aoc/year2021/
day14.rs

1//! # Extended Polymerization
2//!
3//! The key insight to this problem is the same as [`Day 6`]. We track the *total* number of
4//! each pair as the positions don't affect the final result.
5//!
6//! Fixed sized arrays are used for speed as we know that the elements are limited to 26 values
7//! and the possible pairs to 26 × 26 values.
8//!
9//! [`Day 6`]: crate::year2021::day06
10use crate::util::iter::*;
11
12type Elements = [u64; 26];
13type Pairs = [u64; 26 * 26];
14type Rules = Vec<Rule>;
15
16pub struct Rule {
17    from: usize,
18    to_left: usize,
19    to_right: usize,
20    element: usize,
21}
22
23impl Rule {
24    fn parse([a, b, c]: [u8; 3]) -> Rule {
25        Rule { from: pair(a, b), to_left: pair(a, c), to_right: pair(c, b), element: element(c) }
26    }
27}
28
29pub struct Input {
30    elements: Elements,
31    pairs: Pairs,
32    rules: Rules,
33}
34
35/// Count the initial pairs and elements and parse each instruction into a [`Rule`] struct.
36pub fn parse(input: &str) -> Input {
37    let (prefix, suffix) = input.split_once("\n\n").unwrap();
38    let prefix = prefix.trim().as_bytes();
39
40    let mut elements = [0; 26];
41    prefix.iter().for_each(|&b| elements[element(b)] += 1);
42
43    let mut pairs = [0; 26 * 26];
44    prefix.array_windows().for_each(|&[a, b]| pairs[pair(a, b)] += 1);
45
46    let rules: Vec<_> =
47        suffix.bytes().filter(u8::is_ascii_uppercase).chunk::<3>().map(Rule::parse).collect();
48
49    Input { elements, pairs, rules }
50}
51
52/// Apply 10 steps.
53pub fn part1(input: &Input) -> u64 {
54    steps(input, 10)
55}
56
57/// Apply 40 steps.
58pub fn part2(input: &Input) -> u64 {
59    steps(input, 40)
60}
61
62/// Simulate an arbitrary number of steps.
63///
64/// A rule `AC` -> `ABC` implies that for each pair `AC` we create an equal number of pairs
65/// `AB` and `BC`, then increment the amount of element `B`.
66fn steps(input: &Input, rounds: usize) -> u64 {
67    let mut elements = input.elements;
68    let mut pairs = input.pairs;
69    let rules = &input.rules;
70
71    for _ in 0..rounds {
72        let mut next: Pairs = [0; 26 * 26];
73
74        for rule in rules {
75            let n = pairs[rule.from];
76            next[rule.to_left] += n;
77            next[rule.to_right] += n;
78            elements[rule.element] += n;
79        }
80
81        pairs = next;
82    }
83
84    let max = elements.iter().max().unwrap();
85    let min = elements.iter().filter(|&&n| n > 0).min().unwrap();
86    max - min
87}
88
89/// Convert a single uppercase ASCII character to an index between 0 and 25.
90fn element(byte: u8) -> usize {
91    (byte - b'A') as usize
92}
93
94/// Convert two uppercase ASCII characters to an index between 0 and 675.
95fn pair(first: u8, second: u8) -> usize {
96    26 * element(first) + element(second)
97}
aoc/year2021/day14.rs

aoc/year2021/
day14.rs
