diff --git a/2024/day25/main.go b/2024/day25/main.go
new file mode 100644
index 0000000..a661793
--- /dev/null
+++ b/2024/day25/main.go
@@ -0,0 +1,91 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"strings"
+
+	"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)
+
+	ans := part1(input)
+	util.CopyToClipboard(fmt.Sprintf("%v", ans))
+	fmt.Println("Output:", ans)
+}
+
+func part1(input string) int {
+	parts := strings.Split(input, "\n\n")
+
+	locks := [][]int{}
+	keys := [][]int{}
+	maxHeight := 0
+	for _, part := range parts {
+		lines := strings.Split(part, "\n")
+		maxHeight = len(lines) - 1
+
+		// lock detection: first line is all '#'
+		fullLock := strings.Repeat("#", len(lines[0]))
+		if lines[0] == fullLock {
+			lockHeights := []int{}
+			// for each column, find first non-dot from bottom
+			for col := range len(lines[0]) {
+				for row := len(lines) - 1; row >= 0; row-- {
+					if lines[row][col:col+1] != "." {
+						lockHeights = append(lockHeights, row)
+						break
+					}
+				}
+			}
+			locks = append(locks, lockHeights)
+		} else {
+			// key detection: for each column, find first non-dot from top
+			keyHeights := []int{}
+			for col := range len(lines[0]) {
+				for row := range len(lines) {
+					if lines[row][col:col+1] != "." {
+						keyHeights = append(keyHeights, len(lines)-1-row)
+						break
+					}
+				}
+			}
+			keys = append(keys, keyHeights)
+		}
+	}
+
+	count := 0
+
+	for _, lock := range locks {
+		for _, key := range keys {
+			fits := true
+			for col := range len(key) {
+				if lock[col]+key[col] >= maxHeight {
+					fits = false
+					break
+				}
+			}
+			if fits {
+				count += 1
+			}
+		}
+	}
+
+	return count
+}
diff --git a/2024/day25/main_test.go b/2024/day25/main_test.go
new file mode 100644
index 0000000..33ac789
--- /dev/null
+++ b/2024/day25/main_test.go
@@ -0,0 +1,71 @@
+package main
+
+import (
+	"testing"
+)
+
+var example = `#####
+.####
+.####
+.####
+.#.#.
+.#...
+.....
+
+#####
+##.##
+.#.##
+...##
+...#.
+...#.
+.....
+
+.....
+#....
+#....
+#...#
+#.#.#
+#.###
+#####
+
+.....
+.....
+#.#..
+###..
+###.#
+###.#
+#####
+
+.....
+.....
+.....
+#....
+#.#..
+#.#.#
+#####`
+
+func Test_part1(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			want:  3,
+		},
+		{
+			name:  "actual",
+			input: input,
+			want:  3114,
+		},
+	}
+	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)
+			}
+		})
+	}
+}