diff --git a/input/day09 b/input/day09
new file mode 100644
index 0000000..519c592
--- /dev/null
+++ b/input/day09
@@ -0,0 +1,496 @@
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diff --git a/src/day09.scala b/src/day09.scala
new file mode 100644
index 0000000..454cde9
--- /dev/null
+++ b/src/day09.scala
@@ -0,0 +1,57 @@
+package aoc
+package day09
+
+val dayNumber = "09"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): Long =
+  input
+    .split('\n').map { case s"$x,$y" => (x = x.toLong, y = y.toLong) }
+    .combinations(2)
+    .collect { case Array(a, b) => ((a.x - b.x + 1) * (a.y - b.y + 1)).abs }
+    .max
+
+def part2(input: String): Long =
+
+  case class Point(x: Int, y: Int)
+
+  case class Rectangle(xs: Range, ys: Range):
+    def area: Long           = xs.size.toLong * ys.size.toLong
+    def dropEdges: Rectangle = Rectangle(xs.drop(1).dropRight(1), ys.drop(1).dropRight(1))
+    def intersects(other: Rectangle): Boolean =
+      xs.intersects(other.xs) && ys.intersects(other.ys)
+
+  object Rectangle:
+    def between(a: Point, b: Point): Rectangle =
+      val diffX = b.x - a.x
+      val diffY = b.y - a.y
+      Rectangle(
+        a.x to b.x by (if diffX == 0 then 1 else diffX.sign),
+        a.y to b.y by (if diffY == 0 then 1 else diffY.sign)
+      )
+
+  extension (r: Range)
+    def intersects(other: Range): Boolean =
+      r.nonEmpty && other.nonEmpty &&
+        (r.contains(other.min) || other.contains(r.min))
+
+  val points = input.split('\n').collect { case s"$x,$y" => Point(x.toInt, y.toInt) }.toList
+  val edges  =
+    (points.last :: points)
+      .sliding(2)
+      .collect { case List(a, b) => Rectangle.between(a, b) }
+      .toList
+
+  val rectangles = points
+    .combinations(2)
+    .collect { case List(a, b) => Rectangle.between(a, b) }
+    .toList
+
+  rectangles.collect { case r if !edges.exists(r.dropEdges.intersects) => r.area }.max
+
+end part2