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// Copyright 2020 The Go 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 protopath provides functionality for
// representing a sequence of protobuf reflection operations on a message.
package protopath
import (
// NOTE: The Path and Values are separate types here since there are use cases
// where you would like to "address" some value in a message with just the path
// and don't have the value information available.
// This is different from how "".Path operates,
// which combines both path and value information together.
// Since the cmp package itself is the only one ever constructing a cmp.Path,
// it will always have the value available.
// Path is a sequence of protobuf reflection steps applied to some root
// protobuf message value to arrive at the current value.
// The first step must be a Root step.
type Path []Step
// TODO: Provide a Parse function that parses something similar to or
// perhaps identical to the output of Path.String.
// Index returns the ith step in the path and supports negative indexing.
// A negative index starts counting from the tail of the Path such that -1
// refers to the last step, -2 refers to the second-to-last step, and so on.
// It returns a zero Step value if the index is out-of-bounds.
func (p Path) Index(i int) Step {
if i < 0 {
i = len(p) + i
if i < 0 || i >= len(p) {
return Step{}
return p[i]
// String returns a structured representation of the path
// by concatenating the string representation of every path step.
func (p Path) String() string {
var b []byte
for _, s := range p {
b = s.appendString(b)
return string(b)
// Values is a Path paired with a sequence of values at each step.
// The lengths of Path and Values must be identical.
// The first step must be a Root step and
// the first value must be a concrete message value.
type Values struct {
Path Path
Values []protoreflect.Value
// Len reports the length of the path and values.
// If the path and values have differing length, it returns the minimum length.
func (p Values) Len() int {
n := len(p.Path)
if n > len(p.Values) {
n = len(p.Values)
return n
// Index returns the ith step and value and supports negative indexing.
// A negative index starts counting from the tail of the Values such that -1
// refers to the last pair, -2 refers to the second-to-last pair, and so on.
func (p Values) Index(i int) (out struct {
Step Step
Value protoreflect.Value
}) {
// NOTE: This returns a single struct instead of two return values so that
// callers can make use of the the value in an expression:
// vs.Index(i).Value.Interface()
n := p.Len()
if i < 0 {
i = n + i
if i < 0 || i >= n {
return out
out.Step = p.Path[i]
out.Value = p.Values[i]
return out
// String returns a humanly readable representation of the path and last value.
// Do not depend on the output being stable.
// For example:
// ([5].map_field["hello"] = {hello: "world"}
func (p Values) String() string {
n := p.Len()
if n == 0 {
return ""
// Determine the field descriptor associated with the last step.
var fd protoreflect.FieldDescriptor
last := p.Index(-1)
switch last.Step.kind {
case FieldAccessStep:
fd = last.Step.FieldDescriptor()
case MapIndexStep, ListIndexStep:
fd = p.Index(-2).Step.FieldDescriptor()
// Format the full path with the last value.
return fmt.Sprintf("%v = %v", p.Path[:n], msgfmt.FormatValue(last.Value, fd))