diff --git a/src/bin/day9.rs b/src/bin/day9.rs
index 990ea6d..f30a8e7 100644
--- a/src/bin/day9.rs
+++ b/src/bin/day9.rs
@@ -1,8 +1,9 @@
-use std::time::Instant;
+use std::{collections::HashSet, ops::RangeInclusive, time::Instant};
 
 use aoc_2025::{load, print_res};
 use bstr::BString;
 
+#[derive(PartialEq, Eq, Hash, Debug, Clone, Copy)]
 pub struct Point2 {
     x: u64,
     y: u64,
@@ -26,22 +27,285 @@ pub fn parsing(input: &BString) -> color_eyre::Result<Parsed> {
         .collect()
 }
 
+fn rect_area(a: &Point2, b: &Point2) -> u64 {
+    (a.x.abs_diff(b.x) + 1) * (a.y.abs_diff(b.y) + 1)
+}
+
 #[inline(never)]
 pub fn part1(input: Parsed) {
     let max_area = input
         .iter()
         .enumerate()
         .flat_map(|(ia, a)| input.iter().skip(ia + 1).map(move |b| (a, b)))
-        .map(|(a, b)| (a.x.abs_diff(b.x) + 1) * (a.y.abs_diff(b.y) + 1))
+        .map(|(a, b)| rect_area(a, b))
         .max()
         .unwrap();
 
