diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
index 470503b..fc9e325 100644
--- a/2021/aoc/aoc.go
+++ b/2021/aoc/aoc.go
@@ -2,6 +2,8 @@ package aoc
 
 import (
 	"bufio"
+	"io"
+	"io/fs"
 	"os"
 	"strconv"
 	"strings"
@@ -36,3 +38,49 @@ func MustInt(s string) int {
 	}
 	return i
 }
+
+type Inputs struct {
+	fs fs.FS
+}
+
+func (i Inputs) ReadLines(path string) (lines []string) {
+	f, err := i.fs.Open(path)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+	return lines
+}
+
+func (i Inputs) ReadFile(path string) string {
+	f, err := i.fs.Open(path)
+	if err != nil {
+		panic(err)
+	}
+	defer f.Close()
+	b, err := io.ReadAll(f)
+	if err != nil {
+		panic(err)
+	}
+	return strings.TrimSpace(string(b))
+}
+
+func Input() Inputs {
+	if len(os.Args) > 1 {
+		return Inputs{os.DirFS(os.Args[1])}
+	}
+	return Inputs{os.DirFS("./input")}
+}
+
+// Cut will be added to strings in Go 1.18, so hack it in for now!
+func Cut(s, sep string) (before, after string) {
+	if i := strings.Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):]
+	}
+	panic("Separator was not found in string")
+}
diff --git a/2021/day5/main.go b/2021/day5/main.go
new file mode 100644
index 0000000..2d0fba7
--- /dev/null
+++ b/2021/day5/main.go
@@ -0,0 +1,123 @@
+package main
+
+import (
+	"fmt"
+	"math"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+type Point struct {
+	x, y int
+}
+
+func (p Point) Plus(dx, dy int) Point {
+	return Point{x: p.x + dx, y: p.y + dy}
+}
+
+type Line struct {
+	p1, p2 Point
+}
+
+func (l Line) Slope() float64 {
+	dx := float64(l.p2.x - l.p1.x)
+	dy := float64(l.p2.y - l.p1.y)
+	return dy / dx
+}
+
+func (l Line) Delta() (dx, dy int) {
+	switch s := l.Slope(); s {
+	case 0:
+		if l.p1.x < l.p2.x {
+			return 1, 0
+		}
+		return -1, 0
+	case math.Inf(+1):
+		if l.p1.y < l.p2.y {
+			return 0, 1
+		}
+		return 0, -1
+	case +1:
+		if l.p1.x < l.p2.x {
+			return 1, 1
+		}
+		return -1, -1
+	case -1:
+		if l.p1.x < l.p2.x {
+			return 1, -1
+		}
+		return -1, 1
+	default:
+		panic(s)
+	}
+}
+
+func parse(flines []string) (lines []Line) {
+	for _, l := range flines {
+		s, e := aoc.Cut(l, " -> ")
+		x1, y1 := aoc.Cut(s, ",")
+		x2, y2 := aoc.Cut(e, ",")
+		p1 := Point{
+			x: aoc.MustInt(x1),
+			y: aoc.MustInt(y1),
+		}
+		p2 := Point{
+			x: aoc.MustInt(x2),
+			y: aoc.MustInt(y2),
+		}
+		if p1.x > p2.x || p1.y > p2.y {
+			// This point is backwards!
+			p1, p2 = p2, p1
+		}
+		lines = append(lines, Line{p1: p1, p2: p2})
+	}
+	return
+}
+func part1(flines []string) int {
+	grid := make(map[Point]int)
+	for _, l := range parse(flines) {
+		switch l.Slope() {
+		case 0:
+			y := l.p1.y
+			for x := l.p1.x; x <= l.p2.x; x++ {
+				grid[Point{x: x, y: y}]++
+			}
+		case math.Inf(+1), math.Inf(-1):
+			x := l.p1.x
+			for y := l.p1.y; y <= l.p2.y; y++ {
+				grid[Point{x: x, y: y}]++
+			}
+		}
+	}
+	var count int
+	for _, c := range grid {
+		if c > 1 {
+			count++
+		}
+	}
+	return count
+}
+
+func part2(flines []string) int {
+	grid := make(map[Point]int)
+	for _, l := range parse(flines) {
+		dx, dy := l.Delta()
+		for p := l.p1; p != l.p2; p = p.Plus(dx, dy) {
+			grid[p]++
+		}
+		grid[l.p2]++
+	}
+	var count int
+	for _, c := range grid {
+		if c > 1 {
+			count++
+		}
+	}
+	return count
+}
+
+func main() {
+	lines := aoc.Input().ReadLines("day5.txt")
+	fmt.Println(part1(lines))
+	fmt.Println(part2(lines))
+}