Day 14

src/bin/14.rs

@@ -0,0 +1,117 @@
+use std::{collections::HashMap, hash::Hash};
+
+fn main() {
+    let mut puzzle = input();
+    for _ in 0..10 {
+        puzzle.step();
+    }
+    println!("First solution: {}", puzzle.score());
+    for _ in 0..30 {
+        puzzle.step();
+    }
+    println!("Second solution: {}", puzzle.score());
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+struct Digram(char, char);
+
+#[derive(Debug)]
+struct Puzzle {
+    /// polymer is a count of all digrams in it.
+    polymer: HashMap<Digram, usize>,
+    /// start and end need to be kept track of, since all other chars are counted doubly,
+    /// due to the nature of counting the digrams.
+    start: char,
+    end: char,
+    instructions: Vec<Instruction>,
+}
+
+/// ab -> to
+#[derive(Debug, Clone, Copy)]
+struct Instruction {
+    from: Digram,
+    to: char,
+}
+
+impl Puzzle {
+    fn step(&mut self) {
+        let mut new_polymer = HashMap::new();
+        for (&old_digram, &count) in &self.polymer {
+            if let Some(i) = self.find_instruction(old_digram) {
+                for new_digram in i.output() {
+                    let counter = new_polymer.entry(new_digram).or_default();
+                    *counter += count;
+                }
+            } else {
+                let counter = new_polymer.entry(old_digram).or_default();
+                *counter += count;
+            }
+        }
+        self.polymer = new_polymer;
+    }
+
+    fn find_instruction(&self, d: Digram) -> Option<Instruction> {
+        self.instructions.iter().copied().find(|i| i.from == d)
+    }
+
+    fn score(&self) -> usize {
+        // All chars were counted doubly, due to the nature of the digrams, except start and end.
+        // Add one for those manually here.
+        let mut char_count: HashMap<char, usize> =
+            [(self.start, 1), (self.end, 1)].into_iter().collect();
+
+        for (&d, &count) in &self.polymer {
+            for c in [d.0, d.1] {
+                let counter = char_count.entry(c).or_default();
+                *counter += count;
+            }
+        }
+
+        let mut doubly_counted: Vec<usize> = char_count.values().copied().collect();
+        doubly_counted.sort_unstable();
+
+        (doubly_counted.last().unwrap() - doubly_counted.first().unwrap()) / 2
+    }
+}
+
+impl Instruction {
+    fn output(&self) -> [Digram; 2] {
+        [Digram(self.from.0, self.to), Digram(self.to, self.from.1)]
+    }
+}
+
+fn input() -> Puzzle {
+    let file = include_str!("../../input/14.txt");
+
+    let polymer_chars: Vec<char> = file.lines().next().unwrap().trim().chars().collect();
+    let polymer = polymer_chars
+        .windows(2)
+        .into_iter()
+        .map(|w| Digram(w[0], w[1]))
+        .fold(HashMap::new(), |mut acc, d| {
+            let count = acc.entry(d).or_default();
+            *count += 1;
+            acc
+        });
+
+    let instructions = file
+        .lines()
+        .filter(|l| l.contains("->"))
+        .filter_map(|l| l.split_once(" -> "))
+        .map(|(from, to)| {
+            let a = from.chars().next().unwrap();
+            let b = from.chars().nth(1).unwrap();
+            Instruction {
+                from: Digram(a, b),
+                to: to.chars().next().unwrap(),
+            }
+        })
+        .collect();
+
+    Puzzle {
+        polymer,
+        instructions,
+        start: polymer_chars.first().copied().unwrap(),
+        end: polymer_chars.last().copied().unwrap(),
+    }
+}