protected int calculateIdV7(DocumentType owner) {
        String combined = getName() + dataType.getId();

        int newId = BobHash.hash(combined); // Using a portfriendly hash
        if (newId < 0) newId = -newId; // Highest bit is reserved to tell 7-bit id's from 31-bit ones
        validateId(newId, owner);
        return newId;
    }
The field id must be unique within a document type, and also
within a (unknown at this time) hierarchy of document types.
In addition it should be as resilient to doctype content changes
and inheritance hierarchy changes as possible.
All of this is enforced for names, so id's should follow names.
Therefore we hash on name.
document/src/main/java/com/yahoo/document/Field.java