Get unique number

getUniqueNumber.js

// Some math based on https://en.wikipedia.org/wiki/Discrete_logarithm
// The function fastModularExponentiation allow to generate
// pseudo-random unique number from 0 to prime - 1.
// In other words every gotten number is converted to another number and returned numbers are unique
// Warning! When number arg is more than prime then returned numbers are going to be repeated
// thus for biggrer numbers we'll need to choose bigger prime number.

// The implementation if fastModularExponentiation is taken there https://gist.github.com/krzkaczor/0bdba0ee9555659ae5fe
// Primitive root calc: http://www.bluetulip.org/2014/programs/primitive.html

const prime = 9999991;
const primitiveRoot = 22;

function fastModularExponentiation(a, b, n) {
    a %= n; // eslint-disable-line no-param-reassign
    let result = 1;
    let x = a;

    while (b > 0) {
        const leastSignificantBit = b % 2;
        b = Math.floor(b / 2);  // eslint-disable-line no-param-reassign

        if (leastSignificantBit === 1) {
            result *= x;
            result %= n;
        }

        x *= x;
        x %= n;
    }
    return result;
}

function getUniqueNumber(number) {
    return fastModularExponentiation(primitiveRoot, number, prime);
}

// TODO: Move this test to another place
function test() { // eslint-disable-line  no-unused-vars
    const set = {};
    for (let i = 0; i < prime - 1; i++) { // eslint-disable-line no-plusplus
        const n = getUniqueNumber(i);
        if (n in set) {
            throw Error('Not passed');
        }

        set[n] = 1;
    }

    console.log('Passed'); // eslint-disable-line no-console
}

module.exports = getUniqueNumber;