Task: Kolakoski sequence
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")
    }
}

Java: import java.util.Arrays;

public class Kolakoski {
    private static class Crutch {
        final int len;
        int[] s;
        int i;

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

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

    private static int nextInCycle(final int[] self, int index) {
        return self[index % self.length];
    }

    private static int[] kolakoski(final int[] self, int len) {
        Crutch c = new Crutch(len);

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

    private static boolean possibleKolakoski(final 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 void main(String[] args) {
        int[][] ias = new int[][]{
            new int[]{1, 2},
            new int[]{2, 1},
            new int[]{1, 3, 1, 2},
            new int[]{1, 3, 2, 1}
        };
        int[] lens = new int[]{20, 20, 30, 30};

        for (int i=0; i<ias.length; i++) {
            int len = lens[i];
            int[] kol = kolakoski(ias[i], len);

            System.out.printf("First %d members of the sequence generated by %s: \n", len, Arrays.toString(ias[i]));
            System.out.printf("%s\n", Arrays.toString(kol));
            System.out.printf("Possible Kolakoski sequence? %s\n\n", possibleKolakoski(kol));
        }
    }
}