diff --git a/2021/src/day5.rs b/2021/src/day5.rs
index 53dbca1..9f60719 100644
--- a/2021/src/day5.rs
+++ b/2021/src/day5.rs
@@ -1,77 +1,150 @@
+use std::str::FromStr;
+
 use color_eyre::eyre;
-use num::{Integer, integer::gcd};
+use num::integer::gcd;
+
+use crate::lib::{ParseLines, IterExt};
 
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 struct Point {
     x: isize,
     y: isize,
 }
 
 
-#[derive(Copy, Clone)]
-struct Line {
-    start: Point,
-    end: Point,
-}
-
-
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 struct Slope {
     rise: isize,
     run: isize,
 }
 
-
-impl Line {
-    fn slope(&self) -> Slope {
-        let mut rise = self.end.y - self.start.y;
-        let mut run = self.end.x - self.start.x;
-        if rise != 0 && run != 0 {
-            let divisor = gcd(rise, run);
-            rise /= divisor;
-            run /= divisor;
+impl Slope {
+    fn apply_to(&self, pt: Point) -> Point {
+        Point {
+            x: pt.x + self.run,
+            y: pt.y + self.rise,
         }
-        Slope {rise, run}
     }
 }
 
 
+#[derive(Copy, Clone, Debug)]
+struct Line {
+    start: Point,
+    end: Point,
+}
+
+impl Line {
+    fn slope(&self) -> Slope {
+        let dx = self.end.y - self.start.y;
+        let dy = self.end.x - self.start.x;
+
+        let divisor;
+        if dx == 0 || dy == 0 {
+            divisor = (dx + dy).abs();
+        }
+        else {
+            divisor = gcd(dx, dy);
+        }
+
+        Slope {
+            rise: dx / divisor,
+            run: dy / divisor,
+        }
+    }
+
+    fn points(&self) -> PointsIter {
+        PointsIter {
+            line: self,
+            pos: self.start,
+            slope: self.slope(),
+        }
+    }
+}
+
+
+#[derive(Debug)]
 struct PointsIter<'a> {
     line: &'a Line,
     pos: Point,
     slope: Slope,
 }
 
-impl PointsIter<'_> {
-    fn from(line: &Line) -> Self {
-        PointsIter {
-            line,
-            pos: line.start,
-            slope: line.slope(),
-        }
-    }
-}
-
-impl Iterator for PointsIter {
+impl Iterator for PointsIter<'_> {
     type Item = Point;
     fn next(&mut self) -> Option<Self::Item> {
-        if self.pos.x = self.line.start.x && self.pos.y == self.line.start.y {
-            return Some(self.pos);
-        }
-
-        if self.pos.x != self.line.end.x && self.pos.y != self.line.end.y {
-            self.pos.y += self.slope.rise;
-            self.pos.x += self.slope.run;
-            return Some(self.pos);
-        }
-        else {
+        // note, this approach will break if total dX and dY are not multiples of rise/run
+        if self.pos == self.slope.apply_to(self.line.end) {
             return None;
         }
+
+        let orig = self.pos;
+        self.pos = self.slope.apply_to(self.pos);
+        Some(orig)
     }
 }
 
 
-struct GridCounter<const W: usize, const H: usize> {
-    rows: [[usize; W]; H],
+impl FromStr for Line {
+    type Err = eyre::Report;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let (p1, p2) = s.split(" -> ").take_pair()?;
+        let (x1, y1) = p1.split(',').take_pair()?;
+        let (x2, y2) = p2.split(',').take_pair()?;
+        
+        let start = Point {
+            x: x1.parse::<isize>()?,
+            y: y1.parse::<isize>()?,
+        };
+        let end = Point {
+            x: x2.parse::<isize>()?,
+            y: y2.parse::<isize>()?,
+        };
+
+        Ok(Line {start, end})
+    }
+}
+
+
+fn part1(lines: &[Line]) -> usize {
+    let mut grid = vec![[0; 1000]; 1000];
+
+    let points = lines.iter()
+        .filter(|l| l.start.x == l.end.x || l.start.y == l.end.y)
+        .flat_map(|l| l.points());
+
+    for pt in points {
+        grid[pt.y as usize][pt.x as usize] += 1;
+    }
+
+    grid.into_iter()
+        .flat_map(|row| row.into_iter())
+        .filter(|n| *n > 1)
+        .count()
+}
+
+
+fn part2(lines: &[Line]) -> usize {
+    let mut grid = vec![[0; 1000]; 1000];
+
+    for pt in lines.iter().flat_map(|l| l.points()) {
+        grid[pt.y as usize][pt.x as usize] += 1;
+    }
+
+    grid.into_iter()
+        .flat_map(|row| row.into_iter())
+        .filter(|n| *n > 1)
+        .count()
+}
+
+
+pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
+    let mut lines = Vec::new();
+    for line in data.parse_lines::<Line>() {
+        lines.push(line?);
+    }
+
+    Ok((part1(&lines), part2(&lines)))
 }
diff --git a/2021/src/lib.rs b/2021/src/lib.rs
index 1da8eda..f991069 100644
--- a/2021/src/lib.rs
+++ b/2021/src/lib.rs
@@ -3,6 +3,7 @@ use std::str::FromStr;
 use std::fs::File;
 
 use color_eyre::{eyre};
+use eyre::eyre;
 
 
 pub fn load(filename: &str) -> eyre::Result<String> {
@@ -52,3 +53,19 @@ impl ParseLines for String {
     }
 }
 
+
+pub trait IterExt: Iterator {
+    fn take_pair(&mut self) -> eyre::Result<(Self::Item, Self::Item)> {
+        let a = match self.next() {
+            Some(v) => v,
+            None => return Err(eyre!("Not enough values to unpack")),
+        };
+        let b = match self.next() {
+            Some(v) => v,
+            None => return Err(eyre!("Not enough values to unpack")),
+        };
+        Ok((a, b))
+    }
+}
+
+impl<I: Iterator> IterExt for I {}
diff --git a/2021/src/main.rs b/2021/src/main.rs
index c104471..931307a 100644
--- a/2021/src/main.rs
+++ b/2021/src/main.rs
@@ -4,14 +4,14 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day4;
+mod day5;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/04.txt")?;
+    let data = load("data/05.txt")?;
 
     let start = Instant::now();
-    let (one, two) = day4::run(&data)?;
+    let (one, two) = day5::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
 
     let precision = 2.0 - dur.log10().floor();