Solve day 14!

source/day_14/mod.rs

@@ -0,0 +1,97 @@
+use std::collections::HashMap;
+
+use color_eyre::{eyre::eyre, Result};
+use itertools::Itertools;
+
+type PairMap = HashMap<(char, char), char>;
+type PairCounts = HashMap<(char, char), isize>;
+
+pub fn solve() -> Result<()> {
+  let input_data = include_str!("../../data/day_14.txt").trim();
+  println!("Day 14 Part 1: {}", part_1(input_data)?);
+  println!("Day 14 Part 2: {}", part_2(input_data)?);
+  Ok(())
+}
+
+fn parse(input: &str) -> Result<(String, PairMap)> {
+  let mut lines = input.lines();
+  let template = lines
+    .next()
+    .ok_or_else(|| eyre!("Invalid input: {}", input))?
+    .to_string();
+
+  let mut pairs = HashMap::new();
+
+  for line in lines.skip(1) {
+    let (a, b, c) = line
+      .replace(" -> ", "")
+      .chars()
+      .collect_tuple()
+      .ok_or_else(|| eyre!("Invalid line: {}", line))?;
+
+    pairs.insert((a, b), c);
+  }
+
+  Ok((template, pairs))
+}
+
+fn apply(counts: &PairCounts, pairs: &PairMap) -> PairCounts {
+  let mut to_add = PairCounts::new();
+
+  for (pair, count) in counts {
+    if let Some(character) = pairs.get(pair) {
+      *to_add.entry((pair.0, *character)).or_default() += count;
+      *to_add.entry((*character, pair.1)).or_default() += count;
+    }
+  }
+
+  to_add
+}
+
+fn count_totals(counts: &PairCounts) -> HashMap<char, isize> {
+  let mut totals = HashMap::new();
+
+  for ((_, b), count) in counts {
+    // If I count the first character in the pair the end result ends up being
+    // off by one...?
+    *totals.entry(*b).or_default() += count;
+  }
+
+  totals
+}
+
+fn run(input: &str, steps: isize) -> Result<isize> {
+  let (template, pairs) = parse(input)?;
+
+  let mut counts = PairCounts::new();
+  for (a, b) in template.chars().tuple_windows() {
+    *counts.entry((a, b)).or_default() += 1;
+  }
+
+  for _ in 0..steps {
+    counts = apply(&counts, &pairs);
+  }
+
+  let totals = count_totals(&counts)
+    .into_iter()
+    .sorted_by(|(_, a), (_, b)| a.cmp(b));
+
+  let (_, min) = totals
+    .clone()
+    .next()
+    .ok_or_else(|| eyre!("No minimum found"))?;
+  let (_, max) = totals
+    .clone()
+    .last()
+    .ok_or_else(|| eyre!("No maximum found"))?;
+
+  Ok(max - min)
+}
+
+fn part_1(input: &str) -> Result<isize> {
+  run(input, 10)
+}
+
+fn part_2(input: &str) -> Result<isize> {
+  run(input, 40)
+}

source/main.rs

@@ -12,6 +12,7 @@ mod day_07;
 mod day_08;
 mod day_09;
 mod day_10;
+mod day_14;
 
 fn main() -> Result<()> {
   color_eyre::install()?;
@@ -29,6 +30,7 @@ fn main() -> Result<()> {
     day_08::solve,
     day_09::solve,
     day_10::solve,
+    day_14::solve,
   ];
 
   for day in days {