2020: day21: part 1

aoc2020/src/day21.rs

@@ -0,0 +1,87 @@
+use std::collections::{HashMap, HashSet};
+use std::convert::TryInto;
+use std::fmt::Write;
+
+use anyhow::{Context, Result};
+
+const INPUT: &str = include_str!("../input/day21.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+/// Returns all words, and the number of times they appear in the input (useful for part1)
+fn get_all_words(input: &str) -> HashMap<&str, usize> {
+    input.lines().fold(HashMap::new(), |mut map, line| {
+        let left_paren = line.find('(').unwrap();
+
+        line[..(left_paren - 1)].split(' ').for_each(|word| {
+            *map.entry(word).or_default() += 1;
+        });
+
+        map
+    })
+}
+
+fn part1(input: &str) -> Result<usize> {
+    let matchings: AllergenMatchings = input.try_into()?;
+    let all_words = get_all_words(input);
+
+    // identify words that aren't in any allergen possible matching list
+    let not_allergens = all_words
+        .keys()
+        .filter(|&word| !matchings.0.values().any(|set| set.contains(word)));
+
+    Ok(not_allergens.map(|word| all_words[word]).sum())
+}
+
+#[derive(Debug)]
+struct AllergenMatchings<'a>(HashMap<&'a str, HashSet<&'a str>>);
+
+impl<'a> std::convert::TryFrom<&'a str> for AllergenMatchings<'a> {
+    type Error = anyhow::Error;
+
+    fn try_from(s: &'a str) -> Result<Self> {
+        let mut matchings = HashMap::new();
+
+        for line in s.lines() {
+            let left_paren = line.find('(').context("couldn't find open paren in line")?;
+            let right_paren = line
+                .find(')')
+                .context("couldn't find closing paren in line")?;
+
+            let allergens = line[(left_paren + "contains ".len() + 1)..right_paren].split(", ");
+
+            let words: HashSet<&str> = line[..(left_paren - 1)].split(' ').collect();
+
+            // update potential matches for each allergen mentioned in line
+            for allergen in allergens {
+                let set = matchings.entry(allergen).or_insert_with(|| words.clone());
+                *set = set.intersection(&words).copied().collect();
+            }
+        }
+
+        Ok(AllergenMatchings(matchings))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day21_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 5);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 2315);
+    }
+}

aoc2020/src/lib.rs

@@ -19,3 +19,4 @@ pub mod day16;
 pub mod day17;
 pub mod day18;
 pub mod day19;
+pub mod day21;

aoc2020/src/main.rs

@@ -21,6 +21,7 @@ use aoc2020::day16;
 use aoc2020::day17;
 use aoc2020::day18;
 use aoc2020::day19;
+use aoc2020::day21;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -43,6 +44,7 @@ fn main() -> Result<()> {
         day17::run,
         day18::run,
         day19::run,
+        day21::run,
     ];
 
     aoc::run(days)