2020: day21: part 2

This commit is contained in:
Antoine Martin 2020-12-21 19:21:14 +01:00
parent 33fbe9ce5a
commit fbd7ae5be2

View file

@ -10,6 +10,7 @@ pub fn run() -> Result<String> {
let mut res = String::with_capacity(128);
writeln!(res, "part 1: {}", part1(INPUT)?)?;
writeln!(res, "part 2: {}", part2(INPUT)?)?;
Ok(res)
}
@ -39,6 +40,56 @@ fn part1(input: &str) -> Result<usize> {
Ok(not_allergens.map(|word| all_words[word]).sum())
}
fn part2(input: &str) -> Result<String> {
let mut matchings: AllergenMatchings = input.try_into()?;
let mut allergens_to_identify: Vec<&str> = matchings.0.keys().copied().collect();
for _ in 0..matchings.0.len() {
let allergen = allergens_to_identify
.iter()
.min_by_key(|&name| matchings.0[name].len())
.copied()
.expect("should always have at least one allergen to identify");
// the algorithm only works if we can always find an allergen with only one possible
// assignation
assert_eq!(matchings.0[allergen].len(), 1);
let allergen_translation = matchings.0[allergen].iter().copied().next().unwrap();
matchings
.0
.iter_mut()
.filter(|(&allerg, _)| allerg != allergen)
.for_each(|(_, possible_matchings)| {
possible_matchings.remove(allergen_translation);
});
allergens_to_identify.swap_remove(
allergens_to_identify
.iter()
.position(|al| *al == allergen)
.unwrap(),
);
}
// Vec of (allergen, translation)
let mut matchings: Vec<(&str, &str)> = matchings
.0
.iter()
.map(|(&key, possibilities)| (key, possibilities.iter().copied().next().unwrap()))
.collect();
matchings.sort_unstable();
let canonical_ingredient_list: Vec<&str> = matchings
.iter()
.map(|(_, translation)| *translation)
.collect();
Ok(canonical_ingredient_list.join(","))
}
#[derive(Debug)]
struct AllergenMatchings<'a>(HashMap<&'a str, HashSet<&'a str>>);
@ -84,4 +135,17 @@ mod tests {
fn part1_real() {
assert_eq!(part1(INPUT).unwrap(), 2315);
}
#[test]
fn part2_provided() {
assert_eq!(part2(PROVIDED).unwrap(), "mxmxvkd,sqjhc,fvjkl");
}
#[test]
fn part2_real() {
assert_eq!(
part2(INPUT).unwrap(),
"cfzdnz,htxsjf,ttbrlvd,bbbl,lmds,cbmjz,cmbcm,dvnbh"
);
}
}