ent: use stringer tool to generate Op.String() (#439)

Signed-off-by: Alex Snast <alexsn@fb.com>

ent.go

@@ -7,7 +7,6 @@ package ent
 
 import (
 	"context"
-	"fmt"
 
 	"github.com/facebookincubator/ent/schema/edge"
 	"github.com/facebookincubator/ent/schema/field"
@@ -319,19 +318,6 @@ const (
 )
 
 // Is reports whether o is match the given operation.
-func (op Op) Is(o Op) bool { return op&o != 0 }
+func (i Op) Is(o Op) bool { return i&o != 0 }
 
-func (op Op) String() string {
-	if op > 0 || op < Op(len(ops)) {
-		return ops[op]
-	}
-	return fmt.Sprintf("op(%d)", op)
-}
-
-var ops = [...]string{
-	OpCreate:    "Create",
-	OpUpdate:    "Update",
-	OpUpdateOne: "UpdateOne",
-	OpDelete:    "Delete",
-	OpDeleteOne: "DeleteOne",
-}
+//go:generate go run golang.org/x/tools/cmd/stringer -type Op

op_string.go

@@ -0,0 +1,43 @@
+// Code generated by "stringer -type Op"; DO NOT EDIT.
+
+package ent
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[OpCreate-1]
+	_ = x[OpUpdate-2]
+	_ = x[OpUpdateOne-4]
+	_ = x[OpDelete-8]
+	_ = x[OpDeleteOne-16]
+}
+
+const (
+	_Op_name_0 = "OpCreateOpUpdate"
+	_Op_name_1 = "OpUpdateOne"
+	_Op_name_2 = "OpDelete"
+	_Op_name_3 = "OpDeleteOne"
+)
+
+var (
+	_Op_index_0 = [...]uint8{0, 8, 16}
+)
+
+func (i Op) String() string {
+	switch {
+	case 1 <= i && i <= 2:
+		i -= 1
+		return _Op_name_0[_Op_index_0[i]:_Op_index_0[i+1]]
+	case i == 4:
+		return _Op_name_1
+	case i == 8:
+		return _Op_name_2
+	case i == 16:
+		return _Op_name_3
+	default:
+		return "Op(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+}