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Quentin Ménoret
Quentin Ménoret

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Advent of code - Day 21

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.

Thank you for the break after Day #20 horrors 🙏
This one was clear, easy to work with and just challenging enough for a Monday!

Part 2 was actually free if you anticipated a little bit while doing the first part, so after reading the subject, it only took me 30 seconds to adapt the return value (since I already knew which allergens and ingredients were linked).

Anyway, here is my solution for day #21:

const lines = input.split('\n').map((line) => {
    const [_ingredients, _allergens] = line.split(' (')
    return {
      ingredients: _ingredients.split(' '),
      allergens: _allergens.slice(9, -1).split(', '),

  const allAllergens = Array.from(new Set(lines.reduce((acc, line) => [...acc, ...line.allergens], [])))
  const allIngredients = Array.from(new Set(lines.reduce((acc, line) => [...acc, ...line.ingredients], [])))

  const struct = allAllergens.reduce((acc, allergen) => {
    acc[allergen] = [...allIngredients]
    return acc
  }, {}) => { => {
      if (line.allergens.includes(allergen)) {
        struct[allergen] = struct[allergen].filter((ingredient) => line.ingredients.includes(ingredient))

  while (true) {
    let found = false => {
      if (struct[foundAllergen].length === 1) {
        const foundIngredient = struct[foundAllergen][0] => {
          if (allergen === foundAllergen) return
          if (struct[allergen].includes(foundIngredient)) {
            found = true
            const index = struct[allergen].indexOf(foundIngredient)
            struct[allergen].splice(index, 1)

    if (!found) break

      .map((allergen) => struct[allergen][0])
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Feel free to share your solution in the comments!

Photo by Markus Spiske on Unsplash

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