#!/usr/bin/env node

'use strict';

const BLOCKSIZE = 3;
const CHARS1 = [
    ' ',
    ...'0123456789'.split(''),
    ...'abcdefghijklmnopqrstuvwxyz'.split(''),
    '_',
    '-',
];
const SHIFT = BigInt(Math.ceil(Math.log2(CHARS1.length ** BLOCKSIZE)));
const MASK = 2n**SHIFT - 1n;

const CHARSN = [...Array(BLOCKSIZE - 1)].reduce((acc) => acc.map((v1) => CHARS1.map((v2) => ''+v1+v2)).flat(), CHARS1);
const FMAP = new Map(CHARSN.map((v, i) => [''+v, BigInt(i)]));
const RMAP = new Map(CHARSN.map((v, i) => [BigInt(i), ''+v]));

function main(arg1, arg2) {
    if (!arg1) {
        console.error('Usage: hexhost fdxx::4a59954e');
        console.error('Usage: hexhost fdxx:: hostname');
        process.exit(1);
    }

    if (arg2) {
        const prefix = arg1;
        const suffix = encode(arg2).replaceAll(/(.{4})/g, '$1:').replace(/:$/, '');
        console.log(prefix + suffix);
    } else {
        const [, suffix] = arg1.split(/::|:0:/);
        console.log(decode(suffix));
    }
}

function decode(input) {
    input = input && input.replaceAll(':', '');
    if (!input) {
        throw new Error('No suffix found');
    }
    input = BigInt('0x' + input);
    let output = [];
    while (input > 0) {
        const encodedBlock = input & MASK;
        input >>= SHIFT;
        const block = RMAP.get(encodedBlock);
        if (block !== undefined) {
            output.push(block);
        }
    }
    return output.reverse().join('').trim();
}

function encode(input) {
    if (input.length / BLOCKSIZE > (64n / SHIFT)) {
        throw new Error('Input is too long to fit in a /64!');
    }

    input = input.toLowerCase();

    let out = BigInt(0);
    for (let i = 0; i < input.length; i += BLOCKSIZE) {
        const block = input.substring(i, i + BLOCKSIZE).padEnd(BLOCKSIZE);
        const encodedBlock = FMAP.get(block);
        if (encodedBlock !== undefined) {
            out = (out << SHIFT) + encodedBlock;
        }
    }
    return out.toString(16);
}

main(process.argv[2], process.argv[3]);