1 files changed, 58 insertions, 0 deletions
diff --git a/src/bin/day19.rs b/src/bin/day19.rs
new file mode 100644
index 0000000..7c1e6b4
--- /dev/null
+++ b/src/bin/day19.rs
@@ -0,0 +1,58 @@
+use std::collections::{HashMap, HashSet};
+
+fn main() {
+    let input = advent::read_lines(19);
+    println!("19a: {}", distinct_molecules(&input));
+    println!("19b: {}", 0);
+}
+
+fn distinct_molecules(input: &[String]) -> usize {
+    let mut input = input.to_owned();
+    let molecule = input.pop().unwrap();
+    input.pop(); // drop empty line
+
+    let mut replacements = HashMap::new();
+    for line in input {
+        let replacement : Vec<_> = line.split(" => ").collect();
+        let (left, right) = (replacement[0].to_string(), replacement[1].to_string());
+        replacements.entry(left).or_insert(Vec::new()).push(right);
+    }
+
+    let mut result = HashSet::new();
+
+    for (left, rights) in replacements {
+        for right in rights {
+            for i in 0 .. molecule.len() {
+                let check_pos = std::cmp::min(i + left.len(), molecule.len());
+                if molecule[i..check_pos].contains(&left) {
+                    let new_molecule = molecule[0..i].to_string()
+                                     + &right
+                                     + &molecule[(i + left.len())..];
+                    result.insert(new_molecule);
+                }
+            }
+        }
+    }
+
+    result.len()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let mut input = Vec::from([
+            "H => HO".to_string(),
+            "H => OH".to_string(),
+            "O => HH".to_string(),
+            "".to_string(),
+            "HOH".to_string(),
+        ]);
+        assert_eq!(distinct_molecules(&input), 4);
+        input.pop();
+        input.push("HOHOHO".to_string());
+        assert_eq!(distinct_molecules(&input), 7);
+    }
+}