2021 day14/rust: add solution

2021/day14/day14_rs/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "day14_rs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+nom = "7.1.0"

2021/day14/day14_rs/src/main.rs

@@ -0,0 +1,152 @@
+#![warn(clippy::pedantic)]
+use nom::{
+    bytes::complete::{tag, take_till},
+    character::complete::{anychar, newline},
+    combinator::recognize,
+    multi::many1,
+    sequence::{pair, separated_pair, terminated},
+};
+use std::collections::BTreeMap;
+use std::io::{stdin, Read};
+use std::ops::Add;
+
+type Input<'a> = &'a str;
+type IResult<'a, T> = nom::IResult<Input<'a>, T>;
+
+type Rule = ((char, char), char);
+
+fn rule(i: Input) -> IResult<Rule> {
+    separated_pair(pair(anychar, anychar), tag(" -> "), anychar)(i)
+}
+
+fn parse_input(i: Input) -> IResult<(&str, Vec<Rule>)> {
+    separated_pair(
+        terminated(recognize(take_till(|c| c == '\n')), newline),
+        newline,
+        many1(terminated(rule, newline)),
+    )(i)
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct Counter<T> {
+    counts: BTreeMap<T, usize>,
+}
+
+impl<T: Ord> Counter<T> {
+    pub fn new() -> Self {
+        Self {
+            counts: BTreeMap::new(),
+        }
+    }
+
+    pub fn push(&mut self, el: T) {
+        *self.counts.entry(el).or_insert(0) += 1;
+    }
+
+    pub fn most_common(&self) -> Option<(&T, usize)> {
+        self.counts
+            .iter()
+            .map(|(el, count)| (el, *count))
+            .max_by_key(|&(_, count)| count)
+    }
+
+    pub fn least_common(&self) -> Option<(&T, usize)> {
+        self.counts
+            .iter()
+            .map(|(el, count)| (el, *count))
+            .min_by_key(|&(_, count)| count)
+    }
+}
+
+impl<T: Ord> FromIterator<T> for Counter<T> {
+    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
+        let mut counts = Self::new();
+        for el in iter {
+            counts.push(el);
+        }
+        counts
+    }
+}
+
+impl<T: Ord + Copy> Add<&Counter<T>> for Counter<T> {
+    type Output = Self;
+
+    fn add(mut self, other: &Self) -> Self {
+        for (el, count) in &other.counts {
+            *self.counts.entry(*el).or_insert(0) += count;
+        }
+        self
+    }
+}
+
+impl<T: Ord> Add for Counter<T> {
+    type Output = Self;
+
+    fn add(mut self, other: Self) -> Self {
+        for (el, count) in other.counts {
+            *self.counts.entry(el).or_insert(0) += count;
+        }
+        self
+    }
+}
+
+#[derive(Clone, Debug)]
+struct LetterCounter {
+    rules: BTreeMap<(char, char), char>,
+    counts: BTreeMap<(char, char, usize), Counter<char>>,
+}
+
+impl LetterCounter {
+    pub fn new(rules: BTreeMap<(char, char), char>) -> Self {
+        let mut counts = BTreeMap::new();
+        for &(left, right) in rules.keys() {
+            counts.insert((left, right, 0), Counter::from_iter([left]));
+        }
+        Self { rules, counts }
+    }
+
+    pub fn get_counts_right_exclusive(
+        &mut self,
+        left: char,
+        right: char,
+        depth: usize,
+    ) -> &Counter<char> {
+        #[allow(clippy::map_entry)] // lifetimes don't work out
+        if !self.counts.contains_key(&(left, right, depth)) {
+            let middle = self.rules[&(left, right)];
+            let counts_left = self
+                .get_counts_right_exclusive(left, middle, depth - 1)
+                .clone();
+            let counts_right = self.get_counts_right_exclusive(middle, right, depth - 1);
+            let counts = counts_left + counts_right;
+            self.counts.insert((left, right, depth), counts);
+        }
+        &self.counts[&(left, right, depth)]
+    }
+}
+
+fn main() {
+    let mut input = String::new();
+    stdin().lock().read_to_string(&mut input).unwrap();
+
+    let (input, rules) = parse_input(&input).unwrap().1;
+
+    let rules: BTreeMap<_, _> = rules.into_iter().collect();
+    let chars: Vec<_> = input.chars().collect();
+
+    let mut counter = LetterCounter::new(rules);
+
+    let mut run = |steps| {
+        let mut totals = chars.windows(2).fold(Counter::new(), |counts, x| {
+            counts + counter.get_counts_right_exclusive(x[0], x[1], steps)
+        });
+        totals.push(*chars.last().unwrap());
+
+        println!(
+            "{:?}",
+            totals.most_common().unwrap().1 - totals.least_common().unwrap().1
+        );
+    };
+    run(10);
+    run(40);
+}