Linq.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

namespace System.Linq;

public static class EnumerablePolyfill
{
    /// <summary>
    /// Split the elements of a sequence into chunks of size at most <paramref name="size"/>.
    /// </summary>
    /// <remarks>
    /// Every chunk except the last will be of size <paramref name="size"/>.
    /// The last chunk will contain the remaining elements and may be of a smaller size.
    /// </remarks>
    /// <param name="source">
    /// An <see cref="IEnumerable{T}"/> whose elements to chunk.
    /// </param>
    /// <param name="size">
    /// Maximum size of each chunk.
    /// </param>
    /// <typeparam name="TSource">
    /// The type of the elements of source.
    /// </typeparam>
    /// <returns>
    /// An <see cref="IEnumerable{T}"/> that contains the elements the input sequence split into chunks of size <paramref name="size"/>.
    /// </returns>
    /// <exception cref="ArgumentNullException">
    /// <paramref name="source"/> is null.
    /// </exception>
    /// <exception cref="ArgumentOutOfRangeException">
    /// <paramref name="size"/> is below 1.
    /// </exception>
    public static IEnumerable<TSource[]> Chunk<TSource>(this IEnumerable<TSource> source, int size)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (size < 1) throw new ArgumentOutOfRangeException(nameof(size));
        return ChunkIterator(source, size);
    }

    private static IEnumerable<TSource[]> ChunkIterator<TSource>(IEnumerable<TSource> source, int size)
    {
        using var e = source.GetEnumerator();
        while (e.MoveNext())
        {
            var chunk = new TSource[size];
            chunk[0] = e.Current;

            var i = 1;
            for (; i < chunk.Length && e.MoveNext(); i++)
            {
                chunk[i] = e.Current;
            }

            if (i == chunk.Length)
            {
                yield return chunk;
            }
            else
            {
                Array.Resize(ref chunk, i);
                yield return chunk;
                yield break;
            }
        }
    }
}