3 files changed, 193 insertions, 1 deletions
diff --git a/2021/rust/Cargo.toml b/2021/rust/Cargo.toml
index 06ea192..8c18a33 100644
--- a/2021/rust/Cargo.toml
+++ b/2021/rust/Cargo.toml
@@ -12,7 +12,7 @@ members = [
     "day10",
     "day11",
     "day12",
-#    "day13",
+    "day13",
 #    "day14",
 #    "day15",
 #    "day16",
diff --git a/2021/rust/day13/Cargo.toml b/2021/rust/day13/Cargo.toml
new file mode 100644
index 0000000..8408975
--- /dev/null
+++ b/2021/rust/day13/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "day13"
+version = "0.1.0"
+authors = ["cos <cos>"]
+edition = "2021"
+
+[dependencies]
+aoc = { path = "../../../common/rust/aoc" }
+anyhow = "1.0"
diff --git a/2021/rust/day13/src/main.rs b/2021/rust/day13/src/main.rs
new file mode 100644
index 0000000..c87373c
--- /dev/null
+++ b/2021/rust/day13/src/main.rs
@@ -0,0 +1,183 @@
+use {
+    anyhow::{
+        anyhow,
+        Context,
+        Result,
+    },
+    std::{
+        collections::BTreeSet,
+        env::args,
+        fmt::{
+            Display,
+            Error as FmtError,
+            Formatter,
+            Result as FmtResult,
+        },
+        fs::File,
+        io::{
+            BufRead,
+            BufReader,
+        },
+        path::Path,
+    },
+};
+
+#[derive(Clone,Copy,Debug,Eq,Ord,PartialEq,PartialOrd)]
+struct Point {
+    y: usize, // Please note that ordering is derived with y prior to x, as required for the
+    x: usize, // Display trait to work as currently implemented.
+}
+
+impl Point {
+    fn fold(&self, fold: Fold) -> Self {
+        match fold {
+            Fold::X(num) => {
+                if self.x > num {
+                    return Point{x: num - (self.x - num), y: self.y};
+                }
+            }
+            Fold::Y(num) => {
+                if self.y > num {
+                    return Point{x: self.x, y: num-(self.y - num)};
+                }
+            }
+        }
+        *self
+    }
+}
+
+#[derive(Clone,Copy)]
+enum Fold {
+    X(usize),
+    Y(usize),
+}
+
+struct Paper {
+    dots: BTreeSet<Point>,
+}
+
+impl Paper {
+    fn fold(&self, fold: &Fold) -> Result<Paper> {
+        let mut folded = Paper {
+            dots: BTreeSet::new(),
+        };
+
+        for point in &self.dots {
+            let mirror = point.fold(*fold);
+            if self.dots.contains(point) {
+                folded.dots.insert(mirror);
+            }
+        }
+
+        Ok(folded)
+    }
+
+    fn dot_count(&self) -> usize {
+        self.dots.len()
+    }
+}
+
+impl Display for Paper {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        let mut s = String::new();
+        let (mut x, mut y) = (0, 0);
+        let max_x = self.dots.iter().map(|p| p.x).max().ok_or(FmtError)?;
+        for p in self.dots.iter() {
+            if p.y == y {
+                (x..p.x).for_each(|_| s += "." );
+            } else {
+                (x..=max_x).for_each(|_| s += "." );
+                s += "\n";
+                for _ in y..(p.y-1) {
+                    (0..=max_x).for_each(|_| s += "." );
+                    s += "\n";
+                }
+                (0..p.x).for_each(|_| s += "." );
+            }
+            s += "#";
+
+            x = p.x+1;
+            y = p.y;
+            if x > max_x {
+                x = 0;
+                y += 1;
+                s += "\n";
+            }
+        }
+        (x..=max_x).for_each(|_| s += "." );
+        write!(f, "{}", s)
+    }
+}
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<(Paper, Vec<Fold>)> {
+    let reader = BufReader::new(File::open(filename)?);
+    let mut lines = reader.lines();
+    let mut dots: BTreeSet<Point> = BTreeSet::new();
+
+    for line in lines.by_ref() {
+        let l = line.map_err(|err| anyhow!("Failed reading from file: {}", err))?;
+        if l.is_empty() {
+            break;
+        }
+
+        let values: Vec<_> = l.split(',')
+            .map(|v| v.parse().map_err(|err| anyhow!("{}: {}", l, err))).collect::<Result<_>>()?;
+        if values.len() != 2 {
+            return Err(anyhow!("Failed parsing coordinates from line: {}", l));
+        }
+        let (x, y) = match (values.get(0), values.get(1)) {
+            (Some(parsed_x), Some(parsed_y)) => Ok((parsed_x, parsed_y)),
+            (_, _) => Err(anyhow!("Invalid coordinates: {}", l)),
+        }?;
+        dots.insert(Point{x: *x, y: *y});
+    }
+
+    let folds = lines.map(|line| {
+        let l = line?;
+        let s = l.strip_prefix("fold along ")
+            .ok_or_else(|| anyhow!("Could not find fold: {}", l))?;
+        let (axis, position) = s.split_once('=')
+            .ok_or_else(|| anyhow!("Could not parse fold: {}", l))?;
+        let num = position.parse()
+            .map_err(|err| anyhow!("Invalid fold position: {}, {}", l, err))?;
+        let fold = match axis {
+            "x" => Ok(Fold::X(num)),
+            "y" => Ok(Fold::Y(num)),
+            _ => Err(anyhow!("Invalid fold axis: {}", axis)),
+        }?;
+
+        Ok(fold)
+    }).collect::<Result<Vec<_>>>()?;
+
+    Ok((Paper { dots, }, folds))
+}
+
+fn part1(paper: &Paper, folds: &[Fold]) -> Result<usize> {
+    let fold = folds.get(0).ok_or_else(|| anyhow!("Could not find any fold"))?;
+    let folded = paper.fold(fold)?;
+    Ok(folded.dot_count())
+}
+
+fn part2(paper: Paper, folds: &[Fold]) -> Result<String> {
+    let folded = folds.iter().fold(Ok(paper), |p: Result<Paper>, fold| {
+        let folded = p?.fold(fold)?;
+        Ok(folded)
+    })?;
+    Ok(format!("{}", folded))
+}
+
+fn main() -> Result<()> {
+    let ( do_part_1, do_part_2 ) = aoc::do_parts();
+
+    let filename = args().nth(1).ok_or(anyhow!("Missing input filename"))?;
+    let (paper, folds) = read_input(filename).context("Could not read input")?;
+    if do_part_1 {
+        let solution = part1(&paper, &folds).context("No solution for part 1")?;
+        println!("Part1, solution found to be: {}", solution);
+    }
+    if do_part_2 {
+        let solution = part2(paper, &folds).context("No solution for part 2")?;
+        println!("Part2, solution found to be:\n{}", solution);
+    }
+    Ok(())
+}