// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

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

namespace CommunityToolkit.HighPerformance.Streams;

/// <summary>
/// An <see cref="ISpanOwner"/> implementation wrapping a <see cref="MemoryManager{T}"/> of <see cref="byte"/> instance.
/// </summary>
internal readonly struct MemoryManagerOwner : ISpanOwner
{
    /// <summary>
    /// The wrapped <see cref="MemoryManager{T}"/> instance.
    /// </summary>
    private readonly MemoryManager<byte> memoryManager;

    /// <summary>
    /// The starting offset within <see cref="memoryManager"/>.
    /// </summary>
    private readonly int offset;

    /// <summary>
    /// The usable length within <see cref="memoryManager"/>.
    /// </summary>
    private readonly int length;

    /// <summary>
    /// Initializes a new instance of the <see cref="MemoryManagerOwner"/> struct.
    /// </summary>
    /// <param name="memoryManager">The wrapped <see cref="MemoryManager{T}"/> instance.</param>
    /// <param name="offset">The starting offset within <paramref name="memoryManager"/>.</param>
    /// <param name="length">The usable length within <paramref name="memoryManager"/>.</param>
    public MemoryManagerOwner(MemoryManager<byte> memoryManager, int offset, int length)
    {
        this.memoryManager = memoryManager;
        this.offset = offset;
        this.length = length;
    }

    /// <inheritdoc/>
    public int Length
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get => this.length;
    }

    /// <inheritdoc/>
    public Span<byte> Span
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        get
        {
            // We can't use the same trick we use for arrays to optimize the creation of
            // the offset span, as otherwise a bugged MemoryManager<T> instance returning
            // a span of an incorrect size could cause an access violation. Instead, we just
            // get the span and then slice it, which will validate both offset and length.
            return this.memoryManager.GetSpan().Slice(this.offset, this.length);
        }
    }
}