegments));
            lock (_lock)
            {
                while (_freeSegments.Count > minFreeSegments)
                {
                    var ip = _freeSegments.Pop();
                    Segment* segment = (Segment*)ip;

                    // Free the unmanaged memory
                    _allocator.FreeAligned(segment->Ptr, _segmentAlignment);
                    _totalReserved -= segment->Size;
                    _allocator.FreeAligned(segment, 8);
                }                
            }
        }

        /// <summary>
        /// Resets the allocator, returning all active segments to the free pool.
        /// </summary>
        /// <param name="trim">If true, trims the <see cref="Segment"/> in the free pool. <see cref="Trim"/></param>
        public void Reset(bool trim = false)
        {
            ThrowIfDisposed();
            lock (_lock)
            {
                foreach (var ip in _activeSegments)
                {
                    Segment* segment = (Segment*)ip;
                    segment->Offset = 0;
                    _freeSegments.Push(ip);

                    // Zero memory if requested
                    ZeroSegment(segment);
                }

                _activeSegments.Clear();
                _totalUsed = 0;

                if (_freeSegments.Count > 0)
                {
                    _current = (Segment*)_freeSegments.Pop();
                    _activeSegments.Add((IntPtr)_current);
                }
                else
                {
                    // This should not be hit in normal circumstances as we always have _current
                    _current = AllocateNewSegment(_currentSegmentSize);
                    _activeSegments.Add((IntPtr)_current);
                    // Zero newly allocated segment if requested
                    ZeroSegment(_current);
                }

                // Optionally trim excess free segments after reset
                if (trim)
                {
                    Trim();
                }
            }            
        }

        /// <summary>
        /// Frees <paramref name="handle"/>
        /// </summary>
        /// <param name="handle"></param>
        /// <exception cref="NotImplementedException"></exception>
        void IUnmanagedMemoryOwner.Free(IOwnedHandle handle)
        {
            ThrowIfDisposed();
            if (handle.Pointer == null)
            {
                return;
            }
            if (handle.GetOwner() != this)
            {
                throw new InvalidOperationException(
                    "Attempted to free a handle from a different allocator pool.");
            }

            //
        }

        /// <summary>
        /// Releases all unmanaged memory allocated by the <see cref="SegmentedPool"/> and clears internal state.
        /// After calling this method, the pool can no longer be used for allocations.
        /// </summary>
        /// <remarks>
        /// After disposal, all allocations become invalid and any further operations will throw <see cref="ObjectDisposedException"/>.
        /// This method is thread-safe and may be called multiple times safely.
        /// </remarks>
        public void Dispose()
        {
            lock (_lock)
            {
                if (_disposed)
                {
                    return;
                }

                // Free active segments
                foreach (var ip in _activeSegments)
                {
                    Segment* segment = (Segment*)ip;
                    _allocator.FreeAligned(segment->Ptr, _segmentAlignment);
                    _allocator.FreeAligned(segment, 8);
                }

                // Free free segments
                foreach (var ip in _freeSegments)
                {
                    Segment* segment = (Segment*)ip;
                    _allocator.FreeAligned(segment->Ptr, _segmentAlignment);
                    _allocator.FreeAligned(segment, 8);
                }

                _activeSegments.Clear();
                _freeSegments.Clear();
                _current = null;
                _totalReserved = 0;
                _totalUsed = 0;
                _disposed = true;
            }
        }

        /// <summary>
        /// Throws an <see cref="ObjectDisposedException"/> if the <see cref="SegmentedPool"/> has already been disposed.
        /// </summary>
        /// <exception cref="ObjectDisposedException">
        /// Thrown when this <see cref="SegmentedPool"/> instance is no longer valid for use.
        /// </exception>
        private void ThrowIfDisposed()
        {
            ObjectDisposedException.ThrowIf(_disposed, this);
        }


        /// <summary>
        /// Gets a snapshot of the current pool state for diagnostics.
        /// Thread-safe and produces no garbage.
        /// </summary>
        /// <returns>A <see cref="PoolState"/> containing current pool metrics.</returns>
        public PoolState GetPoolState()
        {
            ThrowIfDisposed();
            lock (_lock)
            {
                nuint alignment = _segmentAlignment;
                nuint basePtr = (nuint)_current->Ptr;
                int active = _activeSegments.Count;
                int free = _freeSegments.Count;
                int total = active + free;

