Batch.cs

#region License and Terms
// MoreLINQ - Extensions to LINQ to Objects
// Copyright (c) 2009 Atif Aziz. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#endregion

namespace MoreLinq
{
    using System;
    using System.Collections.Generic;

    static partial class MoreEnumerable
    {
        /// <summary>
        /// Batches the source sequence into sized buckets.
        /// </summary>
        /// <typeparam name="TSource">Type of elements in <paramref name="source"/> sequence.</typeparam>
        /// <param name="source">The source sequence.</param>
        /// <param name="size">Size of buckets.</param>
        /// <returns>A sequence of equally sized buckets containing elements of the source collection.</returns>
        /// <remarks>
        /// <para>
        /// This operator uses deferred execution and streams its results
        /// (buckets are streamed but their content buffered).</para>
        /// <para>
        /// When more than one bucket is streamed, all buckets except the last
        /// is guaranteed to have <paramref name="size"/> elements. The last
        /// bucket may be smaller depending on the remaining elements in the
        /// <paramref name="source"/> sequence.</para>
        /// <para>
        /// Each bucket is pre-allocated to <paramref name="size"/> elements.
        /// If <paramref name="size"/> is set to a very large value, e.g.
        /// <see cref="int.MaxValue"/> to effectively disable batching by just
        /// hoping for a single bucket, then it can lead to memory exhaustion
        /// (<see cref="OutOfMemoryException"/>).
        /// </para>
        /// </remarks>

        public static IEnumerable<TSource[]> Batch<TSource>(this IEnumerable<TSource> source, int size)
        {
            return Batch(source, size, IdFn);
        }

        /// <summary>
        /// Batches the source sequence into sized buckets and applies a projection to each bucket.
        /// </summary>
        /// <typeparam name="TSource">Type of elements in <paramref name="source"/> sequence.</typeparam>
        /// <typeparam name="TResult">Type of result returned by <paramref name="resultSelector"/>.</typeparam>
        /// <param name="source">The source sequence.</param>
        /// <param name="size">Size of buckets.</param>
        /// <param name="resultSelector">The projection to apply to each bucket.</param>
        /// <returns>A sequence of projections on equally sized buckets containing elements of the source collection.</returns>
        /// <remarks>
        /// <para>
        /// This operator uses deferred execution and streams its results
        /// (buckets are streamed but their content buffered).</para>
        /// <para>
        /// <para>
        /// When more than one bucket is streamed, all buckets except the last
        /// is guaranteed to have <paramref name="size"/> elements. The last
        /// bucket may be smaller depending on the remaining elements in the
        /// <paramref name="source"/> sequence.</para>
        /// Each bucket is pre-allocated to <paramref name="size"/> elements.
        /// If <paramref name="size"/> is set to a very large value, e.g.
        /// <see cref="int.MaxValue"/> to effectively disable batching by just
        /// hoping for a single bucket, then it can lead to memory exhaustion
        /// (<see cref="OutOfMemoryException"/>).
        /// </para>
        /// </remarks>

        public static IEnumerable<TResult> Batch<TSource, TResult>(this IEnumerable<TSource> source, int size,
            Func<TSource[], TResult> resultSelector)
        {
            if (source == null) throw new ArgumentNullException(nameof(source));
            if (size <= 0) throw new ArgumentOutOfRangeException(nameof(size));
            if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));

            return _(source, size, resultSelector);

            static IEnumerable<TResult> _(IEnumerable<TSource> source, int size, Func<TSource[], TResult> resultSelector)
            {
                switch (source)
                {
                    case ICollection<TSource> { Count: 0 }:
                    {
                        break;
                    }
                    case ICollection<TSource> collection when collection.Count <= size:
                    {
                        var bucket = new TSource[collection.Count];
                        collection.CopyTo(bucket, 0);
                        yield return resultSelector(bucket);
                        break;
                    }
                    case IReadOnlyCollection<TSource> { Count: 0 }:
                    {
                        break;
                    }
                    case IReadOnlyList<TSource> list when list.Count <= size:
                    {
                        var bucket = new TSource[list.Count];
                        for (var i = 0; i < list.Count; i++)
                            bucket[i] = list[i];
                        yield return resultSelector(bucket);
                        break;
                    }
                    case IReadOnlyCollection<TSource> collection when collection.Count <= size:
                    {
                        size = collection.Count;
                        goto default;
                    }
                    default:
                    {
                        TSource[]? bucket = null;
                        var count = 0;

                        foreach (var item in source)
                        {
                            bucket ??= new TSource[size];
                            bucket[count++] = item;

                            // The bucket is fully buffered before it's yielded
                            if (count != size)
                                continue;

                            yield return resultSelector(bucket);

                            bucket = null;
                            count = 0;
                        }

                        // Return the last bucket with all remaining elements
                        if (count > 0)
                        {
                            Array.Resize(ref bucket, count);
                            yield return resultSelector(bucket);
                        }

                        break;
                    }
                }
            }
        }
    }
}