index.js

/**
 * PseudoCrypt — Short, reversible, pseudo-unique hashes for numeric values.
 * Inspired by KevBurnsJr's original PHP implementation: 
 * https://web.archive.org/web/20130727034425/http://blog.kevburnsjr.com/php-unique-hash
 *
 * Author: Tanish Raj (@cttricks)
 * License: MIT
 * Repository: https://github.com/cttricks/pseudocrypt
 *
 * This module provides two main functions:
 * - encode: Convert a number into a pseudo-unique base62-like string
 * - decode: Retrieve the original number from the encoded string
 *
 * ⚠️ Not cryptographically secure. Use only for ID obfuscation, pretty URLs, etc.
 */

const goldenPrimes = {
    1: '1',
    41: '59',
    2377: '1677',
    147299: '187507',
    9132313: '5952585',
    566201239: '643566407',
    35104476161: '22071637057',
    2176477521929: '294289236153',
    134941606358731: '88879354792675',
    8366379594239857: '7275288500431249',
    518715534842869223: '280042546585394647'
};

const chars62 = [
    ...Array.from({ length: 10 }, (_, i) => 48 + i), // 0-9
    ...Array.from({ length: 26 }, (_, i) => 65 + i), // A-Z
    ...Array.from({ length: 26 }, (_, i) => 97 + i)  // a-z
];

const base62 = (int) => {
    int = BigInt(int);
    let str = '';
    const base = BigInt(62);
    while (int > 0) {
        const mod = int % base;
        str += String.fromCharCode(chars62[Number(mod)]);
        int = int / base;
    }
    return str.split('').reverse().join('');
};

const unBase62 = (str) => {
    return [...str].reverse().reduce((acc, char, i) => {
        const val = BigInt(chars62.indexOf(char.charCodeAt(0)));
        return acc + val * (BigInt(62) ** BigInt(i));
    }, BigInt(0));
};

/**
 * Encodes a number into a short, base62-like hash string.
 * Useful for generating pseudo-unique, URL-safe identifiers.
 *
 * @param {number | string | bigint} num - The numeric value to encode.
 * @param {number} [len=5] - The desired length of the output hash. Must be between 1 and 10.
 * @returns {string} Encoded hash string.
 *
 * @example
 * const shortId = encode(123456); // e.g. "4Gze1"
 */

const encode = (num, len = 5) => {
    num = BigInt(num);
    const ceil = BigInt(62) ** BigInt(len);
    const prime = BigInt(Object.keys(goldenPrimes)[len]);
    const hashed = (num * prime) % ceil;
    return base62(hashed).padStart(len, '0');
};

/**
 * Decodes a hash string back into its original numeric value.
 *
 * @param {string} hash - The encoded hash string (must be generated by `encode()`).
 * @returns {bigint} Original numeric value.
 *
 * @example
 * const original = decode("4Gze1"); // 123456n
 */
const decode = (hash) => {
    const len = hash.length;
    const ceil = BigInt(62) ** BigInt(len);
    const mmi = BigInt(Object.values(goldenPrimes)[len]);
    const val = unBase62(hash);
    return (val * mmi) % ceil;
};

module.exports = { encode, decode };