     print_res!("Max area: {max_area}");
 }
 
+struct Polygon {
+    points: Vec<Point2>,
+
+    x_values: Vec<u64>,
+    y_values: Vec<u64>,
+
+    vertical_edges_by_x_slot: Vec<Vec<RangeInclusive<u64>>>,
+    horizontal_edges_by_y_slot: Vec<Vec<RangeInclusive<u64>>>,
+}
+
+impl Polygon {
+    pub fn new(points: Vec<Point2>) -> Self {
+        let x_values = {
+            let x: HashSet<_> = points.iter().map(|p| p.x).collect();
+            let mut x: Vec<_> = x.into_iter().collect();
+            x.sort_unstable();
+            x
+        };
+
+        let y_values = {
+            let y: HashSet<_> = points.iter().map(|p| p.y).collect();
+            let mut y: Vec<_> = y.into_iter().collect();
+            y.sort_unstable();
+            y
+        };
+
+        let mut this = Polygon {
+            points,
+            vertical_edges_by_x_slot: vec![Vec::new(); x_values.len()],
+            horizontal_edges_by_y_slot: vec![Vec::new(); y_values.len()],
+            x_values,
+            y_values,
+        };
+
+        for (i, p) in this.points.iter().enumerate() {
+            let prev = this.prev(i);
+
+            if prev.x == p.x {
+                let x_stop = this.x_values.iter().position(|&x| x == p.x).unwrap();
+
+                // Vertical edge
+                let min_y = p.y.min(prev.y);
+                let max_y = p.y.max(prev.y);
+
+                this.vertical_edges_by_x_slot[x_stop].push(min_y..=max_y);
+            } else {
+                let y_stop = this.y_values.iter().position(|&y| y == p.y).unwrap();
+
+                assert_eq!(prev.y, p.y);
+
+                let min_x = p.x.min(prev.x);
+                let max_x = p.x.max(prev.x);
+
+                this.horizontal_edges_by_y_slot[y_stop].push(min_x..=max_x);
+            }
+        }
+
+        this
+    }
+
+    pub fn contains(&self, point: Point2) -> bool {
+        // Is the point a corner?
+        if self.points.contains(&point) {
+            return true;
+        }
+
+        // On a horizontal edge
+        if let Some(y_stop) = self.y_values.iter().position(|&y| y == point.y) {
+            let edges = &self.horizontal_edges_by_y_slot[y_stop];
+            if edges.iter().any(|e| e.contains(&point.x)) {
+                return true;
+            }
+        }
+
+        // On a vertical edge
+        if let Some(x_stop) = self.x_values.iter().position(|&x| x == point.x) {
+            let edges = &self.vertical_edges_by_x_slot[x_stop];
+            if edges.iter().any(|e| e.contains(&point.y)) {
+                return true;
+            }
+        }
+
+        // Not on the boundary
+
+        let mut intersection_count = 0;
+
+        // Check all vertical edges starting from this point
+        for (slot, _x) in self
+            .x_values
+            .iter()
+            .enumerate()
+            .skip_while(|&(_, &x)| x < point.x)
+            .take_while(|&(_, &x)| point.x <= x)
+        {
+            let edges = &self.vertical_edges_by_x_slot[slot];
+
+            if let Some(edge) = edges.iter().find(|r| r.contains(&point.y)) {
+                let intersect = if point.y == *edge.start() {
+                    *edge.end() > point.y
+                } else if point.y == *edge.end() {
+                    *edge.start() > point.y
+                } else {
+                    true
+                };
+
+                if intersect {
+                    intersection_count += 1;
+                }
+            }
+        }
+
+        intersection_count % 2 == 1
+    }
+
+    pub fn points(&self) -> &[Point2] {
+        &self.points
+    }
+
+    pub fn x_values(&self) -> &[u64] {
+        &self.x_values
+    }
+
+    pub fn y_values(&self) -> &[u64] {
+        &self.y_values
+    }
+
+    pub fn prev(&self, index: usize) -> Point2 {
+        if index == 0 {
+            *self.points.last().unwrap()
+        } else {
+            self.points[index - 1]
+        }
+    }
+}
+
+fn horizontal_edges_contained(poly: &Polygon, a: Point2, b: Point2) -> bool {
+    assert_ne!(a.x, b.x);
+
+    let min_x = a.x.min(b.x);
+    let max_x = a.x.max(b.x);
+
+    let y_values: &[u64] = if a.y == b.y { &[a.y] } else { &[a.y, b.y] };
+
+    for &y in y_values {
+        // Left corner
+
+        if !poly.contains(Point2 { x: min_x, y }) {
+            return false;
+        }
+
+        if !poly.contains(Point2 { x: min_x + 1, y }) {
+            return false;
+        }
+
+        // Inside edges
+
+        for &x in poly
+            .x_values()
+            .iter()
+            .skip_while(|&&x| x <= min_x)
+            .take_while(|&&x| x < max_x)
+        {
+            if !poly.contains(Point2 { x, y })
+                || !poly.contains(Point2 { x: x - 1, y })
+                || !poly.contains(Point2 { x: x + 1, y })
+            {
+                return false;
+            }
+        }
+
+        // Right corner
+
+        if !poly.contains(Point2 { x: max_x, y }) {
+            return false;
+        }
+
+        if !poly.contains(Point2 { x: max_x - 1, y }) {
+            return false;
+        }
+    }
+
+    true
+}
+
+fn vertical_edges_contain(poly: &Polygon, a: Point2, b: Point2) -> bool {
+    assert_ne!(a.y, b.y);
+
+    let min_y = a.y.min(b.y);
+    let max_y = a.y.max(b.y);
+
+    let x_values: &[u64] = if a.x == b.x { &[a.x] } else { &[a.x, b.x] };
+
+    for &x in x_values {
+        // Bottom corner
+
+        if !poly.contains(Point2 { x, y: min_y }) {
+            return false;
+        }
+
+        if !poly.contains(Point2 { x, y: min_y + 1 }) {
+            return false;
+        }
+
+        // Inside edges
+
+        for &y in poly
+            .y_values()
+            .iter()
+            .skip_while(|&&y| y <= min_y)
+            .take_while(|&&y| y < max_y)
+        {
+            if !poly.contains(Point2 { x, y })
+                || !poly.contains(Point2 { x, y: y - 1 })
+                || !poly.contains(Point2 { x, y: y + 1 })
+            {
+                return false;
+            }
+        }
+
+        // Top corner
+
+        if !poly.contains(Point2 { x, y: max_y }) {
+            return false;
+        }
+
+        if !poly.contains(Point2 { x, y: max_y - 1 }) {
+            return false;
+        }
+    }
+
+    true
+}
+
 #[inline(never)]
 pub fn part2(input: Parsed) {
-    todo!("todo part2")
+    let poly = Polygon::new(input);
+
+    let mut max_area = 0;
+
+    for (ia, &a) in poly.points().iter().enumerate() {
+        for &b in poly.points().iter().skip(ia + 1) {
+            let area = rect_area(&a, &b);
+            // Area would not even be bigger, don’t attempt to validate it
+            if area < max_area {
+                continue;
+            }
+
+            assert!(area > 1);
+
+            if a.x != b.x && !horizontal_edges_contained(&poly, a, b) {
+                continue;
+            }
+
+            if a.y != b.y && !vertical_edges_contain(&poly, a, b) {
+                continue;
+            }
+
+            max_area = area;
+        }
+    }
+
+    print_res!("Max inside area: {max_area}");
 }
 
 pub fn main() -> color_eyre::Result<()> {