1 files changed, 177 insertions, 0 deletions
diff --git a/2021/src/bin/2021day25.rs b/2021/src/bin/2021day25.rs
new file mode 100644
index 0000000..28f3f9d
--- /dev/null
+++ b/2021/src/bin/2021day25.rs
@@ -0,0 +1,177 @@
+use std::hint::unreachable_unchecked;
+
+use aoc2021::*;
+
+struct SeaFloor {
+    width: usize,
+    height: usize,
+    cukes: Vec<u8>,
+}
+
+impl FromStr for SeaFloor {
+    type Err = ();
+
+    fn from_str(input: &str) -> StdResult<Self, Self::Err> {
+        let input = input.trim();
+        let height = input.lines().count();
+        let width = input.lines().next().unwrap().trim().len();
+        let mut cukes = Vec::with_capacity(width * height);
+
+        for ch in input.chars() {
+            match ch {
+                '.' => cukes.push(0),
+                '>' => cukes.push(1),
+                'v' => cukes.push(2),
+                _ => (),
+            }
+        }
+
+        assert_eq!(cukes.len(), width * height);
+
+        Ok(Self {
+            width,
+            height,
+            cukes,
+        })
+    }
+}
+
+//fn print_cukes(width: usize, height: usize, cukes: &[u8]) {
+//    let mut pos = 0;
+//    for row in 0..height {
+//        for col in 0..width {
+//            match cukes[pos]
+//        }
+//    }
+//}
+
+fn facing_east(width: usize, _height: usize, row: usize, col: usize) -> usize {
+    if col == width - 1 {
+        row * width
+    } else {
+        (row * width) + col + 1
+    }
+}
+
+fn facing_south(width: usize, height: usize, row: usize, col: usize) -> usize {
+    if row == height - 1 {
+        col
+    } else {
+        ((row + 1) * width) + col
+    }
+}
+
+#[cfg(debug_assertions)]
+fn print_cukes(label: &str, width: usize, height: usize, cukes: &[u8]) {
+    println!("{}", label);
+    let mut pos = 0;
+    for _ in 0..height {
+        for _ in 0..width {
+            print!(
+                "{}",
+                match cukes[pos] {
+                    0 => '.',
+                    1 => '>',
+                    2 => 'v',
+                    _ => unsafe { unreachable_unchecked() },
+                }
+            );
+            pos += 1;
+        }
+        println!();
+    }
+    println!();
+}
+
+fn part1(input: &SeaFloor) -> usize {
+    let width = input.width;
+    let height = input.height;
+    let mut step1 = input.cukes.clone();
+    let mut step2 = input.cukes.clone();
+    let mut step3 = input.cukes.clone();
+    let mut turns = 0;
+    let mut moved = true;
+    #[cfg(debug_assertions)]
+    print_cukes("Starting positions", width, height, &step1);
+    while moved {
+        step2[..].fill(0);
+        // Move the east-facing cukes from step1 to step2
+        let mut pos = 0;
+        for row in 0..height {
+            for col in 0..width {
+                let facing = facing_east(width, height, row, col);
+                match step1[pos] {
+                    0 => (),
+                    1 => {
+                        if step1[facing] == 0 {
+                            step2[facing] = 1;
+                        } else {
+                            step2[pos] = 1;
+                        }
+                    }
+                    2 => step2[pos] = 2,
+                    _ => unsafe { unreachable_unchecked() },
+                }
+                pos += 1;
+            }
+        }
+        step3[..].fill(0);
+        pos = 0;
+        // move the south facing cukes from step2 back to step 1
+        for row in 0..height {
+            for col in 0..width {
+                let facing = facing_south(width, height, row, col);
+                match step2[pos] {
+                    0 => (),
+                    1 => step3[pos] = 1,
+                    2 => {
+                        if step2[facing] == 0 {
+                            step3[facing] = 2;
+                        } else {
+                            step3[pos] = 2
+                        }
+                    }
+                    _ => unsafe { unreachable_unchecked() },
+                }
+                pos += 1;
+            }
+        }
+        moved = step1[..] != step3[..];
+        std::mem::swap(&mut step1, &mut step3);
+        turns += 1;
+
+        #[cfg(debug_assertions)]
+        print_cukes(&format!("After {} turns", turns), width, height, &step1);
+    }
+
+    turns
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    static TEST_INPUT: &str = r#"v...>>.vv>
+.vv>>.vv..
+>>.>v>...v
+>>v>>.>.v.
+v>v.vv.v..
+>.>>..v...
+.vv..>.>v.
+v.v..>>v.v
+....v..v.>"#;
+
+    #[test]
+    fn testcase1() {
+        let input = SeaFloor::from_str(TEST_INPUT).unwrap();
+        assert_eq!(part1(&input), 58);
+    }
+}
+
+fn main() -> Result<()> {
+    let input = read_input(25)?;
+    let input = SeaFloor::from_str(&input).unwrap();
+    println!("Loaded sea floor {}x{}", input.width, input.height);
+    println!("Part 1: {}", part1(&input));
+    Ok(())
+}