Implement day 12 part 2

2021/inputs/12.txt

@@ -0,0 +1,19 @@
+lg-GW
+pt-start
+pt-uq
+nx-lg
+ve-GW
+start-nx
+GW-start
+GW-nx
+pt-SM
+sx-GW
+lg-end
+nx-SM
+lg-SM
+pt-nx
+end-ve
+ve-SM
+TG-uq
+end-SM
+SM-uq

2021/src/day12.rs

@@ -1,9 +1,102 @@
+use std::collections::HashMap;
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+use crate::common::LineIter;
+
+type EdgeMap = HashMap<String, Vec<String>>;
+
+fn read_edges(input: &mut dyn Read) -> EdgeMap {
+    let mut reader = LineIter::new(input);
+    let mut edges = EdgeMap::new();
+
+    while let Some(line) = reader.next() {
+        let (from, to) = line.split_once('-').unwrap();
+
+        edges
+            .entry(from.to_owned())
+            .or_default()
+            .push(to.to_owned());
+
+        edges
+            .entry(to.to_owned())
+            .or_default()
+            .push(from.to_owned());
+    }
+
+    edges
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+fn is_small(cave: &str) -> bool {
+    cave.chars().all(|c| c.is_ascii_lowercase())
+}
+
+fn dfs_routes<'a>(
+    edges: &'a EdgeMap,
+    route: &'_ mut Vec<&'a str>,
+    pos: &'a str,
+    mut small_twice: bool,
+) -> usize {
+    match pos {
+        "end" => return 1,
+        "start" if !route.is_empty() => return 0,
+        pos if is_small(pos) && route.contains(&pos) => {
+            if small_twice {
+                small_twice = false;
+            } else {
+                return 0;
+            }
+        }
+        _ => (),
+    }
+
+    route.push(pos);
+
+    let routes = edges[pos]
+        .iter()
+        .map(|new_pos| dfs_routes(edges, route, new_pos, small_twice))
+        .sum();
+
+    route.pop();
+
+    routes
+}
+
+fn parts_common(input: &mut dyn Read, small_twice: bool) -> String {
+    let edges = read_edges(input);
+    let mut route = Vec::new();
+
+    dfs_routes(&edges, &mut route, "start", small_twice).to_string()
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    parts_common(input, false)
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    parts_common(input, true)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE1: &[u8] = include_bytes!("samples/12.1.txt");
+    const SAMPLE2: &[u8] = include_bytes!("samples/12.2.txt");
+    const SAMPLE3: &[u8] = include_bytes!("samples/12.3.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE1, 10);
+        test_implementation(part1, SAMPLE2, 19);
+        test_implementation(part1, SAMPLE3, 226);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE1, 36);
+        test_implementation(part2, SAMPLE2, 103);
+        test_implementation(part2, SAMPLE3, 3509);
+    }
 }

2021/src/samples/12.1.txt

@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end

2021/src/samples/12.2.txt

@@ -0,0 +1,10 @@
+dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc

2021/src/samples/12.3.txt

@@ -0,0 +1,18 @@
+fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW