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use nom::{
bytes::complete::tag,
character::complete::{alpha1, anychar, line_ending},
combinator::map,
multi::{many0, separated_list1},
sequence::tuple,
IResult,
};
use std::{collections::BTreeMap, fs};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let input = fs::read_to_string("inputs/day_14.txt")?;
let mut polymer = parse_polymer_expansion(&input).unwrap().1;
for _ in 0..10 {
polymer.expand();
}
dbg!(polymer.most_common_element_count() - polymer.least_common_element_count());
for _ in 0..30 {
polymer.expand();
}
dbg!(polymer.most_common_element_count() - polymer.least_common_element_count());
Ok(())
}
#[derive(Debug)]
struct PolymerExpansion {
polymer: Vec<char>,
rules: Rules,
element_counts: BTreeMap<char, u64>,
}
impl PolymerExpansion {
fn new(polymer: Vec<char>, rules: Rules) -> PolymerExpansion {
let mut element_counts = BTreeMap::new();
for c in &polymer {
*element_counts.entry(*c).or_insert(0) += 1;
}
PolymerExpansion {
polymer,
rules,
element_counts,
}
}
fn expand(&mut self) {
let mut next_polymer = Vec::new();
for i in 0..self.polymer.len() - 1 {
next_polymer.push(self.polymer[i]);
if let Some(extra) = self.rules.get(&[self.polymer[i], self.polymer[i + 1]]) {
next_polymer.push(extra);
*self.element_counts.entry(extra).or_insert(0) += 1;
}
}
next_polymer.push(self.polymer[self.polymer.len() - 1]);
self.polymer = next_polymer;
}
fn most_common_element_count(&self) -> u64 {
self.element_counts.values().max().cloned().unwrap_or(0)
}
fn least_common_element_count(&self) -> u64 {
self.element_counts.values().min().cloned().unwrap_or(0)
}
}
#[derive(Debug)]
struct Rules {
rules: BTreeMap<[char; 2], char>,
}
impl Rules {
fn get(&self, key: &[char; 2]) -> Option<char> {
self.rules.get(key).cloned()
}
}
fn parse_polymer_expansion(input: &str) -> IResult<&str, PolymerExpansion> {
let (input, polymer) = map(alpha1, |s: &str| s.chars().collect())(input)?;
let (input, _) = many0(line_ending)(input)?;
let (input, rules) = parse_rules(input)?;
Ok((input, PolymerExpansion::new(polymer, rules)))
}
fn parse_rules(input: &str) -> IResult<&str, Rules> {
map(separated_list1(line_ending, parse_rule), |rules| Rules {
rules: rules.into_iter().collect(),
})(input)
}
fn parse_rule(input: &str) -> IResult<&str, ([char; 2], char)> {
map(
tuple((anychar, anychar, tag(" -> "), anychar)),
|(p1, p2, _, r)| ([p1, p2], r),
)(input)
}
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