OpenFst Conventions

The OpenFst Libary has various conventions and assumptions about its objects and coding style.

Object Conventions

1. The StateIds of an FST are dense integers, numbered from 0 to NumStates() - 1.
2. A StateIterator returns StateIds in numerical order.
3. A user may not request info about a StateId s from an FST unless the FST has already returned a StateId t ≥ s (e.g. from Start(), NumStates(), StateIterator, or ArcIterator->Value().nextstate).
4. The empty machine (no states) has start state kNoStateId; a non-empty machine (has states) has a valid start state.
5. A Label is a non-negative integer except kNoLabel (or library internals). The label 0 is reserved for epsilon.
6. A Weight satisfies the properties described here.
7. A StateIterator is invalidated if the number of states is modified.
8. An ArcIterator for a state is invalidated by any mutation of the arcs at that state.
9. A MutableArcIterator is invalidated by any mutation of the arcs at that state other than by the iterator itself.
10. A Matcher is invalidated by any mutation of an FST.
11. A reference/pointer to an arc is invalidated at the next operation on an FST, state iterator, arc iterator, or matcher.
12. State iterators, arc iterators and matchers should be destroyed prior to destroying their component FSTs.
13. All FST classes F implement a copy constructor F(const &F).
14. The copy constructor and Copy() method of an FST have constant time and space complexity (shallow copy) unless otherwise noted.
15. One or more stored FST properties may be set to unknown once an FST is mutated depending on the type of mutation.

Coding Conventions

FST Application Code:

1. Your code should rarely use int or float when referring to FST components. Use:
   • StateId for state Ids and number of states.
   • Label for labels.
   • size_t for other array lengths.
   • Weight for weights.

New FST classes:

1. All FST classes will implement working versions of all pure virtual member functions; writing dummy or error-raising versions is not permitted.
2. If class C is templated on an Arc, then C::Arc will give that type.