Alexandroppolus

// ------- utls ----------
function getRand(n) {
    return Math.floor(Math.random() * n);
}

function fact(n) {
    let f = 1;
    for (let i = 2; i <= n; ++i) {
        f *= i;
    }
    return f;
}

function shuffle(arr) {
    for (let i = arr.length - 1; i > 0; --i) {
        const r = getRand(i + 1);
        const t = arr[r];
        arr[r] = arr[i];
        arr[i] = t;
    }
    return arr;
}

// ------- classes ----------
class Str extends String {
    getCount() { return 1; }
}

class Line {
    arr = [];
    toString() {
        return this.arr.slice(0).join('');
    }
    getCount() {
        return this.arr.reduce((r, a) => r * a.getCount(), 1);
    }
}

class Select {
    arr = [];
    toString() {
        return this.arr[getRand(this.arr.length)].toString();
    }
    getCount() {
        return this.arr.reduce((r, a) => r + a.getCount(), 0);
    }
}

class Order extends Line {
    arr = [];
    toString() {
        return shuffle(this.arr).join('');
    }
    getCount() {
        return fact(this.arr.length) * this.arr.reduce((r, a) => r * a.getCount(), 1);
    }
}

// ------- parse ----------

function parse(str) {
    const reg = /[{}|\[\]]/g;
    let start = 0;
    let line = new Line();
    const stack = [{
        arr: [line],
    }];
    // debugger;
    while (true) {
        const match = reg.exec(str);
        if (!match) {
            if (str.length - start > 0) {
                line.arr.push(new Str(str.substring(start)));
            }
            break;
        }

        if (match.index - start > 0) {
            line.arr.push(new Str(str.substring(start, match.index)));
        }

        start = reg.lastIndex;

        const c = match[0];

        if (c === '{' || c === '[') {
            const container = c === '{' ? new Select() : new Order();
            line.arr.push(container);
            stack.push(container);
            line = new Line();
            container.arr.push(line);
        } else {
            if (stack.length < 2) {
                return null;
            }
            const container = stack[stack.length - 1];
            if (line.arr.length === 1) {
                container.arr[container.arr.length - 1] = line.arr[line.arr.length - 1];
            }
            if (c === '|') {
                line = new Line();
                container.arr.push(line);
            } else {
                stack.pop();
                const arr = stack[stack.length - 1].arr;
                line = arr[arr.length - 1];
            }
        }
    }
    return stack.length > 1 ? null : line.arr.length === 1 ? line.arr[0] : line;
}

// --------
const p = parse('{ttt|2}qqq[1|2|3{[4|q]|5}]');

console.log(p.toString());
console.log(p.getCount());
console.log(p);