Implement 2023 day 10 part 1

2023/src/day10.rs

@@ -1,7 +1,100 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::VecDeque;
+
+use anyhow::Context;
+
+use crate::common::Grid;
+use crate::common::IndexSet;
+
+const LEFT: u8 = 1;
+const RIGHT: u8 = 2;
+const UP: u8 = 4;
+const DOWN: u8 = 8;
+
+fn get_connections(c: u8) -> u8 {
+    match c {
+        b'|' => UP | DOWN,
+        b'-' => LEFT | RIGHT,
+        b'F' => DOWN | RIGHT,
+        b'J' => LEFT | UP,
+        b'L' => RIGHT | UP,
+        b'7' => LEFT | DOWN,
+        _ => 0,
+    }
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+    let (start_x, start_y) = map.find(b'S').context("Couldn't find starting point")?;
+    let mut visited = IndexSet::with_capacity(map.width() * map.height());
+    let mut todo = VecDeque::new();
+
+    visited.insert(start_y * map.width() + start_x);
+
+    if start_x > 0 && (get_connections(map[start_y][start_x - 1]) & RIGHT) != 0 {
+        todo.push_back((1, (start_x - 1, start_y)));
+        visited.insert(start_y * map.width() + start_x - 1);
+    }
+
+    if start_x + 1 < map.width() && (get_connections(map[start_y][start_x + 1]) & LEFT) != 0 {
+        todo.push_back((1, (start_x + 1, start_y)));
+        visited.insert(start_y * map.width() + start_x + 1);
+    }
+
+    if start_y > 0 && (get_connections(map[start_y - 1][start_x]) & DOWN) != 0 {
+        todo.push_back((1, (start_x, start_y - 1)));
+        visited.insert((start_y - 1) * map.width() + start_x);
+    }
+
+    if start_y + 1 < map.height() && (get_connections(map[start_y + 1][start_x]) & DOWN) != 0 {
+        todo.push_back((1, (start_x, start_y + 1)));
+        visited.insert((start_y + 1) * map.width() + start_x);
+    }
+
+    let mut max_dist = 1;
+
+    while let Some((dist, (x, y))) = todo.pop_front() {
+        let mut enqueue = |x, y| {
+            if visited.insert(y * map.width() + x) {
+                todo.push_back((dist + 1, (x, y)));
+                // Can elide comparison because we do a BFS and length is strictly increasing
+                max_dist = dist + 1;
+            }
+        };
+
+        let connections = get_connections(map[y][x]);
+
+        if (connections & LEFT) != 0 {
+            enqueue(x - 1, y);
+        }
+
+        if (connections & RIGHT) != 0 {
+            enqueue(x + 1, y);
+        }
+
+        if (connections & UP) != 0 {
+            enqueue(x, y - 1);
+        }
+
+        if (connections & DOWN) != 0 {
+            enqueue(x, y + 1);
+        }
+    }
+
+    Ok(max_dist.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/10.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("8", part1(SAMPLE).unwrap());
+    }
+}