1 files changed, 136 insertions, 0 deletions
diff --git a/2021/src/bin/day_13.rs b/2021/src/bin/day_13.rs
new file mode 100644
index 0000000..547c7a2
--- /dev/null
+++ b/2021/src/bin/day_13.rs
@@ -0,0 +1,136 @@
+use nom::{
+    branch::alt,
+    bytes::complete::tag,
+    character::complete::{char as nom_char, line_ending, u32 as nom_u32},
+    combinator::map,
+    multi::{many0, separated_list1},
+    sequence::tuple,
+    IResult,
+};
+use std::{collections::BTreeSet, fmt, fs};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let input = fs::read_to_string("inputs/day_13.txt")?;
+    let mut page = parse_page(&input).unwrap().1;
+    page.do_next_fold();
+    dbg!(page.count_points());
+    while page.do_next_fold() {}
+    println!("{}", page);
+
+    Ok(())
+}
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
+struct Point {
+    x: u32,
+    y: u32,
+}
+
+#[derive(Debug)]
+enum Fold {
+    X(u32),
+    Y(u32),
+}
+
+#[derive(Debug)]
+struct Page {
+    points: BTreeSet<Point>,
+    folds: Vec<Fold>,
+}
+
+impl Page {
+    fn do_next_fold(&mut self) -> bool {
+        let fold = self.folds.pop();
+        match fold {
+            Some(Fold::X(x)) => {
+                self.points = std::mem::take(&mut self.points)
+                    .into_iter()
+                    .filter(|point| point.x != x)
+                    .map(|point| {
+                        if point.x > x {
+                            Point {
+                                x: x - (point.x - x),
+                                y: point.y,
+                            }
+                        } else {
+                            point
+                        }
+                    })
+                    .collect();
+                true
+            }
+            Some(Fold::Y(y)) => {
+                self.points = std::mem::take(&mut self.points)
+                    .into_iter()
+                    .filter(|point| point.y != y)
+                    .map(|point| {
+                        if point.y > y {
+                            Point {
+                                x: point.x,
+                                y: y - (point.y - y),
+                            }
+                        } else {
+                            point
+                        }
+                    })
+                    .collect();
+                true
+            }
+            None => false,
+        }
+    }
+
+    fn count_points(&self) -> usize {
+        self.points.len()
+    }
+}
+
+impl fmt::Display for Page {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        let width = self.points.iter().map(|p| p.x).max().unwrap_or(0);
+        let height = self.points.iter().map(|p| p.y).max().unwrap_or(0);
+        for y in 0..=height {
+            for x in 0..=width {
+                let p = Point { x, y };
+                if self.points.contains(&p) {
+                    write!(f, "#")?;
+                } else {
+                    write!(f, ".")?;
+                }
+            }
+            writeln!(f)?;
+        }
+        Ok(())
+    }
+}
+
+fn parse_page(input: &str) -> IResult<&str, Page> {
+    let (input, points) = separated_list1(line_ending, parse_point)(input)?;
+    let (input, _) = many0(line_ending)(input)?;
+    let (input, mut folds) = separated_list1(line_ending, parse_fold)(input)?;
+    folds.reverse();
+    Ok((
+        input,
+        Page {
+            points: points.into_iter().collect(),
+            folds,
+        },
+    ))
+}
+
+fn parse_fold(input: &str) -> IResult<&str, Fold> {
+    alt((
+        map(tuple((tag("fold along x="), nom_u32)), |(_, val)| {
+            Fold::X(val)
+        }),
+        map(tuple((tag("fold along y="), nom_u32)), |(_, val)| {
+            Fold::Y(val)
+        }),
+    ))(input)
+}
+
+fn parse_point(input: &str) -> IResult<&str, Point> {
+    map(tuple((nom_u32, nom_char(','), nom_u32)), |(x, _, y)| {
+        Point { x, y }
+    })(input)
+}