URL friendly IDs via Modular Multiplicative Inverse

example.js

// https://www.reddit.com/r/PostgreSQL/comments/6gw866/best_practice_for_id_system_that_is_obscure_for/diu8cr1/
// https://planetcalc.com/3311/

const [a, b, modulo, radix] = [204021643231n, 975874032671n, 1000000000000n, 36];

const encode = (plainId) => ((BigInt(plainId) * a) % modulo).toString(radix);
const decode = (encodedId) => (BigInt(parseInt(encodedId, radix)) * b) % modulo;

for (let i = 1; i <= 20; i++) {
    console.log(`${i} | ${encode(i)} | ${decode(encode(i))}`);
}