1 files changed, 144 insertions, 0 deletions
diff --git a/src/bin/2021/day13.rs b/src/bin/2021/day13.rs
new file mode 100644
index 0000000..75e547b
--- /dev/null
+++ b/src/bin/2021/day13.rs
@@ -0,0 +1,144 @@
+use advent_of_code::prelude::*;
+
+#[derive(Default)]
+pub struct Paper {
+    grid: Grid<bool>,
+}
+
+impl Paper {
+    fn set(&mut self, row: Row, col: Col) {
+        self.grid.grow(Row(row.0 + 1), Col(col.0 + 1));
+        self.grid[row][col] = true;
+    }
+
+    fn fold(&mut self, horizontal: bool, coord: usize) {
+        let mut clone = Self::default();
+        if horizontal {
+            clone.grid.grow(
+                self.grid.rows(),
+                Col(coord.max(self.grid.cols().0 - coord - 1)),
+            );
+            for ((Row(row), Col(col)), cell) in self.grid.indexed_cells() {
+                if *cell {
+                    if coord > self.grid.cols().0 - coord - 1 {
+                        if col < coord {
+                            clone.set(Row(row), Col(col));
+                        } else if col > coord {
+                            clone.set(Row(row), Col(coord * 2 - col));
+                        }
+                    } else {
+                        if col < coord {
+                            clone.set(
+                                Row(row),
+                                Col(self.grid.cols().0 - coord * 2 - 1 + col),
+                            );
+                        } else if col > coord {
+                            clone.set(
+                                Row(row),
+                                Col(self.grid.cols().0 - col - 1),
+                            );
+                        }
+                    }
+                }
+            }
+        } else {
+            clone.grid.grow(
+                Row(coord.max(self.grid.rows().0 - coord - 1)),
+                self.grid.cols(),
+            );
+            for ((Row(row), Col(col)), cell) in self.grid.indexed_cells() {
+                if *cell {
+                    if coord > self.grid.rows().0 - coord - 1 {
+                        if row < coord {
+                            clone.set(Row(row), Col(col));
+                        } else if row > coord {
+                            clone.set(Row(coord * 2 - row), Col(col));
+                        }
+                    } else {
+                        if row < coord {
+                            clone.set(
+                                Row(self.grid.rows().0 - coord * 2 - 1 + row),
+                                Col(col),
+                            );
+                        } else if row > coord {
+                            clone.set(
+                                Row(self.grid.rows().0 - row - 1),
+                                Col(col),
+                            );
+                        }
+                    }
+                }
+            }
+        }
+        *self = clone;
+    }
+
+    fn total(&self) -> usize {
+        self.grid.cells().filter(|b| **b).count()
+    }
+}
+
+impl std::fmt::Display for Paper {
+    fn fmt(
+        &self,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> Result<(), std::fmt::Error> {
+        write!(
+            f,
+            "{}",
+            self.grid.display_packed(|b| if *b { '#' } else { '.' })
+        )
+    }
+}
+
+pub fn parse(fh: File) -> Result<(Paper, Vec<(bool, usize)>)> {
+    let mut paper = Paper::default();
+    let mut folds = vec![];
+    for line in parse::raw_lines(fh) {
+        if line.is_empty() {
+            continue;
+        }
+        if let Some(fold) = line.strip_prefix("fold along ") {
+            let mut fold = fold.split('=');
+            let horizontal = fold.next().unwrap() == "x";
+            let coord: usize = fold.next().unwrap().parse()?;
+            folds.push((horizontal, coord));
+        } else {
+            let mut coords = line.split(',');
+            let x: usize = coords.next().unwrap().parse()?;
+            let y: usize = coords.next().unwrap().parse()?;
+            paper.set(Row(y), Col(x));
+        }
+    }
+    Ok((paper, folds))
+}
+
+pub fn part1(
+    (mut paper, folds): (Paper, Vec<(bool, usize)>),
+) -> Result<usize> {
+    paper.fold(folds[0].0, folds[0].1);
+    Ok(paper.total())
+}
+
+pub fn part2(
+    (mut paper, folds): (Paper, Vec<(bool, usize)>),
+) -> Result<usize> {
+    for fold in folds {
+        paper.fold(fold.0, fold.1);
+    }
+
+    println!("{}", paper);
+    Ok(paper.total())
+}
+
+#[test]
+fn test() {
+    assert_eq!(
+        part1(parse(parse::data(2021, 13).unwrap()).unwrap()).unwrap(),
+        678
+    );
+    assert_eq!(
+        part2(parse(parse::data(2021, 13).unwrap()).unwrap()).unwrap(),
+        95
+    );
+}