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45 | 45 | //! [`Day 16`]: crate::year2020::day16 |
46 | 46 | //! [`count_ones`]: u32::count_ones |
47 | 47 | use crate::util::hash::*; |
| 48 | +use std::collections::BTreeMap; |
48 | 49 |
|
49 | 50 | pub struct Input<'a> { |
50 | 51 | ingredients: FastMap<&'a str, Ingredient>, |
@@ -102,34 +103,32 @@ pub fn part1(input: &Input<'_>) -> u32 { |
102 | 103 | } |
103 | 104 |
|
104 | 105 | pub fn part2(input: &Input<'_>) -> String { |
105 | | - let mut ingredients = input.ingredients.clone(); |
106 | | - ingredients.retain(|_, v| v.candidates != 0); |
107 | | - |
108 | 106 | let inverse_allergens: FastMap<_, _> = |
109 | 107 | input.allergens.iter().map(|(k, v)| (1 << v, k)).collect(); |
110 | | - |
111 | | - // There must be at least one ingredient with only one allergen. |
112 | | - let mut todo: Vec<_> = ingredients |
| 108 | + let mut todo: Vec<_> = input |
| 109 | + .ingredients |
113 | 110 | .iter() |
114 | | - .filter(|(_, v)| v.candidates.count_ones() == 1) |
115 | | - .map(|(k, v)| (*k, v.candidates)) |
| 111 | + .filter_map(|(&k, &v)| (v.candidates != 0).then_some((k, v.candidates))) |
116 | 112 | .collect(); |
117 | | - let mut done = Vec::new(); |
| 113 | + let mut done = BTreeMap::new(); |
118 | 114 |
|
119 | 115 | // Eliminate known allergens from other ingredients. |
120 | | - while let Some(pair @ (next, allergen)) = todo.pop() { |
121 | | - ingredients.remove(next); |
122 | | - done.push(pair); |
123 | | - |
124 | | - for (name, ingredient) in &mut ingredients { |
125 | | - ingredient.candidates &= !allergen; |
126 | | - if ingredient.candidates.count_ones() == 1 { |
127 | | - todo.push((name, ingredient.candidates)); |
| 116 | + while done.len() < todo.len() { |
| 117 | + let mut mask = 0; |
| 118 | + |
| 119 | + // There must be at least one ingredient with only one allergen. |
| 120 | + for (name, candidates) in &todo { |
| 121 | + if candidates.count_ones() == 1 { |
| 122 | + let allergen = inverse_allergens[candidates]; |
| 123 | + done.insert(*allergen, *name); |
| 124 | + |
| 125 | + mask |= candidates; |
128 | 126 | } |
129 | 127 | } |
| 128 | + |
| 129 | + todo.iter_mut().for_each(|(_, candidates)| *candidates &= !mask); |
130 | 130 | } |
131 | 131 |
|
132 | 132 | // Sort by alphabetical order of the allergens. |
133 | | - done.sort_by_cached_key(|(_, v)| inverse_allergens[v]); |
134 | | - done.iter().map(|(k, _)| *k).collect::<Vec<_>>().join(",") |
| 133 | + done.into_values().collect::<Vec<_>>().join(",") |
135 | 134 | } |
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