simple-combinatorics/index.js

A simple permutations and combinatorics library for JavaScript

function permute(n, k = n, noRepeat = true) {
    if (typeof k === 'boolean') {
        noRepeat = k;
        k = n;
    }
    let o = 0;
    if (noRepeat) {
        o = -1;
    }
    let out = n;
    for (let i = 1; i < k; i++) {
        out *= n + o * i;
    }
    return out;
}

function permuteList(arr, k = arr.length, noRepeat = true) {
    if (k === 0) {
        return [[]];
    } else if (typeof k === 'boolean') {
        noRepeat = k;
        k = arr.length;
    }
    let result = [];
    for (let i = 0; i < arr.length; i++) {
        let newAlphabet = [...arr];
        if (noRepeat) {
            newAlphabet.splice(i, 1);
        }
        let smallerCombinations = permuteList(newAlphabet, k - 1, noRepeat);
        for (let j = 0; j < smallerCombinations.length; j++) {
            let combination = [...smallerCombinations[j]];
            combination.push(arr[i]);
            result.push(combination);
        }
    }
    return result;
}

function choose(n, k = n, noRepeat = true) {
    if (typeof k === 'boolean') {
        noRepeat = k;
        k = n;
    }
    let o = 0;
    if (noRepeat) {
        o = -1;
    }
    let out = n;
    let kFac = 1;
    for (let i = 1; i < k; i++) {
        out *= (n + o * i);
        kFac *= (i + 1);
    }
    return out / kFac;
}

function chooseList(arr, k = arr.length, noRepeat = true) {
    if (k === 0) {
        return [[]];
    } else if (typeof k === 'boolean') {
        noRepeat = k;
        k = arr.length;
    }
    let result = [];
    for (let i = 0; i < arr.length; i++) {
        let o = noRepeat ? 1 : 0;
        let newAlphabet = [...arr].slice(i + o, arr.length);
        let smallerCombinations = chooseList(newAlphabet, k - 1, noRepeat);
        for (let j = 0; j < smallerCombinations.length; j++) {
            let combination = [...smallerCombinations[j]];
            combination.push(arr[i]);
            result.push(combination);
        }
    }
    return result;
}

function range(a, b) {
    return Array(Math.abs(b - a) + 1).fill().map((_, i) => i + a);
}

module.exports = {
    permute,
    permuteList,
    choose,
    chooseList,
    range
}