// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
using System;
using System.Buffers;
using System.Diagnostics;
using System.Runtime.CompilerServices;
 
namespace Microsoft.Extensions.Caching.Hybrid.Internal;
 
// Used to convey buffer status; like ArraySegment<byte>, but Offset is always
// zero, and we use the most significant bit of the length (usually the sign flag,
// but we do not need to support negative length) to track whether or not
// to recycle this value.
internal readonly struct BufferChunk
{
    private const int FlagReturnToPool = (1 << 31);
    private readonly int _lengthAndPoolFlag;
 
    public byte[]? OversizedArray { get; } // null for default
 
    public bool HasValue => OversizedArray is not null;
 
    public int Offset { get; }
    public int Length => _lengthAndPoolFlag & ~FlagReturnToPool;
 
    public bool ReturnToPool => (_lengthAndPoolFlag & FlagReturnToPool) != 0;
 
    public BufferChunk(byte[] array)
    {
        Debug.Assert(array is not null, "expected valid array input");
        OversizedArray = array;
        _lengthAndPoolFlag = array!.Length;
        Offset = 0;
 
        // assume not pooled, if exact-sized
        // (we don't expect array.Length to be negative; we're really just saying
        // "we expect the result of assigning array.Length to _lengthAndPoolFlag
        // to give the expected Length *and* not have the MSB set; we're just
        // checking that we haven't fat-fingered our MSB logic)
        Debug.Assert(!ReturnToPool, "do not return right-sized arrays");
        Debug.Assert(Length == array.Length, "array length not respected");
    }
 
    public BufferChunk(byte[] array, int offset, int length, bool returnToPool)
    {
        Debug.Assert(array is not null, "expected valid array input");
        Debug.Assert(length >= 0, "expected valid length");
        OversizedArray = array;
        Offset = offset;
        _lengthAndPoolFlag = length | (returnToPool ? FlagReturnToPool : 0);
        Debug.Assert(ReturnToPool == returnToPool, "return-to-pool not respected");
        Debug.Assert(Length == length, "length not respected");
    }
 
    public byte[] ToArray()
    {
        int length = Length;
        if (length == 0)
        {
            return [];
        }
 
        byte[] copy = new byte[length];
        OversizedArray.AsSpan(Offset, length).CopyTo(copy);
        return copy;
 
        // Note on nullability of Array; the usage here is that a non-null array
        // is always provided during construction, so the only null scenario is for default(BufferChunk).
        // Since the constructor explicitly accesses array.Length, any null array passed to the constructor
        // will cause an exception, even in release (the Debug.Assert only covers debug) - although in
        // reality we do not expect this to ever occur (internal type, usage checked, etc). In the case of
        // default(BufferChunk), we know that Length will be zero, which means we will hit the [] case.
    }
 
    internal void RecycleIfAppropriate()
    {
        if (ReturnToPool)
        {
            ArrayPool<byte>.Shared.Return(OversizedArray!);
        }
 
        Unsafe.AsRef(in this) = default; // anti foot-shotgun double-return guard; not 100%, but worth doing
        Debug.Assert(OversizedArray is null && !ReturnToPool, "expected clean slate after recycle");
    }
 
    internal ArraySegment<byte> AsArraySegment() => Length == 0 ? default! : new(OversizedArray!, Offset, Length);
 
    internal ReadOnlySpan<byte> AsSpan() => Length == 0 ? default : new(OversizedArray!, Offset, Length);
 
    // get the data as a ROS; for note on null-logic of Array!, see comment in ToArray
    internal ReadOnlySequence<byte> AsSequence() => Length == 0 ? default : new ReadOnlySequence<byte>(OversizedArray!, Offset, Length);
 
    internal BufferChunk DoNotReturnToPool()
    {
        BufferChunk copy = this;
        Unsafe.AsRef(in copy._lengthAndPoolFlag) &= ~FlagReturnToPool;
        Debug.Assert(copy.Length == Length, "same length expected");
        Debug.Assert(!copy.ReturnToPool, "do not return to pool");
        return copy;
    }
}