2018 day 5

Author: ictrobot

crates/year2018/src/day05.rs

@@ -0,0 +1,57 @@
+use utils::prelude::*;
+
+/// Reducing the input by removing matching letter pairs.
+#[derive(Clone, Debug)]
+pub struct Day05 {
+    reacted: Vec<u8>,
+}
+
+impl Day05 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        if let Some(index) = input.find(|c: char| !c.is_ascii_alphabetic()) {
+            return Err(InputError::new(input, index, "expected ascii letter"));
+        }
+        if input.is_empty() {
+            return Err(InputError::new(input, 0, "expected at least one letter"));
+        }
+
+        Ok(Self {
+            reacted: Self::react(input.bytes()),
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> usize {
+        self.reacted.len()
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> usize {
+        (b'a'..=b'z')
+            .map(|l| Self::react(self.reacted.iter().copied().filter(|&b| b | 32 != l)).len())
+            .min()
+            .unwrap()
+    }
+
+    fn react(polymer: impl Iterator<Item = u8>) -> Vec<u8> {
+        let mut stack = Vec::with_capacity(polymer.size_hint().1.unwrap_or(0));
+        for b in polymer {
+            if let Some(last) = stack.last() {
+                if last ^ b == 32 {
+                    stack.pop();
+                    continue;
+                }
+            }
+            stack.push(b);
+        }
+        stack
+    }
+}
+
+examples!(Day05 -> (usize, usize) [
+    {input: "aA", part1: 0},
+    {input: "abBA", part1: 0},
+    {input: "abAB", part1: 4},
+    {input: "aabAAB", part1: 6},
+    {input: "dabAcCaCBAcCcaDA", part1: 10, part2: 4},
+]);

crates/year2018/src/lib.rs

@@ -6,4 +6,5 @@ utils::year!(2018 => year2018, ${
     2 => day02::Day02,
     3 => day03::Day03,
     4 => day04::Day04,
+    5 => day05::Day05,
 });