using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

​

public static partial class EnumerableExtensions { 

  public static IEnumerable<T[]> GroupByAsc<T>(this IEnumerable<T> source, 

                                                    Comparer<T> comparer = null) {

    if (source is null)

      throw new ArgumentNullException(nameof(source));

​

    if ((comparer ??= Comparer<T>.Default) is null)

      throw new ArgumentNullException(nameof(comparer), 

        $"Type {typeof(T).Name} doesn't have default comparer");

​

    List<T> list = new List<T>();

​

    foreach (var item in source) 

      if (list.Count > 0 && comparer.Compare(list[list.Count - 1], item) <= 0)

        list.Add(item);

      else {

        if (list.Count > 0)  

          yield return list.ToArray();

​

        list.Clear();

​

        list.Add(item);

      }

    if (list.Count > 0)

      yield return list.ToArray();

  }

}

public class Program {

  private static void LinqDemo() {

    var arr = new int[] {

      10, 20, 30,

      10, 20, 30, 40, 50,

      10, 20

    };  

    int prior = int.MinValue;

    int group = 0;

​

    var result = arr

      .GroupBy(item => { 

         if (item < prior) 

           group += 1;

​

         prior = item;

         return group;  

       });

    Console.WriteLine("Pure Linq:");   

    Console.WriteLine();   

    Console.WriteLine(string.Join(Environment.NewLine, result.Select(g => string.Join(", ", g))));  

  }

​

  private static void ForEachDemo() {

    var arr = new int[] {

      10, 20, 30,

      10, 20, 30, 40, 50,

      10, 20

    };

    var result = arr.GroupByAsc();  

    Console.WriteLine("Extension method (foreach):");  

    Console.WriteLine();  

    Console.WriteLine(string.Join(Environment.NewLine, result.Select(g => string.Join(", ", g))));    

  }

  public static void Main() {

    LinqDemo();

    Console.WriteLine();  

    ForEachDemo();  

  }

}