diff --git a/2025/cmd/root.go b/2025/cmd/root.go
index 140ee44..780d7b0 100644
--- a/2025/cmd/root.go
+++ b/2025/cmd/root.go
@@ -17,6 +17,7 @@ import (
 	"cpatino.com/advent-of-code/2025/day06"
 	"cpatino.com/advent-of-code/2025/day07"
 	"cpatino.com/advent-of-code/2025/day08"
+	"cpatino.com/advent-of-code/2025/day09"
 )
 
 var days = map[int]func(string) (any, any){
@@ -28,6 +29,7 @@ var days = map[int]func(string) (any, any){
 	6: day06.Run,
 	7: day07.Run,
 	8: day08.Run,
+	9: day09.Run,
 }
 
 var rootCmd = &cobra.Command{
diff --git a/2025/day09/day09.go b/2025/day09/day09.go
new file mode 100755
index 0000000..1f2dc29
--- /dev/null
+++ b/2025/day09/day09.go
@@ -0,0 +1,242 @@
+package day09
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"runtime"
+	"sync"
+)
+
+type Point struct {
+	x, y int
+}
+
+var tiles []Point
+
+func Part1(tiles []Point) int {
+	n := len(tiles)
+	if n < 2 {
+		return 0
+	}
+
+	numWorkers := runtime.NumCPU()
+	maxChan := make(chan int, numWorkers)
+	var wg sync.WaitGroup
+
+	chunkSize := (n + numWorkers - 1) / numWorkers
+
+	for w := range numWorkers {
+		start := w * chunkSize
+		end := min(start+chunkSize, n)
+		wg.Add(1)
+
+		go func(start, end int) {
+			defer wg.Done()
+
+			localMax := 0
+			for i := start; i < end; i++ {
+				for j := i + 1; j < n; j++ {
+					t1 := tiles[i]
+					t2 := tiles[j]
+					area := (abs(t1.x-t2.x) + 1) * (abs(t1.y-t2.y) + 1)
+					if area > localMax {
+						localMax = area
+					}
+				}
+			}
+
+			maxChan <- localMax
+		}(start, end)
+	}
+
+	wg.Wait()
+	close(maxChan)
+
+	maxArea := 0
+	for v := range maxChan {
+		if v > maxArea {
+			maxArea = v
+		}
+	}
+	return maxArea
+}
+
+func Part2(tiles []Point) int {
+	if len(tiles) < 2 {
+		return 0
+	}
+
+	red := make(map[Point]bool, len(tiles))
+	for _, p := range tiles {
+		red[p] = true
+	}
+
+	green := make(map[Point]bool)
+	n := len(tiles)
+	for i := range n {
+		a := tiles[i]
+		b := tiles[(i+1)%n]
+		if a.x == b.x {
+			sy, ey := a.y, b.y
+			if sy > ey {
+				sy, ey = ey, sy
+			}
+			for y := sy + 1; y < ey; y++ {
+				pt := Point{a.x, y}
+				if !red[pt] {
+					green[pt] = true
+				}
+			}
+		} else if a.y == b.y {
+			sx, ex := a.x, b.x
+			if sx > ex {
+				sx, ex = ex, sx
+			}
+			for x := sx + 1; x < ex; x++ {
+				pt := Point{x, a.y}
+				if !red[pt] {
+					green[pt] = true
+				}
+			}
+		}
+	}
+
+	type vedge struct{ x, y1, y2 int }
+	var vedges []vedge
+	for i := range n {
+		a := tiles[i]
+		b := tiles[(i+1)%n]
+		if a.x == b.x {
+			sy, ey := a.y, b.y
+			if sy > ey {
+				sy, ey = ey, sy
+			}
+			vedges = append(vedges, vedge{a.x, sy, ey})
+		}
+	}
+
+	isInside := func(p Point) bool {
+		cross := 0
+		for _, e := range vedges {
+			if p.y > e.y1 && p.y <= e.y2 { // edge crosses scanline at p.y
+				if e.x > p.x {
+					cross++
+				}
+			}
+		}
+		return cross%2 == 1
+	}
+	if n < 2 {
+		return 0
+	}
+
+	numWorkers := runtime.NumCPU()
+	maxChan := make(chan int, numWorkers)
+	var wg sync.WaitGroup
+
+	chunkSize := (n + numWorkers - 1) / numWorkers
