2021 day12/rust: add simple solution

2021/data/day12.expected

@@ -0,0 +1,2 @@
+4411
+136767

2021/data/day12.input

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+yb-start
+de-vd
+rj-yb
+rj-VP
+OC-de
+MU-de
+end-DN
+vd-end
+WK-vd
+rj-de
+DN-vd
+start-VP
+DN-yb
+vd-MU
+DN-rj
+de-VP
+yb-OC
+start-rj
+oa-MU
+yb-de
+oa-VP
+jv-MU
+yb-MU
+end-OC

2021/day12/day12_rs/Cargo.toml

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+[package]
+name = "day12_rs"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

2021/day12/day12_rs/src/main.rs

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+#![warn(clippy::pedantic)]
+use std::{
+    collections::{HashMap, HashSet},
+    io::{stdin, BufRead},
+};
+
+type Point<'a> = &'a str;
+
+fn walk<'a>(
+    connections: &HashMap<Point<'a>, HashSet<Point<'a>>>,
+    visited: &[Point<'a>],
+    extra_cave: Option<Point<'a>>,
+    from: Point<'a>,
+) -> usize {
+    let mut res = 0;
+
+    let mut visited = Vec::from(visited);
+    visited.push(from);
+
+    for target in &connections[from] {
+        if *target == "end" {
+            res += 1;
+            continue;
+        }
+
+        let is_uppercase = target.chars().all(|c| c.is_ascii_uppercase());
+        let was_visited = visited.contains(target);
+
+        if is_uppercase || !was_visited {
+            res += walk(connections, &visited, extra_cave, target);
+        } else if extra_cave.is_none() {
+            res += walk(connections, &visited, Some(target), target);
+        }
+    }
+
+    res
+}
+
+fn main() {
+    let lines: Vec<_> = stdin().lock().lines().collect::<Result<_, _>>().unwrap();
+
+    let connections: HashMap<Point, HashSet<Point>> =
+        lines.iter().fold(HashMap::new(), |mut map, line| {
+            let mut parts = line.split('-');
+            let p1 = parts.next().unwrap();
+            let p2 = parts.next().unwrap();
+
+            if p2 != "start" {
+                map.entry(p1).or_default().insert(p2);
+            }
+            if p2 != "end" {
+                map.entry(p2).or_default().insert(p1);
+            }
+            map
+        });
+
+    println!("{}", walk(&connections, &[], Some(""), "start"));
+    println!("{}", walk(&connections, &[], None, "start"));
+}