day 14

2021/src/day14.rs

@@ -0,0 +1,83 @@
+use std::collections::HashMap;
+
+use color_eyre::eyre;
+
+use crate::lib::{IterExt, Counter};
+
+
+type Pairs = Counter<(char, char)>;
+type Rules = HashMap<(char, char), char>;
+
+
+fn load(data: &str) -> eyre::Result<(Pairs, Rules)> {
+    let mut lines = data.lines();
+    let template = lines.next().unwrap().to_string();
+    let mut pairs = Counter::new();
+    for i in 1..template.len() {
+        let a = template.as_bytes()[i - 1] as char;
+        let b = template.as_bytes()[i] as char;
+        pairs.inc((a, b));
+    }
+    lines.next();
+
+    let mut rules = HashMap::new();
+    for line in &mut lines {
+        let (pair_s, tgt_s) = line.split(" -> ").take_pair()?;
+        let pair = pair_s.chars().take_pair()?;
+        let tgt = tgt_s.chars().next().unwrap();
+        rules.insert(pair, tgt);
+    }
+
+    Ok((pairs, rules))
+}
+
+
+fn expand(src: &Pairs, rules: &Rules) -> Pairs {
+    let mut dst = Counter::new();
+    for (&(a, b), count) in src.iter() {
+        if let Some(&tween) = rules.get(&(a, b)) {
+            dst.inc_by((a, tween), count);
+            dst.inc_by((tween, b), count);
+        }
+    }
+    dst
+}
+
+
+fn score(pairs: &Pairs) -> usize {
+    // "score" is defined as the number of occurences of the most common
+    // character minus the number of occurrences of the least common
+    let mut firsts = Counter::new();
+    let mut lasts = Counter::new();
+    for (pair, count) in pairs.iter() {
+        let (a, b) = *pair;
+        firsts.inc_by(a, count);
+        lasts.inc_by(b, count);
+    }
+
+    // the count of a given character is the number of times it appears
+    // in either first or last place of a pair, whichever is greater
+    let mut counts = Counter::new();
+    for (c, count) in firsts.iter().chain(lasts.iter()) {
+        if count > counts.get(c) {
+            counts.set(*c, count);
+        }
+    }
+
+    let max = counts.iter().map(|(_c, n)| n).max().unwrap();
+    let min = counts.iter().map(|(_c, n)| n).min().unwrap();
+    max - min
+}
+
+
+pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
+    let (mut pairs, rules) = load(data)?;
+    
+    for _i in 0..10 {pairs = expand(&pairs, &rules);}
+    let one = score(&pairs);
+
+    for _i in 0..30 {pairs = expand(&pairs, &rules);}
+    let two = score(&pairs);
+
+    Ok((one, two))
+}

2021/src/lib.rs

@@ -1,9 +1,11 @@
 use std::io::Read;
 use std::collections::VecDeque;
-use std::str::FromStr;
-use std::fs::File;
+use std::collections::HashMap;
 use std::fmt::{Display, Formatter};
+use std::fs::File;
+use std::hash::Hash;
 use std::ops::{Index, IndexMut};
+use std::str::FromStr;
 
 use color_eyre::{eyre};
 use eyre::eyre;
@@ -264,3 +266,54 @@ impl<T: Display> Display for Vec2<T> {
         Ok(())
     }
 }
+
+
+#[derive(Debug, Clone)]
+pub struct Counter<K> {
+    data: HashMap<K, usize>
+}
+
+#[allow(dead_code)]
+impl<K> Counter<K> 
+    where K: Eq + Hash,
+{
+    pub fn new() -> Self {
+        Counter {data: HashMap::<K, usize>::new()}
+    }
+
+    pub fn inc(&mut self, key: K) {
+        let v = self.data.entry(key).or_insert(0);
+        *v += 1;
+    }
+
+    pub fn inc_by(&mut self, key: K, increment: usize) {
+        let v = self.data.entry(key).or_insert(0);
+        *v += increment;
+    }
+
+    pub fn dec(&mut self, key: K) {
+        let v = self.data.entry(key).or_insert(0);
+        if *v > 0 {
+            *v -= 1;
+        }
+    }
+
+    pub fn dec_by(&mut self, key: K, decrement: usize) {
+        let v = self.data.entry(key).or_insert(0);
+        if *v >= decrement {
+            *v -= decrement;
+        }
+    }
+
+    pub fn get(&mut self, key: &K) -> usize {
+        *self.data.get(key).unwrap_or(&0)
+    }
+
+    pub fn set(&mut self, key: K, value: usize) {
+        self.data.insert(key, value);
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item =(&'_ K, usize)> {
+        self.data.iter().map(|(k, v)| (k, *v))
+    }
+}

2021/src/main.rs

@@ -4,14 +4,14 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day13;
+mod day14;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/13.txt")?;
+    let data = load("data/14.txt")?;
 
     let start = Instant::now();
-    let (one, two) = day13::run(&data)?;
+    let (one, two) = day14::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
 
     let precision = 2.0 - dur.log10().floor();