solution: day 8

data/examples/08-1.txt

@@ -0,0 +1,5 @@
+LLR
+
+AAA = (BBB, BBB)
+BBB = (AAA, ZZZ)
+ZZZ = (ZZZ, ZZZ)

data/examples/08-2.txt

@@ -0,0 +1,10 @@
+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)

src/bin/08.rs

@@ -0,0 +1,95 @@
+use std::collections::HashMap;
+
+use aoc::lcm;
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let mut map = HashMap::new();
+
+    let (directions, map_data) = input.split_once("\n\n")?;
+    for line in map_data.lines() {
+        let (k, v) = line.split_once(" = ")?;
+        let v = v.strip_prefix('(')?;
+        let v = v.strip_suffix(')')?;
+        let (l, r) = v.split_once(", ")?;
+
+        *map.entry(k).or_default() = (l, r);
+    }
+
+    let mut counter = 0;
+    let mut loc = "AAA";
+
+    for d in directions.chars().cycle() {
+        if loc == "ZZZ" {
+            break;
+        }
+
+        counter += 1;
+        let (l, r) = map[loc];
+
+        match d {
+            'L' => loc = l,
+            'R' => loc = r,
+            _ => unimplemented!(),
+        }
+    }
+
+    Some(counter)
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    let mut map = HashMap::new();
+
+    let (directions, map_data) = input.split_once("\n\n")?;
+    for line in map_data.lines() {
+        let (k, v) = line.split_once(" = ")?;
+        let v = v.strip_prefix('(')?;
+        let v = v.strip_suffix(')')?;
+        let (l, r) = v.split_once(", ")?;
+
+        *map.entry(k).or_default() = (l, r);
+    }
+
+    let mut res = 1;
+
+    for k in map.keys().filter(|x| x.ends_with('A')) {
+        let mut location = *k;
+
+        for (i, d) in directions.chars().cycle().enumerate() {
+            let (l, r) = map[location];
+
+            location = match d {
+                'L' => l,
+                'R' => r,
+                _ => unimplemented!(),
+            };
+
+            if location.ends_with('Z') {
+                res = lcm(res, i + 1);
+                break;
+            }
+        }
+    }
+
+    Some(res as u64)
+}
+
+aoc::solution!(8);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        assert_eq!(
+            part_one(&aoc::template::read_file_part("examples", 8, 1)),
+            Some(6)
+        );
+    }
+    #[test]
+    fn test_part_two() {
+        assert_eq!(
+            part_two(&aoc::template::read_file_part("examples", 8, 2)),
+            Some(6)
+        );
+    }
+}

src/lib.rs

@@ -1,4 +1,28 @@
 #![feature(pattern)]
 
+use std::mem::swap;
+
 pub mod parsers;
 pub mod template;
+
+pub fn lcm(first: usize, second: usize) -> usize {
+    first * second / gcd(first, second)
+}
+
+pub fn gcd(first: usize, second: usize) -> usize {
+    let mut max = first;
+    let mut min = second;
+    if min > max {
+        swap(&mut min, &mut max)
+    }
+
+    loop {
+        let res = max % min;
+        if res == 0 {
+            return min;
+        }
+
+        max = min;
+        min = res;
+    }
+}

src/template.rs

@@ -54,3 +54,14 @@ pub fn read_file(folder: &str, day: u8) -> String {
     let f = fs::read_to_string(filepath);
     f.expect("could not open input file").trim().to_string()
 }
+
+#[must_use]
+pub fn read_file_part(folder: &str, day: u8, part: u8) -> String {
+    let cwd = env::current_dir().unwrap();
+    let filepath = cwd
+        .join("data")
+        .join(folder)
+        .join(format!("{day:02}-{part}.txt"));
+    let f = fs::read_to_string(filepath);
+    f.expect("could not open input file").trim().to_string()
+}