1 files changed, 241 insertions, 0 deletions
diff --git a/reflect/protopath/step.go b/reflect/protopath/step.go
new file mode 100644
index 00000000..95ae85c5
--- /dev/null
+++ b/reflect/protopath/step.go
@@ -0,0 +1,241 @@
+// 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
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"google.golang.org/protobuf/internal/encoding/text"
+	"google.golang.org/protobuf/reflect/protoreflect"
+)
+
+// StepKind identifies the kind of step operation.
+// Each kind of step corresponds with some protobuf reflection operation.
+type StepKind int
+
+const (
+	invalidStep StepKind = iota
+	// RootStep identifies a step as the Root step operation.
+	RootStep
+	// FieldAccessStep identifies a step as the FieldAccess step operation.
+	FieldAccessStep
+	// UnknownAccessStep identifies a step as the UnknownAccess step operation.
+	UnknownAccessStep
+	// ListIndexStep identifies a step as the ListIndex step operation.
+	ListIndexStep
+	// MapIndexStep identifies a step as the MapIndex step operation.
+	MapIndexStep
+	// AnyExpandStep identifies a step as the AnyExpand step operation.
+	AnyExpandStep
+)
+
+func (k StepKind) String() string {
+	switch k {
+	case invalidStep:
+		return "<invalid>"
+	case RootStep:
+		return "Root"
+	case FieldAccessStep:
+		return "FieldAccess"
+	case UnknownAccessStep:
+		return "UnknownAccess"
+	case ListIndexStep:
+		return "ListIndex"
+	case MapIndexStep:
+		return "MapIndex"
+	case AnyExpandStep:
+		return "AnyExpand"
+	default:
+		return fmt.Sprintf("<unknown:%d>", k)
+	}
+}
+
+// Step is a union where only one step operation may be specified at a time.
+// The different kinds of steps are specified by the constants defined for
+// the StepKind type.
+type Step struct {
+	kind StepKind
+	desc protoreflect.Descriptor
+	key  protoreflect.Value
+}
+
+// Root indicates the root message that a path is relative to.
+// It should always (and only ever) be the first step in a path.
+func Root(md protoreflect.MessageDescriptor) Step {
+	if md == nil {
+		panic("nil message descriptor")
+	}
+	return Step{kind: RootStep, desc: md}
+}
+
+// FieldAccess describes access of a field within a message.
+// Extension field accesses are also represented using a FieldAccess and
+// must be provided with a protoreflect.FieldDescriptor
+//
+// Within the context of Values,
+// the type of the previous step value is always a message, and
+// the type of the current step value is determined by the field descriptor.
+func FieldAccess(fd protoreflect.FieldDescriptor) Step {
+	if fd == nil {
+		panic("nil field descriptor")
+	} else if _, ok := fd.(protoreflect.ExtensionTypeDescriptor); !ok && fd.IsExtension() {
+		panic(fmt.Sprintf("extension field %q must implement protoreflect.ExtensionTypeDescriptor", fd.FullName()))
+	}
+	return Step{kind: FieldAccessStep, desc: fd}
+}
+
+// UnknownAccess describes access to the unknown fields within a message.
+//
+// Within the context of Values,
+// the type of the previous step value is always a message, and
+// the type of the current step value is always a bytes type.
+func UnknownAccess() Step {
+	return Step{kind: UnknownAccessStep}
+}
+
+// ListIndex describes index of an element within a list.
+//
+// Within the context of Values,
+// the type of the previous, previous step value is always a message,
+// the type of the previous step value is always a list, and
+// the type of the current step value is determined by the field descriptor.
+func ListIndex(i int) Step {
+	if i < 0 {
+		panic(fmt.Sprintf("invalid list index: %v", i))
+	}
+	return Step{kind: ListIndexStep, key: protoreflect.ValueOfInt64(int64(i))}
+}
+
+// MapIndex describes index of an entry within a map.
+// The key type is determined by field descriptor that the map belongs to.
+//
+// Within the context of Values,
+// the type of the previous previous step value is always a message,
+// the type of the previous step value is always a map, and
+// the type of the current step value is determined by the field descriptor.
