ShrinkEnumerable.cs

using System;
using System.Collections;
using System.Collections.Generic;

namespace Tonari.Linq
{    
    public static class Enumerable_ShrinkEx
    {
        public static IEnumerable<TSource[]> Shrink<TSource>(this IEnumerable<TSource> source)
        {
            return new ShrinkEnumerable<TSource, TSource>(source, x => x, null);
        }

        public static IEnumerable<TSource[]> Shrink<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            return new ShrinkEnumerable<TSource, TKey>(source, keySelector, null);
        }

        public static IEnumerable<TSource[]> Shrink<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            return new ShrinkEnumerable<TSource, TKey>(source, keySelector, comparer);
        }
    }
    
    public class ShrinkEnumerable<TSource, TKey> : IEnumerable<TSource[]>, IEnumerable
    {
        private IEnumerable<TSource> _source;
        private Func<TSource, TKey> _keySelector;
        private IEqualityComparer<TKey> _comparer;

        public ShrinkEnumerable(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            this._source = source;
            this._keySelector = keySelector;
            this._comparer = comparer ?? EqualityComparer<TKey>.Default;
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        public IEnumerator<TSource[]> GetEnumerator()
        {
            var results = new List<TSource[]>();

            var isInitlalized = false;
            var isShrinked = false;
            var @base = default(TSource);
            var prev = default(TSource);
            var result = new List<TSource>();
            foreach(var element in this._source)
            {
                if (isInitlalized)
                {
                    var baseKey = this._keySelector(@base);

                    isShrinked = this._comparer.Equals(baseKey, this._keySelector(prev));

                    if (this._comparer.Equals(baseKey, this._keySelector(element)))
                    {
                        prev = element;
                        result.Add(element);

                        continue;
                    }
                }

                isInitlalized = true;

                if (isShrinked)
                {
                    isShrinked = false;

                    results.Add(result.ToArray());
                    result.Clear();
                }

                @base = prev = element;
                result.Add(element);
            }
            if (isInitlalized)
            {
                results.Add(result.ToArray());
            }

            return results.GetEnumerator();
        }
    }
}

前から順番にたどっていって、重複している要素があれば配列にまとめる。
同じ要素が以前にあっても、連続して同じ要素がなければ別になる。

var array = new [] { 1, 2, 2, 3, 3, 3, 1, 1, 2, 2, 3, };
array.Shrink().ToArray(); //  { { 1, }, { 2, 2, }, { 3, 3, 3, }, { 1, 1, }, { 2, 2, }, { 3, }, };