starlark/starlarkgen/generator.go

// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

package starlarkgen

import (
	"reflect"
	"strings"

	"github.com/golang/protobuf/descriptor"
	protoc "github.com/golang/protobuf/protoc-gen-go/descriptor"
)

// Generate produces the starlark code needed to generate the input protobuf message.
func Generate(msg descriptor.Message) string {
	ast := genMessage(reflect.ValueOf(msg))
	return ast.String()
}

func genMessage(val reflect.Value) Node {
	// Don't output anything for nil or zero values.
	if isNothing(val) {
		return nil
	}

	// Must read these values before calling val.Elem(). Otherwise the value it reflects
	// is no longer a descriptor.Message.
	msg := val.Interface().(descriptor.Message)
	filedesc, desc := descriptor.ForMessage(msg)
	node := &ConstructorCall{
		Package:  filedesc.GetPackage(),
		Typename: desc.GetName(),
	}
	val = val.Elem()
	for _, field := range desc.GetField() {
		fieldValue := val.FieldByName(strings.Title(field.GetJsonName()))
		fieldAst := genField(field, fieldValue)
		// Skip fields that have nil pointer values.
		if fieldAst == nil {
			continue
		}
		// Skip fields that are set to zero-values.
		if primitive, ok := fieldAst.(Primitive); ok && isZero(primitive.Value) {
			continue
		}
		node.Kwargs = append(node.Kwargs, Kwarg{
			Keyword: field.GetName(),
			Value:   fieldAst,
		})
	}
	return node
}

func genField(desc *protoc.FieldDescriptorProto, val reflect.Value) Node {
	if desc.GetTypeName() == "" {
		// This field is a not a proto message (and the instance is not a Go struct). Just
		// write the raw value.
		return Primitive{val}
	}

	// This field is a proto.Message, which has a corresponding Go struct type. Generate
	// it and its children recursively.
	return genMessage(val)
}

func isZero(val reflect.Value) bool {
	zero := reflect.Zero(val.Type()).Interface()
	return reflect.DeepEqual(zero, val.Interface())
}

func isNothing(val reflect.Value) bool {
	return isZero(val) || val.IsNil()
}