                // 1. Calculate Padding (Alignment Overhead) for Current Segment
                // This is dynamic: Offset tracks total bytes written, TotalUsed tracks actual allocation bytes
                nuint padding = _current->Offset - _totalUsed;

                // 2. Calculate Potential Savings (Trim Projection)
                nuint potentialSavings = 0;
                if (free > 16)
                {
                    int excess = free - 16;
                    potentialSavings = (nuint)excess * _currentSegmentSize;
                }

                return new PoolState
                {
                    SegmentAlignment = alignment,
                    SegmentSize = _currentSegmentSize,
                    TotalReserved = _totalReserved,
                    TotalUsed = _totalUsed,
                    CurrentBase = basePtr,
                    CurrentOffset = _current->Offset,
                    BaseAligned = (basePtr & (alignment - 1)) == 0,
                    ActiveSegmentCount = active,
                    FreeSegmentCount = free,
                    TotalSegmentCount = total,
                    PaddingBytes = padding,
                    PotentialSavings = potentialSavings
                };
            }
        }


        /// <summary>
        /// Generates a diagnostic report for the pool.
        /// Thread-safe and produces no garbage.
        /// </summary>
        /// <returns>A formatted diagnostic string.</returns>
        public string GetDiagnosticReport()
        {
            PoolState state = GetPoolState();

            DiagnosticConfig config = new()
            {
                SegmentSize = _currentSegmentSize,
                TotalReserved = _totalReserved
            };

            string suggestion = SegmentedPoolDiagnostics.GenerateSuggestions(state, config);
            state = state with { Suggestion = suggestion };

            return SegmentedPoolDiagnostics.GenerateReport(state);
        }

        /// <summary>
        /// Helper method to construct a <see cref="SegmentInfo"/> from raw segment data.
        /// </summary>
        /// <param name="segment">Pointer to the segment to inspect.</param>
        /// <param name="current">Pointer to the currently active segment for comparison.</param>
        /// <param name="index">The logical index of the segment in the list.</param>
        /// <returns>A populated <see cref="SegmentInfo"/> struct.</returns>
        private SegmentInfo CreateSegmentInfo(Segment* segment, Segment* current, int index)
        {
            nuint alignment = _segmentAlignment;
            nuint ptr = (nuint)segment->Ptr;
            bool isAligned = (ptr & (alignment - 1)) == 0;

            return new SegmentInfo
            {
                Index = index,
                BaseAddress = ptr,
                UsedBytes = segment->Offset,
                Size = segment->Size,
                IsActive = (segment == current),
                AlignmentRequirement = alignment,
                IsAligned = isAligned
            };
        }

        /// <summary>
        /// Gets information about the currently active segment.
        /// This is the primary diagnostic view for memory usage within the active segment.
        /// </summary>
        /// <returns>A <see cref="SegmentInfo"/> for the current segment, or null if disposed.</returns>
        public SegmentInfo GetCurrentSegmentInfo()
        {
            ThrowIfDisposed();
            lock (_lock)
            {
                return CreateSegmentInfo(_current, _current, 0);
            }
        }

        /// <summary>
        /// Gets a list of all segment details for deep diagnostics.
        /// Includes both active and free segments.
        /// </summary>
        /// <returns>A list of <see cref="SegmentInfo"/> containing all segments.</returns>
        public List<SegmentInfo> GetAllSegmentInfos()
        {
            ThrowIfDisposed();
            lock (_lock)
            {
                var totalSegments = _activeSegments.Count + _freeSegments.Count;
                var result = new List<SegmentInfo>(totalSegments);

                int currentIndex = 0;

                for (int i = 0; i < _activeSegments.Count; i++)
                {
                    IntPtr ip = _activeSegments[i];
                    Segment* segment = (Segment*)ip;

                    result.Add(CreateSegmentInfo(segment, _current, currentIndex));
                    currentIndex++;
                }

                foreach (var ip in _freeSegments)
                {
                    Segment* segment = (Segment*)ip;
                    result.Add(CreateSegmentInfo(segment, _current, currentIndex++));
                }

                return result;
            }
        }
    }
}