Implement 2023 day 21 part 1

2023/src/day21.rs

@@ -1,7 +1,69 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::HashSet;
+use std::collections::VecDeque;
+
+use anyhow::Context;
+
+use crate::common::Grid;
+
+fn part1_parametrized(input: &[u8], steps: usize) -> anyhow::Result<String> {
+    let map = Grid::new(input)?;
+    let mut visited = HashSet::new();
+    let mut todo = VecDeque::new();
+    let start = map.find(b'S').context("Couldn't find starting point")?;
+    todo.push_back((0, start));
+    visited.insert(start);
+
+    while let Some((dist, (x, y))) = todo.pop_front() {
+        let mut enqueue = |x, y| {
+            if map[(y, x)] != b'#' && visited.insert((x, y)) && dist < steps {
+                todo.push_back((dist + 1, (x, y)));
+            }
+        };
+
+        if x > 0 {
+            enqueue(x - 1, y);
+        }
+
+        if y > 0 {
+            enqueue(x, y - 1);
+        }
+
+        if x + 1 < map.width() {
+            enqueue(x + 1, y);
+        }
+
+        if y + 1 < map.height() {
+            enqueue(x, y + 1);
+        }
+    }
+
+    Ok(visited
+        .iter()
+        .filter(|&&(x, y)| {
+            let dist = start.0.abs_diff(x) + start.1.abs_diff(y);
+
+            dist % 2 == steps % 2
+        })
+        .count()
+        .to_string())
+}
+
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    part1_parametrized(input, 64)
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/21.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("16", part1_parametrized(SAMPLE, 6).unwrap());
+    }
+}