From 49f0df253f5ba4379f0ad494077b8b09b9f31482 Mon Sep 17 00:00:00 2001
From: Wesley Moore <wes@wezm.net>
Date: Tue, 11 Dec 2018 22:14:28 +1100
Subject: [PATCH] Add day 5 2018 parts 1 and 2

---
 2018/src/bin/day5.rs | 99 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 2018/src/bin/day5.rs

diff --git a/2018/src/bin/day5.rs b/2018/src/bin/day5.rs
new file mode 100644
index 0000000..adf333a
--- /dev/null
+++ b/2018/src/bin/day5.rs
@@ -0,0 +1,99 @@
+use std::fs;
+
+struct Polymer {
+    units: Vec<char>,
+}
+
+fn reacts(chr1: char, chr2: char) -> bool {
+    // The is_ascii_lowercase probably isn't strictly necessary but ¯\_(ツ)_/¯
+    (chr1.is_ascii_uppercase() && chr2 == chr1.to_ascii_lowercase())
+        || (chr1.is_ascii_lowercase() && chr2 == chr1.to_ascii_uppercase())
+}
+
+impl Polymer {
+    fn new() -> Self {
+        Polymer { units: Vec::new() }
+    }
+
+    fn last(&self) -> Option<char> {
+        self.units.last().map(|c| *c)
+    }
+
+    fn push(&mut self, chr: char) {
+        // if the char being pushed is the same as the top of stack then pop it, otherwise push
+        if self.last().map(|top| reacts(top, chr)).unwrap_or(false) {
+            self.units.pop();
+        } else {
+            self.units.push(chr);
+        }
+    }
+
+    fn into_string(self) -> String {
+        self.units.iter().collect()
+    }
+
+    fn len(&self) -> usize {
+        self.units.len()
+    }
+}
+
+fn main() {
+    let input = fs::read_to_string("input/day5.txt").expect("input");
+
+    part1(input.trim());
+    part2(input.trim());
+}
+
+fn part1(input: &str) {
+    let result = reduce(input);
+
+    println!("{}", result.len());
+}
+
+fn part2(input: &str) {
+    let shortest_length = "abcdefghijklmnopqrstuvwxyz"
+        .chars()
+        .map(|letter| {
+            // Remove letter/LETTER and LETTER/letter
+            let input = input
+                .replace(letter, "")
+                .replace(letter.to_ascii_uppercase(), "");
+            reduce(&input)
+        })
+        .min_by(|polymer_a, polymer_b| polymer_a.len().cmp(&polymer_b.len()));
+
+    println!("Part 2: {:?}", shortest_length.map(|polymer| polymer.len()));
+}
+
+fn reduce(input: &str) -> Polymer {
+    input.chars().fold(Polymer::new(), |mut polymer, chr| {
+        polymer.push(chr);
+        polymer
+    })
+}
+
+#[test]
+fn test_reacts() {
+    assert!(reacts('a', 'A'));
+    assert!(reacts('A', 'a'));
+    assert!(!reacts('A', 'A'));
+    assert!(!reacts('a', 'a'));
+    assert!(!reacts('a', 'B'));
+}
+
+#[test]
+fn test_part1() {
+    let input = "dabAcCaCBAcCcaDA";
+    let result = reduce(input);
+
+    assert_eq!(result.len(), 10);
+    assert_eq!(result.into_string(), "dabCBAcaDA".to_string());
+}
+
+#[test]
+fn test_part1_snippet() {
+    let input = "wUuXxrRbeEaAuUMmJUuXxvoFfMmOTjJmMtVvmMVWpdDPwGgAalVv";
+    let result = reduce(input);
+
+    assert_eq!(result.into_string(), "wbJl".to_string());
+}