comments-engine/Data/ArrayPoolList.cs

// SPDX-FileCopyrightText: Copyright 2024 Fabio Iotti
// SPDX-License-Identifier: AGPL-3.0-only

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

namespace CommentsEngine.Data;

class ArrayPoolList<T> : IList<T>, IDisposable
{
    readonly ArrayPool<T> _pool;
    readonly bool _clearArray;

    int _count;
    T[]? _buffer;

    public ArrayPoolList() : this(16) { }

    public ArrayPoolList(int capacity) : this(capacity, ArrayPool<T>.Shared) { }

    public ArrayPoolList(ArrayPool<T> pool, bool clearArray = false) : this(16, pool, clearArray) { }

    public ArrayPoolList(int capacity, ArrayPool<T> pool, bool clearArray = false)
    {
        _pool = pool;
        _clearArray = clearArray;

        _buffer = _pool.Rent(capacity);
    }

    /// <inheritdoc/>
    public T this[int index]
    {
        get
        {
            CheckNotDisposed();

            ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(index, _count);

            return _buffer[index];
        }
        set
        {
            CheckNotDisposed();

            ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(index, _count);

            _buffer[index] = value;
        }
    }

    /// <inheritdoc/>
    public int Count => _count;

    bool ICollection<T>.IsReadOnly => false;

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    [MemberNotNull(nameof(_buffer))]
    void CheckNotDisposed()
        => ObjectDisposedException.ThrowIf(_buffer is null, typeof(ArrayPoolList<T>));

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    void EnsureBufferAtLeast(ref T[] _buffer, int size)
    {
        if (_buffer.Length < size)
        {
            T[] newBuffer = _pool.Rent(1 + size * 2);
            _buffer.CopyTo(newBuffer, 0);
            _pool.Return(_buffer, _clearArray);
            _buffer = newBuffer;
        }
    }

    /// <inheritdoc/>
    public void Add(T item)
    {
        CheckNotDisposed();
        EnsureBufferAtLeast(ref _buffer, _count + 1);

        _buffer[_count++] = item;
    }

    /// <inheritdoc/>
    public void Clear()
    {
        CheckNotDisposed();

        _count = 0;
    }

    /// <inheritdoc/>
    public bool Contains(T item)
        => IndexOf(item) is not -1;

    /// <inheritdoc/>
    public void CopyTo(T[] array, int arrayIndex)
    {
        CheckNotDisposed();

        _buffer.AsSpan(0, _count).CopyTo(array.AsSpan(arrayIndex));
    }

    /// <inheritdoc/>
    public int IndexOf(T item)
    {
        CheckNotDisposed();

        for (int i = 0; i < _buffer.Length && i < _count; i++)
            if (Equals(item, _buffer[i]))
                return i;

        return -1;
    }

    /// <inheritdoc/>
    public void Insert(int index, T item)
    {
        CheckNotDisposed();

        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(index);
        ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(_count, index);

        EnsureBufferAtLeast(ref _buffer, _count + 1);

        Array.Copy(_buffer, index, _buffer, index + 1, _count - index);
        _buffer[index] = item;
        _count++;
    }

    /// <inheritdoc/>
    public bool Remove(T item)
    {
        CheckNotDisposed();

        int index = IndexOf(item);
        if (index is -1)
            return false;

        Array.Copy(_buffer, index, _buffer, index - 1, _count - index);
        _count--;

        return true;
    }

    /// <inheritdoc/>
    public void RemoveAt(int index)
    {
        CheckNotDisposed();

        ArgumentOutOfRangeException.ThrowIfNegativeOrZero(index);
        ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(_count, index);

        Array.Copy(_buffer, index, _buffer, index - 1, _count - index);
        _count--;
    }

    /// <inheritdoc/>
    public IEnumerator<T> GetEnumerator()
    {
        CheckNotDisposed();

        return _buffer.Take(_count).GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
        => GetEnumerator();

    protected virtual void Dispose(bool disposing)
    {
        if (_buffer is not null)
        {
            if (disposing)
                _pool.Return(_buffer, _clearArray);

            _buffer = null;
        }
    }

    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);
    }
}