diff --git a/2018/src/day5.rs b/2018/src/day5.rs
new file mode 100644
index 0000000..e2d8849
--- /dev/null
+++ b/2018/src/day5.rs
@@ -0,0 +1,105 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
+use std::fmt;
+
+const INPUT: &str = include_str!("input/day5.txt");
+// const INPUT: &str = "dabAcCaCBAcCcaDA";
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+enum Polarity {
+    Upper,
+    Lower,
+}
+
+#[derive(Debug, Clone)]
+struct Unit {
+    name: String,
+    polarity: Polarity,
+}
+
+type Polymer = Vec<Unit>;
+impl fmt::Display for Unit {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let c = match self.polarity {
+            Polarity::Upper => self.name.to_uppercase(),
+            Polarity::Lower => self.name.to_lowercase(),
+        };
+        write!(f, "{}", c)
+    }
+}
+
+fn parse_input(s: String) -> Polymer {
+    let mut polymer = Vec::new();
+    for c in s.chars() {
+        polymer.push(Unit {
+            name: c.to_lowercase().to_string(),
+            polarity: if c.is_lowercase() {
+                Polarity::Lower
+            } else {
+                Polarity::Upper
+            },
+        });
+    }
+    polymer
+}
+
+fn can_cancel(u1: &Unit, u2: &Unit) -> bool {
+    u1.name == u2.name && u1.polarity != u2.polarity
+}
+
+// TODO: optimize
+fn reduce(p: &mut Polymer) -> &Polymer {
+    let mut i = 0;
+    while i + 1 < p.len() {
+        if can_cancel(&p[i], &p[i + 1]) {
+            p.remove(i + 1);
+            p.remove(i);
+            return p;
+        }
+        i += 1;
+    }
+    p
+}
+
+fn reduce_fully(p: &mut Polymer) -> &Polymer {
+    let mut old_len = p.len();
+    loop {
+        reduce(p);
+        let new_len = p.len();
+        if new_len == old_len {
+            break;
+        }
+        old_len = new_len;
+    }
+    p
+}
+
+pub fn part1() -> usize {
+    let mut polymer = parse_input(INPUT.to_string());
+    reduce_fully(&mut polymer);
+    polymer.len()
+}
+
+fn unit_types(p: Polymer) -> HashSet<String> {
+    let mut types = HashSet::new();
+    for u in p {
+        types.insert(u.name);
+    }
+    types
+}
+
+fn remove_all(p: Polymer, t: String) -> Polymer {
+    p.into_iter().filter(|u| u.name != t).collect()
+}
+
+pub fn part2() -> &'static str {
+    let original = parse_input(INPUT.to_string());
+    let types = unit_types(original.clone());
+    let mut lengths = HashMap::new();
+    for t in types {
+        let mut cleaned = remove_all(original.clone(), t.clone());
+        reduce_fully(&mut cleaned);
+        lengths.insert(t, cleaned.len());
+    }
+    "TODO"
+}
diff --git a/2018/src/main.rs b/2018/src/main.rs
index bde3e90..8049e74 100644
--- a/2018/src/main.rs
+++ b/2018/src/main.rs
@@ -2,6 +2,7 @@ mod day1;
 mod day2;
 mod day3;
 mod day4;
+// mod day5; // TODO: optimize
 
 fn main() {
     println!("Day 1 part 1: {}", day1::part1());
@@ -12,4 +13,6 @@ fn main() {
     println!("Day 3 part 2: {}", day3::part2());
     println!("Day 4 part 1: {}", day4::part1());
     println!("Day 4 part 2: {}", day4::part2());
+    // println!("Day 5 part 1: {}", day5::part1()); // TODO: optimize
+    // println!("Day 5 part 2: {}", day5::part2()); // TODO: optimize
 }