From c2389e93af8fdda5e4aa5949a7193be93e2ae486 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Sat, 16 Dec 2023 11:38:50 +0100
Subject: [PATCH] Implement 2023 day 16 part 1

---
 2023/samples/.gitkeep   |   0
 2023/src/common.rs      |  15 ++++++
 2023/src/day16.rs       | 112 +++++++++++++++++++++++++++++++++++++++-
 2023/src/samples/16.txt |  10 ++++
 4 files changed, 135 insertions(+), 2 deletions(-)
 delete mode 100644 2023/samples/.gitkeep
 create mode 100644 2023/src/samples/16.txt

diff --git a/2023/samples/.gitkeep b/2023/samples/.gitkeep
deleted file mode 100644
index e69de29..0000000
diff --git a/2023/src/common.rs b/2023/src/common.rs
index 3e848b8..1bf8edd 100644
--- a/2023/src/common.rs
+++ b/2023/src/common.rs
@@ -289,6 +289,21 @@ impl<T: AsRef<[u8]>> Grid<T> {
     }
 }
 
+impl Grid<Vec<u8>> {
+    pub fn zeroed(width: usize, height: usize) -> Self {
+        let mut data = vec![0; (width + 1) * height];
+
+        for line_end in data[width..].iter_mut().step_by(width + 1) {
+            *line_end = b'\n';
+        }
+
+        Self {
+            width: width + 1,
+            data,
+        }
+    }
+}
+
 impl<T: AsMut<[u8]> + AsRef<[u8]>> Grid<T> {
     pub fn rows_mut(&mut self) -> impl Iterator<Item = &mut [u8]> {
         let width = self.width();
diff --git a/2023/src/day16.rs b/2023/src/day16.rs
index 7c1760f..e8ab584 100644
--- a/2023/src/day16.rs
+++ b/2023/src/day16.rs
@@ -1,7 +1,115 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use crate::common::Grid;
+
+#[derive(Clone, Copy)]
+enum Direction {
+    Up = 0,
+    Left = 1,
+    Down = 2,
+    Right = 3,
+}
+
+impl Direction {
+    fn bit(self) -> u8 {
+        1 << self as u8
+    }
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+    let mut state = Grid::zeroed(map.width(), map.height());
+    state[0][0] = Direction::Right.bit();
+
+    let mut todo = Vec::new();
+    todo.push((Direction::Right, 0, 0));
+
+    let mut energized = 1;
+
+    while let Some((dir, x, y)) = todo.pop() {
+        let mut enqueue = |dir: Direction, x: usize, y| {
+            let state = &mut state[y][x];
+            if state == &0 {
+                energized += 1;
+            }
+            if *state & dir.bit() == 0 {
+                *state |= dir.bit();
+                todo.push((dir, x, y));
+            }
+        };
+
+        let new_dir = match (map[y][x], dir) {
+            (b'/', Direction::Left) => Direction::Down,
+            (b'/', Direction::Up) => Direction::Right,
+            (b'/', Direction::Down) => Direction::Left,
+            (b'/', Direction::Right) => Direction::Up,
+            (b'\\', Direction::Left) => Direction::Up,
+            (b'\\', Direction::Up) => Direction::Left,
+            (b'\\', Direction::Down) => Direction::Right,
+            (b'\\', Direction::Right) => Direction::Down,
+            (b'|', Direction::Right) | (b'|', Direction::Left) => {
+                if y > 0 {
+                    enqueue(Direction::Up, x, y - 1);
+                }
+                if y + 1 < map.height() {
+                    enqueue(Direction::Down, x, y + 1);
+                }
+                continue;
+            }
+            (b'-', Direction::Up) | (b'-', Direction::Down) => {
+                if x > 0 {
+                    enqueue(Direction::Left, x - 1, y);
+                }
+                if x + 1 < map.width() {
+                    enqueue(Direction::Right, x + 1, y);
+                }
+                continue;
+            }
+            (_, dir) => dir,
+        };
+
+        match new_dir {
+            Direction::Up => {
+                if y > 0 {
+                    enqueue(new_dir, x, y - 1);
+                }
+            }
+            Direction::Left => {
+                if x > 0 {
+                    enqueue(new_dir, x - 1, y);
+                }
+            }
+            Direction::Down => {
+                if y + 1 < map.height() {
+                    enqueue(new_dir, x, y + 1);
+                }
+            }
+            Direction::Right => {
+                if x + 1 < map.width() {
+                    enqueue(new_dir, x + 1, y);
+                }
+            }
+        }
+    }
+
+    Ok(energized.to_string())
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/16.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("46", part1(SAMPLE).unwrap());
+    }
+
+    // #[test]
+    // fn sample_part2() {
+    //     assert_eq!("64", part2(SAMPLE).unwrap());
+    // }
+}
diff --git a/2023/src/samples/16.txt b/2023/src/samples/16.txt
new file mode 100644
index 0000000..d6805ce
--- /dev/null
+++ b/2023/src/samples/16.txt
@@ -0,0 +1,10 @@
+.|...\....
+|.-.\.....
+.....|-...
+........|.
+..........
+.........\
+..../.\\..
+.-.-/..|..
+.|....-|.\
+..//.|....