combination-and-permutation/dist/combination/index.js

Combination or Permutation computation

import { fixed } from "./fixed.js";
function computeReady(n) {
    const rates = fixed.get(n);
    const nw = Math.random() * rates[0];
    let chosenCount = 0;
    for (let m = 1; m <= n; m++) {
        if (nw < rates[m]) {
            chosenCount = m;
            break;
        }
    }
    if (chosenCount == 0)
        chosenCount = n;
    const ok = new Set();
    while (ok.size < chosenCount) {
        const index = Math.floor(Math.random() * n);
        ok.add(index + 1);
    }
    const ret = [];
    for (const s of ok)
        ret.push(s);
    return ret.sort((a, b) => a - b);
}
export function compute(n) {
    if (fixed.get(n) != null) {
        return computeReady(n);
    }
    let total = 0;
    const rates = new Array(n + 1);
    rates[0] = 0;
    for (let m = 1; m <= n; m++) {
        let now = 1;
        for (let j = 0; j < m; j++) {
            now = now * (n - j);
        }
        for (let j = 2; j <= m; j++) {
            now /= j;
        }
        rates[m] = now;
        total += now;
    }
    for (let m = 1; m <= n; m++) {
        rates[m] = rates[m - 1] + rates[m];
    }
    const nw = Math.random() * total;
    let chosenCount = 0;
    for (let m = 1; m <= n; m++) {
        if (nw < rates[m]) {
            chosenCount = m;
            break;
        }
    }
    if (chosenCount == 0)
        chosenCount = n;
    const ok = new Set();
    while (ok.size < chosenCount) {
        const index = Math.floor(Math.random() * n);
        ok.add(index + 1);
    }
    const ret = [];
    for (const s of ok)
        ret.push(s);
    return ret.sort((a, b) => a - b);
}
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