[рони]

<pre>
<script>
const randomNetto = length => {
    let ar = [],
        sum = 0;
    for (let i = 1; i <= length / 2; i++) {
        var n = 1,
            j = length;
        for (let k = i; k; k--) {
            n *= j--;
            n /= k;
        }
        sum += n;
        ar.push(sum);
    }
    ar = ar.map(a => a / sum);
    return random => {
        let len = 0;
        for (let k of ar) {
            len++;
            if (random < k) break;
        }
        return len
    }
}

function splitSet(set) {
    const random = a => Math.trunc(Math.random() * a),
        sort = (a, b) => a - b;
    let {
        length
    } = set = set.slice(0), ar = [], count = randomNetto(length), n = count(Math.random());
    for (let i = 0; i < n; i++) {
        let k = random(length - i);
        ar.push(...set.splice(k, 1))
    }
    ar.sort(sort);
    return ar[0] == 1 ? [ar, set] : [set, ar];
}
let v1 = 0;
let v2 = 0;
let v3 = 0;
let v4 = 0;
let v5 = 0
for (let i = 0; i < 63000; i++) {
    let [s1, s2] = splitSet([1, 2, 3, 4, 5, 6, 7])
    //document.write(`[[${s1}],[${s2}]], ${s1.length}, ${s2.length}<br>`)
    if (s1.length == 1 && s1[0] == 1) v1++
    if (s1.length == 2 && s1[0] == 1 && s1[1] == 2) v2++
    if (s1.length == 3 && s1[0] == 1 && s1[1] == 2 && s1[2] == 3) v3++
    if (s1.length == 4 && s1[0] == 1 && s1[1] == 2 && s1[2] == 3 && s1[3] == 4) v4++
    if (s1.length == 5 && s1[0] == 1 && s1[1] == 2 && s1[2] == 3 && s1[3] == 4 && s1[4] == 5) v5++
}
document.write(`[1] - ${v1}   [1,2] - ${v2} [1,2,3] - ${v3} [1,2,3,4] - ${v4}  [1,2,3,4,5] - ${v5}`)
</script>
</pre>