2018: day05: part 2

aoc2018/src/day05.rs

@@ -1,17 +1,24 @@
+use std::collections::HashSet;
+
 use super::Result;
 
 const INPUT: &str = include_str!("../input/day05.txt");
 
 pub fn run() -> Result<()> {
     println!("part 1: {}", part1(INPUT));
+    println!("part 2: {}", part2(INPUT));
 
     Ok(())
 }
 
-fn same_type(a: char, b: char) -> bool {
+fn same_type(a: &char, b: &char) -> bool {
     a.to_ascii_lowercase() == b.to_ascii_lowercase()
 }
 
+fn remove_type(input: &str, c: &char) -> String {
+    input.chars().filter(|ch| !same_type(c, ch)).collect()
+}
+
 fn collapse(input: &str) -> String {
     let mut res = String::with_capacity(input.len());
 
@@ -22,7 +29,7 @@ fn collapse(input: &str) -> String {
         match last {
             Some(elem) => {
                 // if same type but different polarity
-                if same_type(elem, next) && elem != next {
+                if same_type(&elem, &next) && elem != next {
                     // drop both elem and next
                     last = res.pop();
                 } else {
@@ -50,6 +57,19 @@ fn part1(input: &str) -> usize {
     res.len()
 }
 
+fn part2(input: &str) -> usize {
+    let mut set = HashSet::new();
+
+    for c in input.chars() {
+        set.insert(c.to_ascii_lowercase());
+    }
+
+    set.iter()
+        .map(|c| collapse(&remove_type(input, c)).len())
+        .min()
+        .unwrap()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -65,4 +85,14 @@ mod tests {
     fn part1_real() {
         assert_eq!(part1(INPUT), 10638);
     }
+
+    #[test]
+    fn part2_provided() {
+        assert_eq!(part2(PROVIDED), 4);
+    }
+
+    #[test]
+    fn part2_real() {
+        assert_eq!(part2(INPUT), 4944);
+    }
 }