//-----------------------------------------------------------------------
// -----------------------------------------------------------------------
// file="HLSFAllocator.SegmentEntry.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 System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.Serialization;
using System.Threading;

namespace StgSharp.HighPerformance.Memory
{
    public unsafe partial class HybridLayerSegregatedFitAllocator
    {

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private void AcquireEntrySpinLock(Entry* entry)
        {
            if (entry == null) {
                return;
            }

            while (Interlocked.CompareExchange(ref entry->SpinLock, Environment.CurrentManagedThreadId, 0) != 0) {
                Thread.Sleep(0);
            }
        }

        /// <summary>
        ///   Determine segment index based on size and level
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private int DetermineSegmentIndex(uint size, int level)
        {
            if (level < 0 || level >= levelSize.Length) {
                return 0;
            }

            int segmentIndex = (int)size / levelSize[level];
            return Math.Min(2, segmentIndex); // Ensure not exceeding maximum segment index
        }

        private void MergeEntry(Entry* remain, Entry* other)
        {
            if (other == null) {
                return;
            }

            // Determine merged starting position and size
            nuint newPosition = remain->Position < other->Position ? other->Position : other->Position;
            uint newSize = remain->BlockSize + other->BlockSize;

            // Update current Entry
            remain->Position = newPosition;
            remain->BlockSize = newSize;
        }

        private void RecycleEntry(Entry* entry)
        {
            if (entry is null) {
                return;
            }
            entry-> PreviousNear = EmptyHandle;
            entry-> Position = EmptyHandle;
            _nodes.Free((nuint)entry);
        }

        private void ReleaseEntrySpinLock(Entry* entry)
        {
            if (entry == null) {
                return;
            }
            _ = Interlocked.CompareExchange(ref entry->SpinLock, 0, Environment.CurrentManagedThreadId);
        }

        /// <summary>
        ///   Remove node from position linked list
        /// </summary>
        private void RemoveFromPositionChain(Entry* entry)
        {
            Entry* prev = (Entry*)entry->PreviousNear;
            Entry* next = (Entry*)entry->NextNear;

            if (prev != null) {
                prev->NextNear = entry->NextNear;
            }
            if (next != null) {
                next->PreviousNear = entry->PreviousNear;
            }

            // Clear node pointers
            entry->NextNear = EmptyHandle;
            entry->PreviousNear = EmptyHandle;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private bool TryAcquireEntrySpinLock(Entry* entry)
        {
            int thread = Environment.CurrentManagedThreadId;
            return entry != null &&
                   (Volatile.Read(ref entry->SpinLock) == thread ||
                    Interlocked.CompareExchange(ref entry->SpinLock, thread, 0) == 0);
        }

        /// <summary>
        ///   Used in Free operations, efficiently remove from buckets and merge adjacent blocks
        /// </summary>
        private Entry* TryMergeAndRemoveFromBuckets(Entry* entry)
        {
            if (entry == null) {
                return null;
            }

            Entry* result = entry;
            uint originalSize = entry->BlockSize;
            int originalLevel = GetLevelFromSize(originalSize);
            int originalSegment = DetermineSegmentIndex(originalSize, originalLevel);

            // Forward merge: check predecessor nodes in position linked list
            Entry* prev = (Entry*)entry->PreviousNear;
            while (prev != null && prev != _spareMemory && prev->State == EntryState.Empty && result->IsAdjacentTo(prev))
            {
                // Try acquire spin lock for prev entry
                if (!TryAcquireEntrySpinLock(prev))
                {
                    break; // Cannot acquire lock, might be operated by other thread
                }

                try
                {
                    // Double-check state after acquiring lock
                    if (prev->State != EntryState.Empty || !result->IsAdjacentTo(prev))
                    {
                        break;
                    }

                    // Calculate prev's bucket position
                    uint prevSize = prev->BlockSize;
                    int prevLevel = GetLevelFromSize(prevSize);
                    int prevSegment = DetermineSegmentIndex(prevSize, prevLevel);

