@Override
            public void mergeFrom(Input input, Map<Object, Object> message)
                    throws IOException
            {
                PMapWrapper entry = null;
                for (int number = input.readFieldNumber(this);; number = input
                        .readFieldNumber(this))
                {
                    switch (number)
                    {
                        case 0:
                            return;
                        case 1:
                            if (entry == null)
                            {
                                // lazy initialize
                                entry = new PMapWrapper(message);
                            }

                            if (entry != input.mergeObject(entry, ENTRY_SCHEMA))
                            {
                                // an entry will always be unique
                                // it can never be a reference.
                                throw new IllegalStateException(
                                        "A Map.Entry will always be "
                                                + "unique, hence it cannot be a reference "
                                                + "obtained from "
                                                + input.getClass().getName());
                            }
                            break;
                        default:
                            throw new ProtostuffException(
                                    "The map was incorrectly serialized.");
                    }
                }
            }
This base class handles all the IO for reading and writing polymorphic fields. When a field's type is
polymorphic/dynamic (e.g interface/abstract/object), the type (id) needs to be written (ahead) before its
value/content to be able to deserialize it correctly.
<p>
The underlying impl will determine how the type (id) should be written.
<p>
An {@link IdStrategy} is standalone if the {@link #primaryGroup} is not set.

@author Leo Romanoff
@author David Yu
protostuff-runtime/src/main/java/io/protostuff/runtime/IdStrategy.java