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Finite state transducers
<p>
This package implements <a href="http://en.wikipedia.org/wiki/Finite_state_transducer">
Finite State Transducers</a> with the following characteristics:
<ul>
   <li>Fast and low memory overhead construction of the minimal FST 
       (but inputs must be provided in sorted order)</li>
   <li>Low object overhead and quick deserialization (byte[] representation)</li>
   <li>Optional two-pass compression: {@link org.apache.lucene.util.fst.FST#pack FST.pack()}</li>
   <li>{@link org.apache.lucene.util.fst.Util#getByOutput Lookup-by-output} when the 
       outputs are in sorted order (e.g., ordinals or file pointers)</li>
   <li>Pluggable {@link org.apache.lucene.util.fst.Outputs Outputs} representation</li>
   <li>{@link org.apache.lucene.util.fst.Util#shortestPaths N-shortest-paths} search by
       weight</li>
   <li>Enumerators ({@link org.apache.lucene.util.fst.IntsRefFSTEnum IntsRef} and {@link org.apache.lucene.util.fst.BytesRefFSTEnum BytesRef}) that behave like {@link java.util.SortedMap SortedMap} iterators
</ul>
<p>
FST Construction example:
<pre class="prettyprint">
    // Input values (keys). These must be provided to Builder in Unicode sorted order!
    String inputValues[] = {"cat", "dog", "dogs"};
    long outputValues[] = {5, 7, 12};
    
    PositiveIntOutputs outputs = PositiveIntOutputs.getSingleton();
    Builder&lt;Long&gt; builder = new Builder&lt;Long&gt;(INPUT_TYPE.BYTE1, outputs);
    BytesRef scratchBytes = new BytesRef();
    IntsRef scratchInts = new IntsRef();
    for (int i = 0; i &lt; inputValues.length; i++) {
      scratchBytes.copyChars(inputValues[i]);
      builder.add(Util.toIntsRef(scratchBytes, scratchInts), outputValues[i]);
    }
    FST&lt;Long&gt; fst = builder.finish();
</pre>
Retrieval by key:
<pre class="prettyprint">
    Long value = Util.get(fst, new BytesRef("dog"));
    System.out.println(value); // 7
</pre>
Retrieval by value:
<pre class="prettyprint">
    // Only works because outputs are also in sorted order
    IntsRef key = Util.getByOutput(fst, 12);
    System.out.println(Util.toBytesRef(key, scratchBytes).utf8ToString()); // dogs
</pre>
Iterate over key-value pairs in sorted order:
<pre class="prettyprint">
    // Like TermsEnum, this also supports seeking (advance)
    BytesRefFSTEnum&lt;Long&gt; iterator = new BytesRefFSTEnum&lt;Long&gt;(fst);
    while (iterator.next() != null) {
      InputOutput&lt;Long&gt; mapEntry = iterator.current();
      System.out.println(mapEntry.input.utf8ToString());
      System.out.println(mapEntry.output);
    }
</pre>
N-shortest paths by weight:
<pre class="prettyprint">
    Comparator&lt;Long&gt; comparator = new Comparator&lt;Long&gt;() {
      public int compare(Long left, Long right) {
        return left.compareTo(right);
      }
    };
    Arc&lt;Long&gt; firstArc = fst.getFirstArc(new Arc&lt;Long&gt;());
    MinResult&lt;Long&gt; paths[] = Util.shortestPaths(fst, firstArc, comparator, 2);
    System.out.println(Util.toBytesRef(paths[0].input, scratchBytes).utf8ToString()); // cat
    System.out.println(paths[0].output); // 5
    System.out.println(Util.toBytesRef(paths[1].input, scratchBytes).utf8ToString()); // dog
    System.out.println(paths[1].output); // 7
</pre>
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