day21

src/day21.rs

@@ -0,0 +1,75 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+use std::collections::{HashMap, HashSet};
+
+#[derive(Debug)]
+struct Food<'a> {
+    ingredients: HashSet<&'a str>,
+    allergens: HashSet<&'a str>,
+}
+impl<'a> Food<'a> {
+    fn parse(input: &'a str) -> Option<Self> {
+        let (ingredients, allergens) = split_once(input, " (contains ")?;
+        let allergens = allergens.strip_suffix(")")?;
+        Some(Food {
+            ingredients: ingredients.split(" ").collect(),
+            allergens: allergens.split(", ").collect(),
+        })
+    }
+}
+
+#[aoc(day21, part1)]
+fn solve_d2_p1(input: &str) -> usize {
+    //let input = EXAMPLE;
+    let foods: Vec<_> = input.split('\n').map(|l| Food::parse(l).unwrap()).collect();
+
+    let mut allergen_causes = HashMap::new();
+    for food in &foods {
+        for &allergen in &food.allergens {
+            let ingredients = allergen_causes
+                .entry(allergen)
+                .or_insert_with(|| food.ingredients.clone());
+            ingredients.retain(|ingredient| food.ingredients.contains(ingredient));
+        }
+    }
+    let possible_allergens: HashSet<_> = allergen_causes.values().flatten().collect();
+    foods
+        .iter()
+        .flat_map(|food| &food.ingredients)
+        .filter(|ingredient| !possible_allergens.contains(ingredient))
+        .count()
+}
+
+#[aoc(day21, part2)]
+fn solve_d2_p2(input: &str) -> String {
+    let foods: Vec<_> = input.split('\n').map(|l| Food::parse(l).unwrap()).collect();
+
+    let mut allergen_causes = HashMap::new();
+    for food in &foods {
+        for &allergen in &food.allergens {
+            let ingredients = allergen_causes
+                .entry(allergen)
+                .or_insert_with(|| food.ingredients.clone());
+            ingredients.retain(|ingredient| food.ingredients.contains(ingredient));
+        }
+    }
+    let mut dangerous_ingredients = Vec::new();
+    while let Some(&allergen) = allergen_causes
+        .iter()
+        .find(|(_k, v)| v.len() == 1)
+        .map(|(k, _v)| k)
+    {
+        let (allergen, mut ingredient) = allergen_causes.remove_entry(allergen).unwrap();
+        let ingredient = ingredient.drain().next().unwrap();
+        dangerous_ingredients.push((allergen, ingredient));
+        for ingredient_set in allergen_causes.values_mut() {
+            ingredient_set.remove(&ingredient);
+        }
+    }
+    dangerous_ingredients.sort_by(|(a, _), (b, _)| a.cmp(b));
+    dangerous_ingredients
+        .into_iter()
+        .map(|(_allergen, ingredient)| ingredient)
+        .collect::<Vec<_>>()
+        .join(",")
+}

src/lib.rs

@@ -11,6 +11,7 @@ pub mod day18;
 pub mod day19;
 pub mod day2;
 pub mod day20;
+pub mod day21;
 pub mod day3;
 pub mod day4;
 pub mod day5;