diff --git a/internal/2017/DayFour/code_test.go b/internal/2017/DayFour/code_test.go
new file mode 100644
index 0000000..e925434
--- /dev/null
+++ b/internal/2017/DayFour/code_test.go
@@ -0,0 +1,41 @@
+package dayfour
+
+import "testing"
+
+func TestPartOne(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []string
+		expected int
+	}{
+		{
+			name:     "aa bb cc dd ee is valid",
+			input:    []string{"aa bb cc dd ee"},
+			expected: 1,
+		},
+		{
+			name:     "aa bb cc dd aa is not valid",
+			input:    []string{"aa bb cc dd aa"},
+			expected: 0,
+		},
+		{
+			name:     "aa bb cc dd aaa is valid",
+			input:    []string{"aa bb cc dd aaa"},
+			expected: 1,
+		},
+		{
+			name:     "multiple passphrases",
+			input:    []string{"aa bb cc dd ee", "aa bb cc dd aa", "aa bb cc dd aaa"},
+			expected: 2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := PartOne(tt.input)
+			if got != tt.expected {
+				t.Errorf("PartOne() = %d, want %d", got, tt.expected)
+			}
+		})
+	}
+}