Dev.to daily challenge (2019-07-10)

20190710.js

const assert = expr => {
    if (!expr) { throw new Error(`assertion failed`); }
};

const permute = arr => {
    if (arr.length < 2) {
        return (typeof arr === "string" ? [arr] : arr);
    }

    if (typeof arr === "string") {
        return permute(arr.split("")).map(arr => arr.join(""));
    }

    const result = [];
    for (let i = 0; i < arr.length; i++) {
        const unused = arr.map(i => i);
        const candidate = unused.splice(i, 1);
        const permuteUnused = permute(unused);
        for (let r of permuteUnused) {
            result.push([...candidate, ...Array.isArray(r) ? r : [r]]);
        };
    }
    return result;
};

assert(JSON.stringify(permute("1")) === `["1"]`)
assert(JSON.stringify(permute("12")) === `["12","21"]`)
assert(JSON.stringify(permute("123")) === `["123","132","213","231","312","321"]`);

const numericalOrder = (a, b) => {
    const nA = Number(a);
    const nB = Number(b);
    return nA < nB ? -1
    : nA > nB ? 1
    : 0;
};

const nextLargerNumber = n => {
    const digits = n.toString();
    const nextValidNumbers = permute(digits)
        .sort(numericalOrder)
        .filter(a => Number(a) > n);
    if (nextValidNumbers.length < 1) {
        return null;
    }
    return Number(nextValidNumbers[0]);
};

assert(nextLargerNumber(1) === null);
assert(nextLargerNumber(12) === 21);
assert(nextLargerNumber(21) === null);
assert(nextLargerNumber(13) === 31);
assert(nextLargerNumber(31) === null);
assert(nextLargerNumber(25) === 52);
assert(nextLargerNumber(513) === 531);
assert(nextLargerNumber(2019) === 2091);
assert(nextLargerNumber(123456789) === 123456798);
assert(nextLargerNumber(987654321) === null)