From c2478f286599daeba3b5c3c98ad8c8728a03e290 Mon Sep 17 00:00:00 2001
From: Ian Liu <ian.liu@tupl.com>
Date: Wed, 1 Jan 2025 00:38:37 -0800
Subject: [PATCH] add: day 9

---
 data/examples/09.txt |   8 +++
 src/bin/09.rs        | 117 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 125 insertions(+)
 create mode 100644 data/examples/09.txt
 create mode 100644 src/bin/09.rs

diff --git a/data/examples/09.txt b/data/examples/09.txt
new file mode 100644
index 0000000..60bd43b
--- /dev/null
+++ b/data/examples/09.txt
@@ -0,0 +1,8 @@
+R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20
diff --git a/src/bin/09.rs b/src/bin/09.rs
new file mode 100644
index 0000000..db73144
--- /dev/null
+++ b/src/bin/09.rs
@@ -0,0 +1,117 @@
+use std::collections::HashSet;
+
+advent_of_code::solution!(9);
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let mut plane = Plane::new(2);
+    plane.run(input);
+
+    Some(plane.tail_visited.len() as u32)
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let mut plane = Plane::new(10);
+    plane.run(input);
+
+    Some(plane.tail_visited.len() as u32)
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+struct Pos(isize, isize);
+
+struct Plane {
+    rope: Vec<Pos>,
+    tail_visited: HashSet<Pos>,
+}
+
+impl Plane {
+    fn new(rope_len: usize) -> Self {
+        Self {
+            rope: vec![Pos(0, 0); rope_len],
+            tail_visited: HashSet::from([Pos(0, 0)]),
+        }
+    }
+
+    fn run(&mut self, input: &str) {
+        input.lines().for_each(|line| {
+            let (direction, cnt) = line.split_once(" ").unwrap();
+
+            for _ in 0..cnt.parse().unwrap() {
+                self.move_head(direction);
+            }
+        });
+    }
+
+    fn move_head(&mut self, direction: &str) {
+        self.update_head(direction);
+        self.update_rope();
+        self.record_tail();
+    }
+
+    fn update_head(&mut self, direction: &str) {
+        let delta = Plane::get_delta_from_direction(direction);
+
+        self.rope[0] = Plane::get_new_pos(&self.rope[0], delta);
+    }
+
+    fn update_rope(&mut self) {
+        for i in 0..self.rope.len() - 1 {
+            let [front_pos, back_pos] = [self.rope[i], self.rope[i + 1]];
+
+            if let Some(delta) = Plane::get_delta_if_not_touching(&front_pos, &back_pos) {
+                self.rope[i + 1] = Plane::get_new_pos_for_knot(&back_pos, &delta);
+            } else {
+                break;
+            }
+        }
+    }
+
+    fn record_tail(&mut self) {
+        self.tail_visited.insert(*self.rope.last().unwrap());
+    }
+
+    fn get_delta_from_direction(direction: &str) -> (isize, isize) {
+        match direction {
+            "U" => (-1, 0),
+            "D" => (1, 0),
+            "L" => (0, -1),
+            "R" => (0, 1),
+            _ => unreachable!(),
+        }
+    }
+
+    fn get_new_pos(pos: &Pos, delta: (isize, isize)) -> Pos {
+        Pos(pos.0 + delta.0, pos.1 + delta.1)
+    }
+
+    fn get_new_pos_for_knot(pos: &Pos, delta: &(isize, isize)) -> Pos {
+        Pos(pos.0 + delta.0.signum(), pos.1 + delta.1.signum())
+    }
+
+    fn get_delta_if_not_touching(front_pos: &Pos, back_pos: &Pos) -> Option<(isize, isize)> {
+        let delta = (front_pos.0 - back_pos.0, front_pos.1 - back_pos.1);
+
+        Plane::is_delta_too_large(&delta).then_some(delta)
+    }
+
+    fn is_delta_too_large(delta: &(isize, isize)) -> bool {
+        delta.0.abs() >= 2 || delta.1.abs() >= 2
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(88));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(36));
+    }
+}