Day 5 part 1

Cargo.lock

@@ -3,6 +3,7 @@ name = "advent-of-code-2018"
 version = "0.1.0"
 dependencies = [
  "chrono 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "lazy_static 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "regex 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 

Cargo.toml

@@ -6,4 +6,5 @@ edition = "2018"
 
 [dependencies]
 chrono = "0.4"
+lazy_static = "1.2.0"
 regex = "1"

inputs/5_test.txt

@@ -0,0 +1 @@
+dabAcCaCBAcCcaDA

src/day5.rs

@@ -0,0 +1,70 @@
+extern crate regex;
+
+use std::error::Error;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+use regex::Regex;
+
+const INPUT: &str = "inputs/5.txt";
+
+pub fn solve_part1() -> Result<usize, Box<Error>> {
+    let polymer = read_polymer(INPUT)?;
+    Ok(reduce_polymer_completely(polymer).len())
+}
+
+fn read_polymer(filename: &str) -> Result<String, Box<Error>> {
+    let file = File::open(filename)?;
+    let polymer = BufReader::new(file).lines().next().unwrap_or(Ok("".to_string()));
+    Ok(polymer?)
+}
+
+fn reduce_polymer(polymer: &String) -> String {
+    lazy_static! {
+        static ref REACTING_UNITS: Regex = Regex::new(concat!(
+            r"aA|bB|cC|dD|eE|fF|gG|hH|iI|jJ|kK|lL|mM|nN|",
+            r"oO|pP|qQ|rR|sS|tT|uU|vV|wW|xX|yY|zZ|",
+            r"Aa|Bb|Cc|Dd|Ee|Ff|Gg|Hh|Ii|Jj|Kk|Ll|Mm|Nn|",
+            r"Oo|Pp|Qq|Rr|Ss|Tt|Uu|Vv|Ww|Xx|Yy|Zz")).unwrap();
+    }
+    REACTING_UNITS.replace_all(&polymer, "").to_string()
+}
+
+fn reduce_polymer_completely(polymer: String) -> String {
+    let reduced = reduce_polymer(&polymer);
+    if reduced == polymer {
+        reduced
+    } else {
+        reduce_polymer_completely(reduced)
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT: &str = "inputs/5_test.txt";
+
+    #[test]
+    fn reduces_polymer() {
+        assert_eq!(reduce_polymer(&"aA".to_string()), "");
+        assert_eq!(reduce_polymer(&"aAbB".to_string()), "");
+        assert_eq!(reduce_polymer(&"aAfgbB".to_string()), "fg");
+        assert_eq!(reduce_polymer(&"abAB".to_string()), "abAB");
+        assert_eq!(reduce_polymer(&"aabAAB".to_string()), "aabAAB");
+        assert_eq!(reduce_polymer(&"dabAcCaCBAcCcaDA".to_string()), "dabAaCBAcaDA");
+        assert_eq!(reduce_polymer(&"dabAaCBAcCcaDA".to_string()), "dabCBAcaDA");
+        assert_eq!(reduce_polymer(&"dabCBAcCcaDA".to_string()), "dabCBAcaDA");
+    }
+
+    #[test]
+    fn reduces_polymer_completely() {
+        assert_eq!(reduce_polymer_completely("dabAcCaCBAcCcaDA".to_string()), "dabCBAcaDA");
+    }
+
+    #[test]
+    fn reads_polymer() {
+        assert_eq!(read_polymer(TEST_INPUT).unwrap(), "dabAcCaCBAcCcaDA");
+    }
+}

src/main.rs

@@ -1,7 +1,11 @@
+#[macro_use]
+extern crate lazy_static;
+
 mod day1;
 mod day2;
 mod day3;
 mod day4;
+mod day5;
 
 fn main() {
     println!("Day 1:");
@@ -16,4 +20,6 @@ fn main() {
     println!("Day 4:");
     println!("{}", day4::solve_part1().unwrap());
     println!("{}", day4::solve_part2().unwrap());
+    println!("Day 5:");
+    println!("{}", day5::solve_part1().unwrap());
 }