1 files changed, 98 insertions, 0 deletions
diff --git a/2021/src/bin/day_25.rs b/2021/src/bin/day_25.rs
new file mode 100644
index 0000000..742c911
--- /dev/null
+++ b/2021/src/bin/day_25.rs
@@ -0,0 +1,98 @@
+use nom::{
+    branch::alt,
+    character::complete::{char as nom_char, line_ending},
+    combinator::{map, value},
+    multi::{many1, separated_list1},
+    IResult,
+};
+use std::fs;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("inputs/day_25.txt")?;
+    let mut seafloor = parse_seafloor(&input).unwrap().1;
+    for i in 1.. {
+        let next = seafloor.next();
+        if next == seafloor {
+            dbg!(i);
+            break;
+        }
+        seafloor = next;
+    }
+    Ok(())
+}
+
+#[derive(Debug, PartialEq, Eq)]
+struct Seafloor(Vec<Vec<Option<Cucumber>>>);
+
+impl Seafloor {
+    fn next(&self) -> Seafloor {
+        self.next_east().next_south()
+    }
+
+    fn next_east(&self) -> Seafloor {
+        let mut results = Vec::new();
+        for y in 0..self.0.len() {
+            let mut current_row = Vec::new();
+            for x in 0..self.0[y].len() {
+                let old = &self.0[y][x];
+                let old_left = &self.0[y][if x == 0 { self.0[y].len() - 1 } else { x - 1 }];
+                let old_right = &self.0[y][(x + 1) % self.0[y].len()];
+
+                let new = if *old == None && *old_left == Some(Cucumber::East) {
+                    old_left.clone()
+                } else if *old == Some(Cucumber::East) && *old_right == None {
+                    None
+                } else {
+                    old.clone()
+                };
+                current_row.push(new);
+            }
+            results.push(current_row);
+        }
+        Seafloor(results)
+    }
+
+    fn next_south(&self) -> Seafloor {
+        let mut results = Vec::new();
+        for y in 0..self.0.len() {
+            let mut current_row = Vec::new();
+            for x in 0..self.0[y].len() {
+                let old = &self.0[y][x];
+                let old_up = &self.0[if y == 0 { self.0.len() - 1 } else { y - 1 }][x];
+                let old_down = &self.0[(y + 1) % self.0.len()][x];
+
+                let new = if *old == None && *old_up == Some(Cucumber::South) {
+                    old_up.clone()
+                } else if *old == Some(Cucumber::South) && *old_down == None {
+                    None
+                } else {
+                    old.clone()
+                };
+                current_row.push(new);
+            }
+            results.push(current_row);
+        }
+        Seafloor(results)
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum Cucumber {
+    East,
+    South,
+}
+
+fn parse_seafloor(input: &str) -> IResult<&str, Seafloor> {
+    map(
+        separated_list1(line_ending, many1(parse_cucumber)),
+        Seafloor,
+    )(input)
+}
+
+fn parse_cucumber(input: &str) -> IResult<&str, Option<Cucumber>> {
+    alt((
+        value(None, nom_char('.')),
+        value(Some(Cucumber::East), nom_char('>')),
+        value(Some(Cucumber::South), nom_char('v')),
+    ))(input)
+}