How to use org.apache.lucene.util.fst

 private void append(Builder<BytesRef> builder, FST<BytesRef> subIndex, IntsRefBuilder scratchIntsRef) throws IOException {
  final BytesRefFSTEnum<BytesRef> subIndexEnum = new BytesRefFSTEnum<>(subIndex);
  BytesRefFSTEnum.InputOutput<BytesRef> indexEnt;
  while((indexEnt = subIndexEnum.next()) != null) {
   //if (DEBUG) {
   //  System.out.println("      add sub=" + indexEnt.input + " " + indexEnt.input + " output=" + indexEnt.output);
   //}
   builder.add(Util.toIntsRef(indexEnt.input, scratchIntsRef), indexEnt.output);
  }
 }
}

@Override
public Pair<A,B> subtract(Pair<A,B> output, Pair<A,B> inc) {
 assert valid(output);
 assert valid(inc);
 return newPair(outputs1.subtract(output.output1, inc.output1),
         outputs2.subtract(output.output2, inc.output2));
}

/** doFloor controls the behavior of advance: if it's true
 *  doFloor is true, advance positions to the biggest
 *  term before target.  */
protected FSTEnum(FST<T> fst) {
 this.fst = fst;
 fstReader = fst.getBytesReader();
 NO_OUTPUT = fst.outputs.getNoOutput();
 fst.getFirstArc(getArc(0));
 output[0] = NO_OUTPUT;
}

private void pushLast() throws IOException {
 FST.Arc<T> arc = arcs[upto];
 assert arc != null;
 while (true) {
  setCurrentLabel(arc.label);
  output[upto] = fst.outputs.add(output[upto-1], arc.output);
  if (arc.label == FST.END_LABEL) {
   // Final node
   break;
  }
  incr();
  arc = fst.readLastTargetArc(arc, getArc(upto), fstReader);
 }
}

@Override
public Pair<A,B> add(Pair<A,B> prefix, Pair<A,B> output) {
 assert valid(prefix);
 assert valid(output);
 return newPair(outputs1.add(prefix.output1, output.output1),
         outputs2.add(prefix.output2, output.output2));
}

public PairOutputs(Outputs<A> outputs1, Outputs<B> outputs2) {
 this.outputs1 = outputs1;
 this.outputs2 = outputs2;
 NO_OUTPUT = new Pair<>(outputs1.getNoOutput(), outputs2.getNoOutput());
}

@Override
public Pair<A,B> common(Pair<A,B> pair1, Pair<A,B> pair2) {
 assert valid(pair1);
 assert valid(pair2);
 return newPair(outputs1.common(pair1.output1, pair2.output1),
         outputs2.common(pair1.output2, pair2.output2));
}

/**
 * Checks if <code>arc</code>'s target state is in expanded (or vector) format. 
 * 
 * @return Returns <code>true</code> if <code>arc</code> points to a state in an
 * expanded array format.
 */
boolean isExpandedTarget(Arc<T> follow, BytesReader in) throws IOException {
 if (!targetHasArcs(follow)) {
  return false;
 } else {
  in.setPosition(follow.target);
  return in.readByte() == ARCS_AS_FIXED_ARRAY;
 }
}

@Override
public Pair<A,B> read(DataInput in) throws IOException {
 A output1 = outputs1.read(in);
 B output2 = outputs2.read(in);
 return newPair(output1, output2);
}

@Override
public void write(Pair<A,B> output, DataOutput writer) throws IOException {
 assert valid(output);
 outputs1.write(output.output1, writer);
 outputs2.write(output.output2, writer);
}

@Override
public String outputToString(Pair<A,B> output) {
 assert valid(output);
 return "<pair:" + outputs1.outputToString(output.output1) + "," + outputs2.outputToString(output.output2) + ">";
}

/** Seeks to biggest term that's <= target. */
public InputOutput<T> seekFloor(BytesRef target) throws IOException {
 this.target = target;
 targetLength = target.length;
 super.doSeekFloor();
 return setResult();
}

/** Seeks to biggest term that's <= target. */
public InputOutput<T> seekFloor(IntsRef target) throws IOException {
 this.target = target;
 targetLength = target.length;
 super.doSeekFloor();
 return setResult();
}

/** Seeks to smallest term that's >= target. */
public InputOutput<T> seekCeil(IntsRef target) throws IOException {
 this.target = target;
 targetLength = target.length;
 super.doSeekCeil();
 return setResult();
}

/** Seeks to smallest term that's >= target. */
public InputOutput<T> seekCeil(BytesRef target) throws IOException {
 this.target = target;
 targetLength = target.length;
 super.doSeekCeil();
 return setResult();
}

/** Returns a {@link BytesReader} for this FST, positioned at
 *  position 0. */
public BytesReader getBytesReader() {
 if (bytesArray != null) {
  return new ReverseBytesReader(bytesArray);
 } else {
  return bytes.getReverseReader();
 }
}

FST(INPUT_TYPE inputType, Outputs<T> outputs, int bytesPageBits) {
 this.inputType = inputType;
 this.outputs = outputs;
 version = VERSION_CURRENT;
 bytesArray = null;
 bytes = new BytesStore(bytesPageBits);
 // pad: ensure no node gets address 0 which is reserved to mean
 // the stop state w/ no arcs
 bytes.writeByte((byte) 0);
 emptyOutput = null;
}

 @Override
 public long ramBytesUsed(Pair<A,B> output) {
  long ramBytesUsed = BASE_NUM_BYTES;
  if (output.output1 != null) {
   ramBytesUsed += outputs1.ramBytesUsed(output.output1);
  }
  if (output.output2 != null) {
   ramBytesUsed += outputs2.ramBytesUsed(output.output2);
  }
  return ramBytesUsed;
 }
}

/**
 * Creates an unbounded TopNSearcher
 * @param fst the {@link org.apache.lucene.util.fst.FST} to search on
 * @param topN the number of top scoring entries to retrieve
 * @param maxQueueDepth the maximum size of the queue of possible top entries
 * @param comparator the comparator to select the top N
 */
public TopNSearcher(FST<T> fst, int topN, int maxQueueDepth, Comparator<T> comparator) {
 this(fst, topN, maxQueueDepth, comparator, new TieBreakByInputComparator<>(comparator));
}

@Override
public Long add(Long prefix, Long output) {
 assert valid(prefix);
 assert valid(output);
 if (prefix == NO_OUTPUT) {
  return output;
 } else if (output == NO_OUTPUT) {
  return prefix;
 } else {
  return prefix + output;
 }
}