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Advent of code - Day 19

qmenoret profile image Quentin Ménoret ・2 min read

Are you participating in the Advent of code this year?

If you don't know what the advent of code is, it's a website where you'll find a daily challenge (every day it gets harder). It's a really fun event, you should participate!

I try to solve the exercises using either JavaScript or TypeScript and will share my solutions daily (with one day delay so no one can cheat!). I only share the solution for the second part.

Again a fun one where you have to generate a RegEx to validate some strings. Part one was super nice, I just had a function to generate the RegEx and... voilà. For part 2, I had to hardcode the two rules for 11 and 8, then merge then for the RegEx to work. Not beautiful, but it works!

Here is my solution for day #19:

const [rawRules, input] = rawInput.split('\n\n')

const rules = rawRules
  .map((rule) => rule.split(': '))
  .map((rule) => {
    return {
      id: rule[0],
      possibilities: rule[1],
  .reduce((acc, rule) => {
    acc[] = rule
    return acc
  }, {})

function transformToRegex(rules, possibilities) {
  if (possibilities === '42 | 42 8') return `(${transformToRegex(rules, rules[42].possibilities)})+`
  return possibilities
    .split(' | ')
    .map((possibility) => {
      return possibility
        .split(' ')
        .map((p) => {
          if (p.match(/\".+\"/)) return p.slice(1, -1)
          const expression = transformToRegex(rules, rules[p].possibilities)
          return expression.includes('|') ? `(${expression})` : expression

const rule11Part1 = transformToRegex(rules, '42')
const rule11Part2 = transformToRegex(rules, '31')

let rule11 = `${rule11Part1}${rule11Part2}`
for (let i = 0; i < 20; i++) {
  rule11 = `${rule11Part1}(${rule11})?${rule11Part2}`

const firstRegex = transformToRegex(rules, rules[8].possibilities)
const fullRegex = new RegExp(`^${firstRegex}${rule11}$`)

console.log(input.split('\n').filter((str) => str.match(fullRegex)).length)
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Feel free to share your solution in the comments!

Photo by Markus Spiske on Unsplash

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