Task: Kolakoski sequence
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace KolakoskiSequence {
    class Crutch {
        public readonly int len;
        public int[] s;
        public int i;

        public Crutch(int len) {
            this.len = len;
            s = new int[len];
            i = 0;
        }

        public void Repeat(int count) {
            for (int j = 0; j < count; j++) {
                if (++i == len) return;
                s[i] = s[i - 1];
            }
        }
    }

    static class Extension {
        public static int NextInCycle(this int[] self, int index) {
            return self[index % self.Length];
        }

        public static int[] Kolakoski(this int[] self, int len) {
            Crutch c = new Crutch(len);

            int k = 0;
            while (c.i < len) {
                c.s[c.i] = self.NextInCycle(k);
                if (c.s[k] > 1) {
                    c.Repeat(c.s[k] - 1);
                }
                if (++c.i == len) return c.s;
                k++;
            }
            return c.s;
        }

        public static bool PossibleKolakoski(this int[] self) {
            int[] rle = new int[self.Length];
            int prev = self[0];
            int count = 1;
            int pos = 0;
            for (int i = 1; i < self.Length; i++) {
                if (self[i] == prev) {
                    count++;
                }
                else {
                    rle[pos++] = count;
                    count = 1;
                    prev = self[i];
                }
            }
            
            for (int i = 0; i < pos; i++) {
                if (rle[i] != self[i]) {
                    return false;
                }
            }
            return true;
        }

        public static string AsString(this int[] self) {
            StringBuilder sb = new StringBuilder("[");
            int count = 0;
            foreach (var item in self) {
                if (count > 0) {
                    sb.Append(", ");
                }
                sb.Append(item);
                count++;
            }
            return sb.Append("]").ToString();
        }
    }

    class Program {
        static void Main(string[] args) {
            int[][] ias = {
                new int[]{1, 2},
                new int[]{2, 1},
                new int[]{1, 3, 1, 2},
                new int[]{1, 3, 2, 1}
            };
            int[] lens = { 20, 20, 30, 30 };

            for (int i = 0; i < ias.Length; i++) {
                int len = lens[i];
                int[] kol = ias[i].Kolakoski(len);

                Console.WriteLine("First {0} members of the sequence by {1}: ", len, ias[i].AsString());
                Console.WriteLine(kol.AsString());
                Console.WriteLine("Possible Kolakoski sequence? {0}", kol.PossibleKolakoski());
                Console.WriteLine();
            }
        }
    }
}

C++: #include <iostream>
#include <vector>

using Sequence = std::vector<int>;

std::ostream& operator<<(std::ostream& os, const Sequence& v) {
  os << "[ ";
  for (const auto& e : v) {
    std::cout << e << ", ";
  }
  os << "]";
  return os;
}

int next_in_cycle(const Sequence& s, size_t i) {
  return s[i % s.size()];
}

Sequence gen_kolakoski(const Sequence& s, int n) {
  Sequence seq;
  for (size_t i = 0; seq.size() < n; ++i) {
    const int next = next_in_cycle(s, i);
    Sequence nv(i >= seq.size() ? next : seq[i], next);
    seq.insert(std::end(seq), std::begin(nv), std::end(nv));
  }
  return { std::begin(seq), std::begin(seq) + n };
}

bool is_possible_kolakoski(const Sequence& s) {
  Sequence r;
  for (size_t i = 0; i < s.size();) {
    int count = 1;
    for (size_t j = i + 1; j < s.size(); ++j) {
      if (s[j] != s[i]) break;
      ++count;
    }
    r.push_back(count);
    i += count;
  }
  for (size_t i = 0; i < r.size(); ++i) if (r[i] != s[i]) return false;
  return true;
}

int main() {
  std::vector<Sequence> seqs = {
    { 1, 2 },
    { 2, 1 },
    { 1, 3, 1, 2 },
    { 1, 3, 2, 1 }
  };
  for (const auto& s : seqs) {
    auto kol = gen_kolakoski(s, s.size() > 2 ? 30 : 20);
    std::cout << "Starting with: " << s << ": " << std::endl << "Kolakoski sequence: " 
      << kol << std::endl << "Possibly kolakoski? " << is_possible_kolakoski(kol) << std::endl;		
  }
  return 0;
}