1 files changed, 30 insertions, 0 deletions
diff --git a/pkg/gitdiff/assert_test.go b/pkg/gitdiff/assert_test.go
new file mode 100644
index 0000000..878f13c
--- /dev/null
+++ b/pkg/gitdiff/assert_test.go
@@ -0,0 +1,30 @@
+package gitdiff
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func assertError(t *testing.T, expected interface{}, actual error, action string) {
+	if actual == nil {
+		t.Fatalf("expected error %s, but got nil", action)
+	}
+
+	switch exp := expected.(type) {
+	case bool:
+		if !exp {
+			t.Fatalf("unexpected error %s: %v", action, actual)
+		}
+	case string:
+		if !strings.Contains(actual.Error(), exp) {
+			t.Fatalf("incorrect error %s: %q does not contain %q", action, actual.Error(), exp)
+		}
+	case error:
+		if !errors.Is(actual, exp) {
+			t.Fatalf("incorrect error %s: expected %T (%v), actual: %T (%v)", action, exp, exp, actual, actual)
+		}
+	default:
+		t.Fatalf("unsupported expected error type: %T", exp)
+	}
+}