+
+	for w := range numWorkers {
+		start := w * chunkSize
+		end := start + chunkSize
+		if end > n {
+			end = n
+		}
+		wg.Add(1)
+		go func(start, end int) {
+			defer wg.Done()
+			localMax := 0
+			for i := start; i < end; i++ {
+				for j := i + 1; j < n; j++ {
+					t1, t2 := tiles[i], tiles[j]
+					sx, ex := t1.x, t2.x
+					if sx > ex {
+						sx, ex = ex, sx
+					}
+
+					sy, ey := t1.y, t2.y
+					if sy > ey {
+						sy, ey = ey, sy
+					}
+
+					valid := true
+					for x := sx; x <= ex && valid; x++ {
+						for _, y := range []int{sy, ey} {
+							pt := Point{x, y}
+							if !red[pt] && !green[pt] && !isInside(pt) {
+								valid = false
+								break
+							}
+						}
+					}
+
+					for y := sy + 1; y < ey && valid; y++ {
+						for _, x := range []int{sx, ex} {
+							pt := Point{x, y}
+							if !red[pt] && !green[pt] && !isInside(pt) {
+								valid = false
+								break
+							}
+						}
+					}
+
+					if valid {
+						area := (ex - sx + 1) * (ey - sy + 1)
+						if area > localMax {
+							localMax = area
+						}
+					}
+				}
+			}
+			maxChan <- localMax
+		}(start, end)
+	}
+
+	wg.Wait()
+	close(maxChan)
+
+	maxArea := 0
+	for v := range maxChan {
+		if v > maxArea {
+			maxArea = v
+		}
+	}
+	return maxArea
+}
+
+func abs(a int) int {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+func Run(filename string) (any, any) {
+	file, err := os.Open(filename)
+	if err != nil {
+		panic(err)
+	}
+	defer file.Close()
+
+	tiles = nil
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		var x, y int
+		_, err := fmt.Sscanf(line, "%d,%d", &x, &y)
+		if err != nil {
+			panic(fmt.Sprintf("failed to parse line: %q: %v", line, err))
+		}
+		tiles = append(tiles, Point{x, y})
+	}
+
+	if err := scanner.Err(); err != nil {
+		panic(err)
+	}
+
+	part1 := Part1(tiles)
+	part2 := Part2(tiles)
+
+	return part1, part2
+}
diff --git a/2025/day09/day09_test.go b/2025/day09/day09_test.go
new file mode 100755
index 0000000..aa2b8f8
--- /dev/null
+++ b/2025/day09/day09_test.go
@@ -0,0 +1,34 @@
+package day09
+
+import (
+	"os"
+	"testing"
+)
+
+type TestCase struct {
+	FileName string
+	Part1    any
+	Part2    any
+}
+
+func TestDay09(t *testing.T) {
+	tests := []TestCase{
+		{"test.txt", 50, 24},
+		{"input.txt", 4777824480, 1542119040},
+	}
+
+	for _, test := range tests {
+		if _, err := os.Stat(test.FileName); os.IsNotExist(err) {
+			t.Fatalf("test file does not exist: %v", test.FileName)
+		}
+
+		part1, part2 := Run(test.FileName)
+		if test.Part1 != nil && part1 != test.Part1 {
+			t.Fatalf("%s: unexpected part1:\nwant:\t%v\ngot:\t%v", test.FileName, test.Part1, part1)
+		}
+
+		if test.Part2 != nil && part2 != test.Part2 {
+			t.Fatalf("%s: unexpected part2:\nwant:\t%v\ngot:\t%v", test.FileName, test.Part2, part2)
+		}
+	}
+}
diff --git a/2025/day09/test.txt b/2025/day09/test.txt
new file mode 100755
index 0000000..c8563ea
--- /dev/null
+++ b/2025/day09/test.txt
@@ -0,0 +1,8 @@
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+7,3