1 files changed, 130 insertions, 0 deletions
diff --git a/src/bin/day13.rs b/src/bin/day13.rs
new file mode 100644
index 0000000..280f6c0
--- /dev/null
+++ b/src/bin/day13.rs
@@ -0,0 +1,130 @@
+use regex::Regex;
+use std::collections::HashSet;
+
+fn main() {
+    let input = advent::read_lines(13);
+    println!("13a: {}", fold_once(&input));
+    println!("13b:");
+    fold_all(&input);
+}
+
+#[derive(PartialEq,Eq,Hash,Clone,Copy)]
+struct Position {
+    x: usize,
+    y: usize,
+}
+
+struct Fold {
+    pos: Position,
+}
+
+fn parse_instructions<T: AsRef<str>>(input: &[T]) -> (HashSet<Position>, Vec<Fold>) {
+    let re_pos = Regex::new(r"^(\d+),(\d+)$").unwrap();
+    let re_fold = Regex::new(r"^fold along ([xy])=(\d+)$").unwrap();
+    let mut positions = HashSet::new();
+    let mut folds = Vec::new();
+
+    for line in input {
+        if let Some(pos) = re_pos.captures(line.as_ref()) {
+            positions.insert(Position {
+                x: pos.get(1).unwrap().as_str().parse::<usize>().unwrap(),
+                y: pos.get(2).unwrap().as_str().parse::<usize>().unwrap(),
+            });
+            continue;
+        }
+        if let Some(fold) = re_fold.captures(line.as_ref()) {
+            let axis = fold.get(1).unwrap().as_str();
+            let coord = fold.get(2).unwrap().as_str().parse::<usize>().unwrap();
+            match axis {
+                "x" => folds.push(Fold { pos: Position { x: coord, y: 0 } }),
+                "y" => folds.push(Fold { pos: Position { x: 0, y: coord } }),
+                _ => panic!("invalid axis"),
+            }
+        }
+    }
+    (positions, folds)
+}
+
+fn fold(points: &HashSet<Position>, fold: &Fold) -> HashSet<Position> {
+    let mut new_points = HashSet::new();
+
+    for point in points {
+        if fold.pos.x == 0 {
+            if point.y < fold.pos.y {
+                new_points.insert(*point);
+            } else {
+                new_points.insert(Position { x: point.x, y: fold.pos.y - (point.y - fold.pos.y) });
+            }
+        } else if fold.pos.y == 0 {
+            if point.x < fold.pos.x {
+                new_points.insert(*point);
+            } else {
+                new_points.insert(Position { x: fold.pos.x - (point.x - fold.pos.x), y: point.y });
+            }
+        } else {
+            panic!("invalid fold");
+        }
+    }
+
+    new_points
+}
+
+fn fold_once<T: AsRef<str>>(input: &[T]) -> usize {
+    let (positions, folds) = parse_instructions(input);
+
+    fold(&positions, &folds[0]).iter().len()
+}
+
+fn fold_all<T: AsRef<str>>(input: &[T]) {
+    let (mut positions, folds) = parse_instructions(input);
+
+    for f in folds {
+        positions = fold(&positions, &f);
+    }
+    let max_x = positions.iter().map(|&pos| pos.x).max().unwrap();
+    let max_y = positions.iter().map(|&pos| pos.y).max().unwrap();
+
+    for y in 0 ..= max_y {
+        for x in 0 ..= max_x {
+            if positions.contains(&Position { x, y }) {
+                print!("#");
+            } else {
+                print!(".");
+            }
+        }
+        println!();
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "6,10",
+            "0,14",
+            "9,10",
+            "0,3",
+            "10,4",
+            "4,11",
+            "6,0",
+            "6,12",
+            "4,1",
+            "0,13",
+            "10,12",
+            "3,4",
+            "3,0",
+            "8,4",
+            "1,10",
+            "2,14",
+            "8,10",
+            "9,0",
+            "",
+            "fold along y=7",
+            "fold along x=5",
+        ];
+        assert_eq!(fold_once(&input), 17);
+    }
+}