// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Buffers;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace System.IO.Pipelines
{
    internal sealed class BufferSegment : ReadOnlySequenceSegment<byte>
    {
        private IMemoryOwner<byte>? _memoryOwner;
        private byte[]? _array;
        private BufferSegment? _next;
        private int _end;

        /// <summary>
        /// The End represents the offset into AvailableMemory where the range of "active" bytes ends. At the point when the block is leased
        /// the End is guaranteed to be equal to Start. The value of Start may be assigned anywhere between 0 and
        /// Buffer.Length, and must be equal to or less than End.
        /// </summary>
        public int End
        {
            get => _end;
            set
            {
                Debug.Assert(value <= AvailableMemory.Length);

                _end = value;
                Memory = AvailableMemory.Slice(0, value);
            }
        }

        /// <summary>
        /// Reference to the next block of data when the overall "active" bytes spans multiple blocks. At the point when the block is
        /// leased Next is guaranteed to be null. Start, End, and Next are used together in order to create a linked-list of discontiguous
        /// working memory. The "active" memory is grown when bytes are copied in, End is increased, and Next is assigned. The "active"
        /// memory is shrunk when bytes are consumed, Start is increased, and blocks are returned to the pool.
        /// </summary>
        public BufferSegment? NextSegment
        {
            get => _next;
            set
            {
                Next = value;
                _next = value;
            }
        }

        public void SetOwnedMemory(IMemoryOwner<byte> memoryOwner)
        {
            _memoryOwner = memoryOwner;
            AvailableMemory = memoryOwner.Memory;
        }

        public void SetOwnedMemory(byte[] arrayPoolBuffer)
        {
            _array = arrayPoolBuffer;
            AvailableMemory = arrayPoolBuffer;
        }

        // Resets memory and internal state, should be called when removing the segment from the linked list
        public void Reset()
        {
            ResetMemory();

            Next = null;
            RunningIndex = 0;
            _next = null;
        }

        // Resets memory only, should be called when keeping the BufferSegment in the linked list and only swapping out the memory
        public void ResetMemory()
        {
            IMemoryOwner<byte>? memoryOwner = _memoryOwner;
            if (memoryOwner != null)
            {
                _memoryOwner = null;
                memoryOwner.Dispose();
            }
            else
            {
                Debug.Assert(_array != null);
                ArrayPool<byte>.Shared.Return(_array);
                _array = null;
            }


            Memory = default;
            _end = 0;
            AvailableMemory = default;
        }

        // Exposed for testing
        internal object? MemoryOwner => (object?)_memoryOwner ?? _array;

        public Memory<byte> AvailableMemory { get; private set; }

        public int Length => End;

        public int WritableBytes
        {
            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            get => AvailableMemory.Length - End;
        }

        public void SetNext(BufferSegment segment)
        {
            Debug.Assert(segment != null);
            Debug.Assert(Next == null);

            NextSegment = segment;

            segment = this;

            while (segment.Next != null)
            {
                Debug.Assert(segment.NextSegment != null);
                segment.NextSegment.RunningIndex = segment.RunningIndex + segment.Length;
                segment = segment.NextSegment;
            }
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static long GetLength(BufferSegment startSegment, int startIndex, BufferSegment endSegment, int endIndex)
        {
            return (endSegment.RunningIndex + (uint)endIndex) - (startSegment.RunningIndex + (uint)startIndex);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static long GetLength(long startPosition, BufferSegment endSegment, int endIndex)
        {
            return (endSegment.RunningIndex + (uint)endIndex) - startPosition;
        }
    }
}