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# Efficiency

By reading the Quick Tour and Conventions and working through the examples, users typically can create correct implementations of applications. What requires more experience is creating efficient implementations.

## Efficient Representations

• Fst Type: The specific FstType can affect both time and space in an application.
• Mutable: VectorFst is recommended for general mutations, while EditFst can be used for small changes to an existing FST.
• Immutable: ConstFst uses similar space to VectorFst but is much faster to read and write and possibly to access since it has less fragmentation. The CompactFst types permit smaller memory and file representations of acceptors, of unweighted acceptors and transducers and weighted and unweighted strings. These types admit optional memory-mapping (coming soon).
• Specialized: For some applications, specialized FST types can be created that are tailored to the structure of the automaton. One relatively simple way to do this is to extend the CompactFst types by defining a new compactor. A more general, but complex, way is to define an entirely new FST type. For example, NGramFst (coming soon) can be used to very compactly represent n-gram language models. This type admits memory-mapping.
• Arc Type: A particularly simple way to improve the representation size is to use an Arc that has StateId, Label, and Weight types appropriately sized to a problem.
• Interface Type: When creating state or arc iterators, the specific interface type can greatly affect efficiency. Using the most specific Fst type as the template argument of the iterators will access specializations that avoid virtual function and other overhead of more generic calls. E.g., prefer ArcIterator<VectorFst<Arc>>(fst, s) to ArcIterator<Fst<Arc>>(fst, s), ArcIterator<ExpandedFst<Arc>>(fst, s), or ArcIterator<MutableFst<Arc>>(fst, s) if performance is critical.

## Efficient Algorithms

#### General Issues

• Association Order: Although an operation may be associative, efficiency may greatly vary depending on the order of application. For example, prefer (A ∪ B) ∪ C to A ∪ (B ∪ C) if A and B are small and C is large. Similar considerations apply to concatenation and intersection/composition.
• Optimization: What optimizations should be applied is very problem-dependent. Epsilon removal, determinization and minimization of an unweighted automaton will give in the smallest, irredundant result. However this could be exponentially larger than the (non-deterministic) input, so applying some or none of these operations may be preferred. In the weighted and/or transducer case, the input may not even be determinizable although it can be determinized and minimized as an appropriately encoded automaton.
• Eager vs Delayed Computation: If only a small portion of the result is visited (e.g., of a composition), then prefer delayed computation (e.g., invoke ComposeFst). On the other hand, if much of the result is visited, then prefer the eager version (e.g., invoke Compose) due to lower overhead.
• Caching: Various operations such as the delayed FST operations and the CompactFst classes may use caching to avoid recomputation on repeated access. The size of the cache and whether (LRU) garbage collection is performed is controlled by options. In some cases, caching can also be disabled per state by arc iterator flags.
• Properties: An FST maintains stored property bits such as whether it is an acceptor, acyclic, or input-label sorted. Many algorithms check for various of these properties, which will be computed in linear time if they are not already stored. This linear pass can thus be avoided if the needed bits are already set (e.g., by the user).

#### Algorithm-Specific Issues

• Composition: The choice of the matcher, filter, and state table, all which may be changed by options, can have significant affect on the efficiency of composition:
• Matchers:
• SortedMatcher: By default, the composition algorithm matches labels using binary search. This requires one or both FSTs to be appropriately label-sorted and this, in turn, requires property checking. If only one FST is sorted or, in fact, only one FST has the sorted property bit set, then that FST will be used for the binary search. In this case, it is important that this FST has the higher out-degree for efficient matching.
• SigmaMatcher,RhoMatcher: These matchers can be used to reduce time and space for matching {\em all} or {\em otherwise unmatched} of the labels at a state.
• ArcLookAheadMatcher,LabelLookAheadMatcher: These matchers can be used to reduce time by checking for a match with the following label or the following non-epsilon label on a path.
• Filters:
• SequenceComposeFilter,AltSequenceComposeFilter,MatchComposeFilter: These filters control how epsilons are matched and can give very different sized (if equivalent) composition results.
• State Tables: By default, a hash mapping is used from state pair to composition state ID. Other data structures can give better performance in specialized cases. Several alternative state tables are provided by the library.
• Epsilon Removal: By default, the epsilon removal algorithm reassigns/copies non-epsilon transitions that follow epsilon paths. An alternate choice is to reassign/copy non-epsilon transitions that precede epsilon paths, which can be requested with the reverse=true option. This latter choice is equivalent to performing the former on the reverse of the FST. The two methods can give very different sized results depending on the input.
• Shortest Path/Distance: The shortest path/distance algorithms use a queue discipline to determine the order of expansion of states:
• LifoQueue: when the input is unweighted
• StateOrderQueue: when the input is topologically-ordered
• TopOrderQueue: when the input is acyclic, but not topologically-ordered
• ShortestFirstQueue: when the input is cyclic, but has no negative weights
• FifoQueue: otherwise
• Thus, the queue is selected based on the FST properties. This choice, with possible property testing, can be overridden by passing the queue as an option. With the ShortestFirstQueue, the shortest path algorithm can be passed the first_path=true option to stop when the first solution is discovered (correct if the weights are between 0 and 1). This is a case where delayed computation of the FST being searched may be preferred.

-- MichaelRiley - 12 May 2012

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Topic revision: r4 - 2012-05-13 - MichaelRiley

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