+func MapIndex(k protoreflect.MapKey) Step {
+	if !k.IsValid() {
+		panic("invalid map index")
+	}
+	return Step{kind: MapIndexStep, key: k.Value()}
+}
+
+// AnyExpand describes expansion of a google.protobuf.Any message into
+// a structured representation of the underlying message.
+//
+// Within the context of Values,
+// the type of the previous step value is always a google.protobuf.Any message, and
+// the type of the current step value is always a message.
+func AnyExpand(md protoreflect.MessageDescriptor) Step {
+	if md == nil {
+		panic("nil message descriptor")
+	}
+	return Step{kind: AnyExpandStep, desc: md}
+}
+
+// MessageDescriptor returns the message descriptor for Root or AnyExpand steps,
+// otherwise it returns nil.
+func (s Step) MessageDescriptor() protoreflect.MessageDescriptor {
+	switch s.kind {
+	case RootStep, AnyExpandStep:
+		return s.desc.(protoreflect.MessageDescriptor)
+	default:
+		return nil
+	}
+}
+
+// FieldDescriptor returns the field descriptor for FieldAccess steps,
+// otherwise it returns nil.
+func (s Step) FieldDescriptor() protoreflect.FieldDescriptor {
+	switch s.kind {
+	case FieldAccessStep:
+		return s.desc.(protoreflect.FieldDescriptor)
+	default:
+		return nil
+	}
+}
+
+// ListIndex returns the list index for ListIndex steps,
+// otherwise it returns 0.
+func (s Step) ListIndex() int {
+	switch s.kind {
+	case ListIndexStep:
+		return int(s.key.Int())
+	default:
+		return 0
+	}
+}
+
+// MapIndex returns the map key for MapIndex steps,
+// otherwise it returns an invalid map key.
+func (s Step) MapIndex() protoreflect.MapKey {
+	switch s.kind {
+	case MapIndexStep:
+		return s.key.MapKey()
+	default:
+		return protoreflect.MapKey{}
+	}
+}
+
+// Kind reports which kind of step this is.
+func (s Step) Kind() StepKind {
+	return s.kind
+}
+
+func (s Step) String() string {
+	return string(s.appendString(nil))
+}
+
+func (s Step) appendString(b []byte) []byte {
+	switch s.kind {
+	case RootStep:
+		b = append(b, '(')
+		b = append(b, s.desc.FullName()...)
+		b = append(b, ')')
+	case FieldAccessStep:
+		b = append(b, '.')
+		if fd := s.desc.(protoreflect.FieldDescriptor); fd.IsExtension() {
+			b = append(b, '(')
+			b = append(b, strings.Trim(fd.TextName(), "[]")...)
+			b = append(b, ')')
+		} else {
+			b = append(b, fd.TextName()...)
+		}
+	case UnknownAccessStep:
+		b = append(b, '.')
+		b = append(b, '?')
+	case ListIndexStep:
+		b = append(b, '[')
+		b = strconv.AppendInt(b, s.key.Int(), 10)
+		b = append(b, ']')
+	case MapIndexStep:
+		b = append(b, '[')
+		switch k := s.key.Interface().(type) {
+		case bool:
+			b = strconv.AppendBool(b, bool(k)) // e.g., "true" or "false"
+		case int32:
+			b = strconv.AppendInt(b, int64(k), 10) // e.g., "-32"
+		case int64:
+			b = strconv.AppendInt(b, int64(k), 10) // e.g., "-64"
+		case uint32:
+			b = strconv.AppendUint(b, uint64(k), 10) // e.g., "32"
+		case uint64:
+			b = strconv.AppendUint(b, uint64(k), 10) // e.g., "64"
+		case string:
+			b = text.AppendString(b, k) // e.g., `"hello, world"`
+		}
+		b = append(b, ']')
+	case AnyExpandStep:
+		b = append(b, '.')
+		b = append(b, '(')
+		b = append(b, s.desc.FullName()...)
+		b = append(b, ')')
+	default:
+		b = append(b, "<invalid>"...)
+	}
+	return b
+}