class Crutch {
    constructor(len) {
        this.len = len;
        this.s = new Array(len);
        this.i = 0;
    }
    repeat(count) {
        for (let j = 0; j < count; j++) {
            if (++this.i === this.len) return;
            this.s[this.i] = this.s[this.i - 1];
        }
    }
}

function nextInCycle(self, index) {
    return self[index % self.length];
}

function kolakoski(self, len) {
    const c = new Crutch(len);
    let k = 0;
    while (c.i < len) {
        c.s[c.i] = nextInCycle(self, k);
        if (self[k] > 1) {
            c.repeat(self[k] - 1);
        }
        if (++c.i === len) return c.s;
        k++;
    }
    return c.s;
}

function possibleKolakoski(self) {
    const rle = [];
    let prev = self[0];
    let count = 1;
    let pos = 0;
    for (let i = 1; i < self.length; i++) {
        if (self[i] === prev) {
            count++;
        } else {
            rle[pos++] = count;
            count = 1;
            prev = self[i];
        }
    }
    for (let i = 0; i < pos; i++) {
        if (rle[i] !== self[i]) {
            return false;
        }
    }
    return true;
}

const ias = [
    [1, 2],
    [2, 1],
    [1, 3, 1, 2],
    [1, 3, 2, 1]
];
const lens = [20, 20, 30, 30];

for (let i = 0; i < ias.length; i++) {
    const len = lens[i];
    const kol = kolakoski(ias[i], len);
    console.log(`First ${len} members of the sequence generated by [${ias[i]}]:`);
    console.log(`[${kol}]`);
    console.log(`Possible Kolakoski sequence? ${possibleKolakoski(kol)}\n`);
}