diff --git a/2021/day09/main.go b/2021/day09/main.go
new file mode 100644
index 0000000..7fda21e
--- /dev/null
+++ b/2021/day09/main.go
@@ -0,0 +1,166 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"sort"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	if part == 1 {
+		ans := part1(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	} else {
+		ans := part2(input)
+		util.CopyToClipboard(fmt.Sprintf("%v", ans))
+		fmt.Println("Output:", ans)
+	}
+}
+
+func part1(input string) int {
+	grid := parseInput(input)
+
+	diffs := [][]int{
+		{-1, 0},
+		{1, 0},
+		{0, -1},
+		{0, 1},
+	}
+
+	var totalRisk int
+	for r, rows := range grid {
+		for c, v := range rows {
+			lowerThanAll := true
+			for _, d := range diffs {
+				dr, dc := r+d[0], c+d[1]
+				// in bounds
+				if dr >= 0 && dr < len(grid) && dc >= 0 && dc < len(grid[0]) {
+					// neighbor is higher or even, r,c is not a low point
+					if grid[dr][dc] <= v {
+						lowerThanAll = false
+						break
+					}
+				}
+			}
+
+			if lowerThanAll {
+				totalRisk += 1 + v
+			}
+		}
+	}
+
+	return totalRisk
+}
+
+func part2(input string) int {
+	grid := parseInput(input)
+
+	diffs := [][]int{
+		{-1, 0},
+		{1, 0},
+		{0, -1},
+		{0, 1},
+	}
+
+	// copy pasta but collecting the coordinates to check
+	var lowPoints [][2]int
+	for r, rows := range grid {
+		for c, v := range rows {
+			lowerThanAll := true
+			for _, d := range diffs {
+				dr, dc := r+d[0], c+d[1]
+				if dr >= 0 && dr < len(grid) && dc >= 0 && dc < len(grid[0]) {
+					if grid[dr][dc] <= v {
+						lowerThanAll = false
+						break
+					}
+				}
+			}
+
+			if lowerThanAll {
+				lowPoints = append(lowPoints, [2]int{r, c})
+			}
+		}
+	}
+
+	// go through all lowpoints and get basin sizes via helper func
+	var basins []int
+	for _, lp := range lowPoints {
+		basins = append(basins, getBasinSize(grid, lp[0], lp[1], map[[2]int]bool{}))
+	}
+
+	// return 3 largest basins multiplied together
+	ans := 1
+	sort.Ints(basins)
+	for i := 0; i < 3; i++ {
+		ans *= basins[len(basins)-1-i]
+	}
+
+	return ans
+}
+
+var diffs = [][]int{
+	{-1, 0},
+	{1, 0},
+	{0, -1},
+	{0, 1},
+}
+
+func getBasinSize(grid [][]int, r, c int, basinCoords map[[2]int]bool) int {
+	// assume that every cell will be involved in one basin, just have to stop at nines
+	if grid[r][c] == 9 {
+		return 0
+	}
+
+	coord := [2]int{r, c}
+	// stop if already seen
+	if basinCoords[coord] {
+		return 0
+	}
+	// mark as seen
+	basinCoords[coord] = true
+
+	for _, d := range diffs {
+		dr, dc := r+d[0], c+d[1]
+		if dr >= 0 && dr < len(grid) && dc >= 0 && dc < len(grid[0]) {
+			// recurse
+			getBasinSize(grid, dr, dc, basinCoords)
+		}
+	}
+
+	// final size of coords map is the basin size
+	return len(basinCoords)
+}
+
+func parseInput(input string) (ans [][]int) {
+	for _, line := range strings.Split(input, "\n") {
+		var nums []int
+		for _, v := range strings.Split(line, "") {
+			nums = append(nums, cast.ToInt(v))
+		}
+		ans = append(ans, nums)
+	}
+	return ans
+}
diff --git a/2021/day09/main_test.go b/2021/day09/main_test.go
new file mode 100644
index 0000000..786f623
--- /dev/null
+++ b/2021/day09/main_test.go
@@ -0,0 +1,63 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `2199943210
+3987894921
+9856789892
+8767896789
+9899965678`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  15,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  580,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part1(tt.input); got != tt.want {
+				t.Errorf("part1() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_part2(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  1134,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  856716,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := part2(tt.input); got != tt.want {
+				t.Errorf("part2() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}