ArrayBufferWriter.cs

/*---------------------------------------------------------------------------------------------
 *  Copyright (c) Microsoft Corporation. All rights reserved.
 *--------------------------------------------------------------------------------------------*/

namespace System.Buffers;

internal sealed class ArrayBufferWriter<T> : IBufferWriter<T>
{
    private const int DefaultInitialBufferSize = 256;
    private T[] _buffer;
    private int _index;

    public ArrayBufferWriter()
        : this(DefaultInitialBufferSize)
    {
    }

    public ArrayBufferWriter(int initialCapacity)
    {
        if (initialCapacity < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(initialCapacity));
        }

        _buffer = initialCapacity == 0 ? [] : new T[initialCapacity];
    }

    public ReadOnlyMemory<T> WrittenMemory => _buffer.AsMemory(0, _index);

    public ReadOnlySpan<T> WrittenSpan => _buffer.AsSpan(0, _index);

    public int WrittenCount => _index;

    public int Capacity => _buffer.Length;

    public int FreeCapacity => _buffer.Length - _index;

    public void Clear()
    {
        _buffer.AsSpan(0, _index).Clear();
        _index = 0;
    }

    public void Advance(int count)
    {
        if (count < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(count));
        }

        if (count > FreeCapacity)
        {
            throw new InvalidOperationException("Cannot advance past the end of the buffer.");
        }

        _index += count;
    }

    public Memory<T> GetMemory(int sizeHint = 0)
    {
        CheckAndResizeBuffer(sizeHint);
        return _buffer.AsMemory(_index);
    }

    public Span<T> GetSpan(int sizeHint = 0)
    {
        CheckAndResizeBuffer(sizeHint);
        return _buffer.AsSpan(_index);
    }

    private void CheckAndResizeBuffer(int sizeHint)
    {
        if (sizeHint < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(sizeHint));
        }

        if (sizeHint == 0)
        {
            sizeHint = 1;
        }

        if (sizeHint <= FreeCapacity)
        {
            return;
        }

        var growBy = Math.Max(sizeHint, _buffer.Length);
        var newSize = checked(_buffer.Length + growBy);
        Array.Resize(ref _buffer, newSize);
    }
}