// 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
{
    // Copied from https://github.com/dotnet/corefx/blob/de3902bb56f1254ec1af4bf7d092fc2c048734cc/src/System.IO.Pipelines/src/System/IO/Pipelines/BufferSegment.cs
    internal sealed class BufferSegment : ReadOnlySequenceSegment<byte>
    {
        private object? _memoryOwner;
        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)
        {
            _memoryOwner = arrayPoolBuffer;
            AvailableMemory = arrayPoolBuffer;
        }

        public void ResetMemory()
        {
            Debug.Assert(_memoryOwner != null);

            if (_memoryOwner is IMemoryOwner<byte> owner)
            {
                owner.Dispose();
            }
            else
            {
                byte[] poolArray = (byte[])_memoryOwner;
                ArrayPool<byte>.Shared.Return(poolArray);
            }

            // Order of below field clears is significant as it clears in a sequential order
            // https://github.com/dotnet/corefx/pull/35256#issuecomment-462800477
            Next = null;
            RunningIndex = 0;
            Memory = default;
            _memoryOwner = null;
            _next = null;
            _end = 0;
            AvailableMemory = default;
        }

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

        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)
            {
                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;
        }
    }
}