                    // Remove prev node from bucket (O(1) operation!)
                    BucketNode* prevBucketNode = ValidateOrRepairBucketNode(prev);
                    if (prevBucketNode != null && RemoveFromBucket(prevLevel, prevSegment, prevBucketNode))
                    {
                        // Merge into result
                        result->MergeWith(prev);

                        // Remove prev from position linked list
                        RemoveFromPositionChain(prev);

                        // Free prev's Entry node
                        _nodes.Free((nuint)prev);

                        // Continue searching forward
                        prev = (Entry*)result->PreviousNear;
                        continue;
                    }
                }
                finally
                {
                    ReleaseEntrySpinLock(prev);
                }

                break; // Cannot remove, might be operated by other thread
            }

            // Backward merge: check successor nodes in position linked list
            Entry* next = (Entry*)entry->NextNear;
            while (next != null && next->State == EntryState.Empty && result->IsAdjacentTo(next))
            {
                // Try acquire spin lock for next entry
                if (!TryAcquireEntrySpinLock(next))
                {
                    break; // Cannot acquire lock, might be operated by other thread
                }

                try
                {
                    // Double-check state after acquiring lock
                    if (next->State != EntryState.Empty || !result->IsAdjacentTo(next))
                    {
                        break;
                    }

                    // Calculate next's bucket position
                    uint nextSize = next->BlockSize;
                    int nextLevel = GetLevelFromSize(nextSize);
                    int nextSegment = DetermineSegmentIndex(nextSize, nextLevel);

                    // Remove next node from bucket (O(1) operation!)
                    BucketNode* nextBucketNode = ValidateOrRepairBucketNode(next);
                    if (nextBucketNode != null && RemoveFromBucket(nextLevel, nextSegment, nextBucketNode))
                    {
                        // Merge into result
                        result->MergeWith(next);

                        // Remove next from position linked list
                        RemoveFromPositionChain(next);

                        // Free next's Entry node
                        _nodes.Free((nuint)next);

                        // Continue searching backward
                        next = (Entry*)result->NextNear;
                        continue;
                    }
                }
                finally
                {
                    ReleaseEntrySpinLock(next);
                }

                break; // Cannot remove, might be operated by other thread
            }

            return result;
        }

        /// <summary>
        ///   Entry structure - streamlined version (only position and state information) Stored in
        ///   external SLAB
        /// </summary>
        [StructLayout(LayoutKind.Explicit, Size = 40)]
        internal struct Entry
        {

            [FieldOffset(24)] internal int SpinLock;        // Spin lock for thread safety protection
            [FieldOffset(28)] internal int State;           // Node state  
            [FieldOffset(32)] internal uint BlockSize;      // Block size information
            [FieldOffset(36)] internal uint Reverse;
            [FieldOffset(8)]  internal ulong NextNear;      // Position linked list: next pointer
            [FieldOffset(16)] internal ulong PreviousNear;  // Position linked list: previous pointer
            [FieldOffset(0)]  public nuint Position;        // Memory block position

            /// <summary>
            ///   Check if two Entries are adjacent in memory
            /// </summary>
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            public readonly bool IsAdjacentTo(Entry* other)
            {
                if (other == null) {
                    return false;
                }

                // Check if this immediately follows other
                return other->Position + other->BlockSize == this.Position ||
                       this.Position + this.BlockSize == other->Position;
            }

            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            public static bool IsNullOrEmptyIndex(ulong index)
            {
                return index == EmptyHandle;
            }

            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            public static bool IsNullOrEmptyIndex(Entry* entry)
            {
                return entry == null;
            }

            /// <summary>
            ///   Merge two adjacent Entries (caller must ensure they are adjacent and both Empty
            ///   state)
            /// </summary>
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            public void MergeWith(Entry* other) { }

        }

        private static class EntryState
        {

            public const int Allocated = 1;       // Already allocated to user
            public const int Empty = 0;           // Free, can be allocated
            public const int ThreadOccupied = 2;  // Occupied by thread (being operated on)

        }

    }
}