[protobuf] Trying to rewrite a protobuf message but changing a couple of values

[Joan Balagueró]

Hello,

I have a protobuf message like this into a byte array:

        1: { // META element
            1: 2
            2: 1
            3: 1
            4: {            // CutOffTime element within META
                1: 10
                2: 3
               }
            5: 1
            6: 40
        }
        2: 9836 // HOTEL element
        3: 724 // MARKET element


We need to traverse this message and write it to a 'CodeOutputStream', but 
changing the values of the 'cutoff' element to, for example to '4: { 1: 7, 
2: 4 }'. I'm not able to do it, I need some help.

A basic algorithm that writes the same protobuf to another byte array but 
without changing anything works. Here I try the special case of 'META' (key 
= 1) that contains the 'cutoff' element.

Map<Integer, UnknownFieldSet.Field> rootFields = 
UnknownFieldSet.parseFrom(document).asMap();

            for (Map.Entry<Integer, UnknownFieldSet.Field> entry : 
rootFields.entrySet()) {
                if (entry.getKey() == 1) {
                    ByteString bs = 
entry.getValue().getLengthDelimitedList().get(0);
                    output.writeByteArray(1, bs.toByteArray());
                }
                else {
                    entry.getValue().writeTo(entry.getKey(), output);
                }
            }


Now I try to go a bit further, trying to read the cuotff element, change 
the values and rewrite them to the 'output'. And here is when I'm not able 
to solve the problem. Below my try that does not work, it generates a byte 
array
of 69 bytes instead of 73 (I'm losing 4 bytes):

Map<Integer, UnknownFieldSet.Field> rootFields = 
UnknownFieldSet.parseFrom(document).asMap();
            
for (Map.Entry<Integer, UnknownFieldSet.Field> entry : 
rootFields.entrySet()) {
                if (entry.getKey() == 1) {
                    ByteString bs = 
entry.getValue().getLengthDelimitedList().get(0);
                    Map<Integer, UnknownFieldSet.Field> ufs = 
UnknownFieldSet.parseFrom(bs).asMap();

                    for (Map.Entry<Integer, UnknownFieldSet.Field> item : 
ufs.entrySet()) {
                        if (item.getKey() == 4) {
                            Map<Integer, UnknownFieldSet.Field> cutoff = 
UnknownFieldSet.parseFrom(item.getValue().getLengthDelimitedList().get(0)).asMap();
                            cutoff.put(1, 
UnknownFieldSet.Field.newBuilder().addVarint(7).build()).writeTo(1, output);
                            cutoff.put(2, 
UnknownFieldSet.Field.newBuilder().addVarint(4).build()).writeTo(1, output);
                        }
                        else {
                            item.getValue().writeTo(item.getKey(), output);
                        }
                    }
                }
                else {
                    entry.getValue().writeTo(entry.getKey(), output);
                }
            }

Any help would be really appreciated.

Thanks,

Joan.

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