Day 19 part 2.

src/day18.rs

@@ -65,7 +65,6 @@ fn part2(input: &Field) -> usize {
     current.into_iter().flatten().filter(|x| *x).count()
 }
 
-
 fn count_neighbors(field: &Field, x: usize, y: usize) -> usize {
     let mut result = 0;
     if field[x - 1][y - 1] {

src/day19.rs

@@ -1,4 +1,5 @@
-use std::collections::HashSet;
+use std::collections::{HashSet, VecDeque};
+
 use aoc_runner_derive::{aoc, aoc_generator};
 
 type Transformation = (String, String);
@@ -6,27 +7,35 @@ type Transformation = (String, String);
 #[aoc_generator(day19)]
 fn parse(input: &str) -> (Vec<Transformation>, String) {
     let (a, b) = input.split_once("\n\n").unwrap();
-    let t = a.lines().map(|line| {
+    let t = a
+        .lines()
+        .map(|line| {
             let (lhs, rhs) = line.split_once(" => ").unwrap();
             (lhs.to_string(), rhs.to_string())
-    }).collect();
+        })
+        .collect();
     (t, b.to_string())
 }
 
 #[aoc(day19, part1)]
 fn part1((transformations, start): &(Vec<Transformation>, String)) -> usize {
-    let all: HashSet<String> = transformations.iter()
+    let all = possibilities(transformations, start);
+    all.len()
+}
+
+fn possibilities(transformations: &[Transformation], start: &str) -> HashSet<String> {
+    let all: HashSet<String> = transformations
+        .iter()
         .flat_map(|(from, to)| {
             let start_indexes = starts(start, from);
-            start_indexes.into_iter()
+            start_indexes.into_iter().map(|si| {
-                .map(|si| {
+                let mut s = start.to_string();
-                    let mut s = start.clone();
                 s.replace_range(si..si + from.len(), to);
                 s
             })
-        }).collect();
+        })
-
+        .collect();
-    all.len()
+    all
 }
 
 fn starts(a: &str, pattern: &str) -> Vec<usize> {
@@ -39,6 +48,29 @@ fn starts(a: &str, pattern: &str) -> Vec<usize> {
     result
 }
 
+#[aoc(day19, part2)]
+fn part2((transformations, target): &(Vec<Transformation>, String)) -> usize {
+    search("e", transformations, target)
+}
+
+fn search(base: &str, transformations: &[Transformation], target: &str) -> usize {
+    let mut seen = HashSet::new();
+    seen.insert(base.to_string());
+    let mut queue = VecDeque::new();
+    queue.push_back((base.to_string(), 0));
+    while let Some((from, depth)) = queue.pop_front() {
+        for possibility in possibilities(transformations, &from) {
+            if possibility == target {
+                return depth + 1;
+            }
+            if seen.insert(possibility.clone()) {
+                queue.push_back((possibility, depth + 1));
+            }
+        }
+    }
+    unreachable!("No solution was found");
+}
+
 #[cfg(test)]
 mod test {
     #[test]
@@ -50,4 +82,28 @@ O => HH
 HOH";
         assert_eq!(super::part1(&super::parse(input)), 4);
     }
+
+    #[test]
+    fn part2a() {
+        let input = "e => H
+e => O
+H => HO
+H => OH
+O => HH
+
+HOH";
+        assert_eq!(super::part2(&super::parse(input)), 3);
+    }
+
+    #[test]
+    fn part2b() {
+        let input = "e => H
+e => O
+H => HO
+H => OH
+O => HH
+
+HOHOHO";
+        assert_eq!(super::part2(&super::parse(input)), 6);
+    }
 }