@Nullable
private static <M extends Message> M filterFrom(
M msg, boolean clearEmpty, Predicate<FieldDescriptor> filter, int firstDiscarded) {
// At least one field was discarded; we have to work harder and maybe create
// a new message that will contain only the retained filters. Use a lazy-allocated
// builder to also optimize the scenario of all fields being discarded.
Message.Builder builder = (firstDiscarded == 0) ? null : msg.newBuilderForType();
Iterator<Map.Entry<FieldDescriptor, Object>> iter = msg.getAllFields().entrySet().iterator();
for (int i = 0; i < firstDiscarded; ++i) {
filterValue(clearEmpty, filter, builder, iter.next());
}
iter.next(); // Ignore object at firstDiscarded position
while (iter.hasNext()) {
Map.Entry<FieldDescriptor, Object> entry = iter.next();
if (filter.test(entry.getKey())) {
builder = (builder == null) ? msg.newBuilderForType() : builder;
filterValue(clearEmpty, filter, builder, entry);
}
}
if (builder == null) {
if (clearEmpty) {
return null;
} else {
@SuppressWarnings("unchecked")
M ret = (M) msg.getDefaultInstanceForType();
return ret;
}
} else {
@SuppressWarnings("unchecked")
M ret = (M) builder.build();
return ret;
}
}
