source: code/trunk/vendor/github.com/golang/protobuf/proto/discard.go@ 822

// Copyright 2019 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 proto

import (
        "google.golang.org/protobuf/reflect/protoreflect"
)

// DiscardUnknown recursively discards all unknown fields from this message
// and all embedded messages.
//
// When unmarshaling a message with unrecognized fields, the tags and values
// of such fields are preserved in the Message. This allows a later call to
// marshal to be able to produce a message that continues to have those
// unrecognized fields. To avoid this, DiscardUnknown is used to
// explicitly clear the unknown fields after unmarshaling.
func DiscardUnknown(m Message) {
        if m != nil {
                discardUnknown(MessageReflect(m))
        }
}

func discardUnknown(m protoreflect.Message) {
        m.Range(func(fd protoreflect.FieldDescriptor, val protoreflect.Value) bool {
                switch {
                // Handle singular message.
                case fd.Cardinality() != protoreflect.Repeated:
                        if fd.Message() != nil {
                                discardUnknown(m.Get(fd).Message())
                        }
                // Handle list of messages.
                case fd.IsList():
                        if fd.Message() != nil {
                                ls := m.Get(fd).List()
                                for i := 0; i < ls.Len(); i++ {
                                        discardUnknown(ls.Get(i).Message())
                                }
                        }
                // Handle map of messages.
                case fd.IsMap():
                        if fd.MapValue().Message() != nil {
                                ms := m.Get(fd).Map()
                                ms.Range(func(_ protoreflect.MapKey, v protoreflect.Value) bool {
                                        discardUnknown(v.Message())
                                        return true
                                })
                        }
                }
                return true
        })

        // Discard unknown fields.
        if len(m.GetUnknown()) > 0 {
                m.SetUnknown(nil)
        }
}