Implement day 25

2021/src/day25.rs

@@ -1,5 +1,72 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+fn read_input(input: &mut dyn Read) -> (usize, Vec<u8>) {
+    let mut result = Vec::new();
+    input.read_to_end(&mut result).unwrap();
+
+    let width = result.iter().position(|&c| c == b'\n').unwrap();
+    result.retain(|c| !c.is_ascii_whitespace());
+
+    (width, result)
+}
+
+fn advance(width: usize, state: &mut [u8]) -> bool {
+    debug_assert_eq!(state.len() % width, 0);
+
+    let mut helper = state.to_owned();
+    let mut changes = false;
+
+    // Move the eastbound herd
+    for (src, dest) in state
+        .chunks_exact(width)
+        .zip(helper.chunks_exact_mut(width))
+    {
+        for x in 0..width {
+            if src[x] == b'>' && src[(x + 1) % width] == b'.' {
+                dest[x] = b'.';
+                dest[(x + 1) % width] = b'>';
+                changes = true;
+            }
+        }
+    }
+
+    state.copy_from_slice(&helper);
+
+    // Now the southbound herd. Y in the outer loop for better cache locality
+    for y_offset in (0..helper.len()).step_by(width) {
+        let n_offset = (y_offset + width) % helper.len();
+
+        for x in 0..width {
+            if helper[x + y_offset] == b'v' && helper[x + n_offset] == b'.' {
+                state[x + y_offset] = b'.';
+                state[x + n_offset] = b'v';
+                changes = true;
+            }
+        }
+    }
+
+    changes
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let (width, mut state) = read_input(input);
+
+    (1..)
+        .find(|_| !advance(width, &mut state))
+        .unwrap()
+        .to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/25.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 58);
+    }
 }

2021/src/samples/25.txt

@@ -0,0 +1,9 @@
+v...>>.vv>
+.vv>>.vv..
+>>.>v>...v
+>>v>>.>.v.
+v>v.vv.v..
+>.>>..v...
+.vv..>.>v.
+v.v..>>v.v
+....v..v.>