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();
            }
        }
    }
}

Go: package main

import "fmt"

func nextInCycle(c []int, index int) int {
    return c[index % len(c)]
}

func kolakoski(c []int, slen int) []int {
    s := make([]int, slen)
    i, k := 0, 0
    for {
        s[i] = nextInCycle(c, k)
        if s[k] > 1 {
            for j := 1; j < s[k]; j++ {
                i++
                if i == slen {
                    return s
                }
                s[i] = s[i - 1]
            }
        }
        i++
        if i == slen {
            return s
        }
        k++
    }
}

func possibleKolakoski(s []int) bool {
    slen := len(s)
    rle := make([]int, 0, slen)
    prev := s[0]
    count := 1
    for i := 1; i < slen; i++ {
        if s[i] == prev {
            count++
        } else {
            rle = append(rle, count)
            count = 1
            prev = s[i]
        }
    }
    
    for i := 0; i < len(rle); i++ {
        if rle[i] != s[i] {
            return false
        }
    }
    return true
}

func printInts(ia []int, suffix string) {
    fmt.Print("[")
    alen := len(ia)
    for i := 0; i < alen; i++ {
        fmt.Print(ia[i])
        if i < alen - 1 {
            fmt.Print(", ")
        }
    }
    fmt.Printf("]%s\n", suffix)
}

func main() {
    ias := make([][]int, 4)
    ias[0] = []int{1, 2}
    ias[1] = []int{2, 1}
    ias[2] = []int{1, 3, 1, 2}
    ias[3] = []int{1, 3, 2, 1}
    slens := []int{20, 20, 30, 30}
    for i, ia := range ias {
        slen := slens[i]
        kol := kolakoski(ia, slen)
        fmt.Printf("First %d members of the sequence generated by ", slen)
        printInts(ia, ":")
        printInts(kol, "")
        p := possibleKolakoski(kol)
        poss := "Yes"
        if !p {
            poss = "No"
        }
        fmt.Println("Possible Kolakoski sequence?", poss, "\n")
    }
}