3 files changed, 151 insertions, 1 deletions
diff --git a/2021/rust/Cargo.toml b/2021/rust/Cargo.toml
index aed5051..3ad3961 100644
--- a/2021/rust/Cargo.toml
+++ b/2021/rust/Cargo.toml
@@ -4,7 +4,7 @@ members = [
     "day02",
     "day03",
     "day04",
-#    "day05",
+    "day05",
 #    "day06",
 #    "day07",
 #    "day08",
diff --git a/2021/rust/day05/Cargo.toml b/2021/rust/day05/Cargo.toml
new file mode 100644
index 0000000..6a8f51f
--- /dev/null
+++ b/2021/rust/day05/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "day05"
+version = "0.1.0"
+authors = ["cos <cos>"]
+edition = "2021"
+
+[dependencies]
+aoc = { path = "../../../common/rust/aoc" }
+anyhow = "1.0"
+regex = "1.5"
diff --git a/2021/rust/day05/src/main.rs b/2021/rust/day05/src/main.rs
new file mode 100644
index 0000000..b3bd170
--- /dev/null
+++ b/2021/rust/day05/src/main.rs
@@ -0,0 +1,140 @@
+use {
+    anyhow::{
+        anyhow,
+        Context,
+        Result,
+    },
+    regex::{
+        Regex,
+    },
+    std::{
+        collections::HashMap,
+        env::args,
+        fs::File,
+        io::{
+            BufRead,
+            BufReader,
+        },
+        iter::repeat,
+        path::Path,
+    },
+};
+
+#[derive(Debug,Eq,Hash,PartialEq)]
+struct Coord {
+    x: usize,
+    y: usize,
+}
+
+#[derive(Debug)]
+struct Line {
+    start: Coord,
+    end: Coord,
+}
+
+impl Line {
+    fn coords(&self) -> Vec<Coord> {
+        let (iter_x, iter_y): (Box<dyn Iterator<Item=_>>, Box<dyn Iterator<Item=_>>) = (
+            match (self.start.x, self.end.x) {
+                (start, end) if start < end => Box::new(start..=end),
+                (start, end) if start > end => Box::new((end..=start).rev()),
+                (start, _end) /* equals */  => Box::new(repeat(start)),
+            },
+            match (self.start.y, self.end.y) {
+                (start, end) if start < end => Box::new(start..=end),
+                (start, end) if start > end => Box::new((end..=start).rev()),
+                (start, _end) /* equals */  => Box::new(repeat(start)),
+            },
+        );
+        iter_x.zip(iter_y).map(|(x, y)| Coord{x, y}).collect()
+    }
+}
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<Line>> {
+    let reader = BufReader::new(File::open(filename)?);
+    let re = Regex::new(r#"(?x)
+        (?P<x1>[0-9]+),
+        (?P<y1>[0-9]+)[[:blank:]]*->[[:blank:]]*
+        (?P<x2>[0-9]+),
+        (?P<y2>[0-9]+)
+        "#)?;
+
+    reader.lines().map(|row| {
+        let string = row?;
+        let caps = re.captures(&string).ok_or_else(|| anyhow!("Could not parse: {}", string))?;
+        match (caps.name("x1"), caps.name("y1"), caps.name("x2"), caps.name("y2")) {
+            (Some(x1), Some(y1), Some(x2), Some(y2)) => {
+                let x = x1.as_str().parse()?;
+                let y = y1.as_str().parse()?;
+                let start = Coord{x, y};
+                let x = x2.as_str().parse()?;
+                let y = y2.as_str().parse()?;
+                let end = Coord {x, y};
+            Ok(Line{start, end})
+            },
+            _ => Err(anyhow!("Could not parse: {}", string)),
+        }
+    }).collect()
+}
+
+fn count_overlaps(points: &HashMap<Coord, usize>) -> Result<usize>{
+    let mut overlap = 0;
+
+    let min_x = points.keys().map(|coord| coord.x).min().ok_or(anyhow!("Problem finding min x"))?;
+    let max_x = points.keys().map(|coord| coord.x).max().ok_or(anyhow!("Problem finding max x"))?;
+    let min_y = points.keys().map(|coord| coord.y).min().ok_or(anyhow!("Problem finding min y"))?;
+    let max_y = points.keys().map(|coord| coord.y).max().ok_or(anyhow!("Problem finding max y"))?;
+
+    for y in min_y..=max_y {
+        for x in min_x..=max_x {
+            if let Some(v) = points.get(&Coord {x, y}) {
+                if *v >= 2 {
+                    overlap += 1;
+                }
+            }
+        }
+    }
+
+    Ok(overlap)
+}
+
+fn part1<'a, I: IntoIterator<Item = &'a Line>>(input: I) -> Result<usize> {
+    let mut points = HashMap::new();
+
+    for line in input.into_iter().filter(|l| l.start.x == l.end.x || l.start.y == l.end.y) {
+        for coord in line.coords() {
+            let point = points.entry(coord).or_insert(0);
+            *point += 1;
+        }
+    }
+
+    count_overlaps(&points)
+}
+
+fn part2<'a, I: IntoIterator<Item = &'a Line>>(input: I) -> Result<usize> {
+    let mut points = HashMap::new();
+
+    for line in input.into_iter() {
+        for coord in line.coords() {
+            let point = points.entry(coord).or_insert(0);
+            *point += 1;
+        }
+    }
+    count_overlaps(&points)
+}
+
+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 input = read_input(filename).context("Could not read input")?;
+    if do_part_1 {
+        let solution = part1(&input).context("No solution for part 1")?;
+        println!("Part1, solution found to be: {}", solution);
+    }
+    if do_part_2 {
+        let solution = part2(&input).context("No solution for part 2")?;
+        println!("Part2, solution found to be: {}", solution);
+    }
+    Ok(())
+}