/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.lucene.util.fst;


import java.io.IOException;

import org.apache.lucene.util.ArrayUtil;
import org.apache.lucene.util.IntsRef;
import org.apache.lucene.util.IntsRefBuilder;
import org.apache.lucene.util.fst.FST.INPUT_TYPE; // javadoc

// TODO: could we somehow stream an FST to disk while we
// build it?

Builds a minimal FST (maps an IntsRef term to an arbitrary
output) from pre-sorted terms with outputs.  The FST
becomes an FSA if you use NoOutputs.  The FST is written
on-the-fly into a compact serialized format byte array, which can
be saved to / loaded from a Directory or used directly
for traversal.  The FST is always finite (no cycles).
NOTE: The algorithm is described at
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.24.3698
The parameterized type T is the output type. See the subclasses of Outputs. 
FSTs larger than 2.1GB are now possible (as of Lucene
4.2).  FSTs containing more than 2.1B nodes are also now
possible, however they cannot be packed.
@lucene.experimental
/**
 * Builds a minimal FST (maps an IntsRef term to an arbitrary
 * output) from pre-sorted terms with outputs.  The FST
 * becomes an FSA if you use NoOutputs.  The FST is written
 * on-the-fly into a compact serialized format byte array, which can
 * be saved to / loaded from a Directory or used directly
 * for traversal.  The FST is always finite (no cycles).
 *
 * <p>NOTE: The algorithm is described at
 * http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.24.3698</p>
 *
 * <p>The parameterized type T is the output type.  See the
 * subclasses of {@link Outputs}.
 *
 * <p>FSTs larger than 2.1GB are now possible (as of Lucene
 * 4.2).  FSTs containing more than 2.1B nodes are also now
 * possible, however they cannot be packed.
 *
 * @lucene.experimental
 */

public class Builder<T> {

  // The amount of Arc array oversizing used to enable direct addressing of Arcs by their labels
  static final int DIRECT_ARC_LOAD_FACTOR = 4;

  private final NodeHash<T> dedupHash;
  final FST<T> fst;
  private final T NO_OUTPUT;

  // private static final boolean DEBUG = true;

  // simplistic pruning: we prune node (and all following
  // nodes) if less than this number of terms go through it:
  private final int minSuffixCount1;

  // better pruning: we prune node (and all following
  // nodes) if the prior node has less than this number of
  // terms go through it:
  private final int minSuffixCount2;

  private final boolean doShareNonSingletonNodes;
  private final int shareMaxTailLength;

  private final IntsRefBuilder lastInput = new IntsRefBuilder();

  // NOTE: cutting this over to ArrayList instead loses ~6%
  // in build performance on 9.8M Wikipedia terms; so we
  // left this as an array:
  // current "frontier"
  private UnCompiledNode<T>[] frontier;

  // Used for the BIT_TARGET_NEXT optimization (whereby
  // instead of storing the address of the target node for
  // a given arc, we mark a single bit noting that the next
  // node in the byte[] is the target node):
  long lastFrozenNode;

  // Reused temporarily while building the FST:
  int[] reusedBytesPerArc = new int[4];

  long arcCount;
  long nodeCount;

  boolean allowArrayArcs;

  BytesStore bytes;

  
Instantiates an FST/FSA builder without any pruning. A shortcut to Builder(INPUT_TYPE, int, int, boolean, boolean, int, Outputs, boolean, int) with pruning options turned off. /**
   * Instantiates an FST/FSA builder without any pruning. A shortcut to {@link
   * #Builder(FST.INPUT_TYPE, int, int, boolean, boolean, int, Outputs, boolean, int)} with
   * pruning options turned off.
   */
  public Builder(FST.INPUT_TYPE inputType, Outputs<T> outputs) {
    this(inputType, 0, 0, true, true, Integer.MAX_VALUE, outputs, true, 15);
  }

  
Instantiates an FST/FSA builder with all the possible tuning and construction
tweaks. Read parameter documentation carefully.
Params:
inputType –  The input type (transition labels). Can be anything from INPUT_TYPE enumeration. Shorter types will consume less memory. Strings (character sequences) are represented as INPUT_TYPE.BYTE4 (full unicode codepoints).
minSuffixCount1 – 
   If pruning the input graph during construction, this threshold is used for telling
   if a node is kept or pruned. If transition_count(node) >= minSuffixCount1, the node
   is kept.
minSuffixCount2 – 
   (Note: only Mike McCandless knows what this one is really doing...)
doShareSuffix – 
   If true, the shared suffixes will be compacted into unique paths.
   This requires an additional RAM-intensive hash map for lookups in memory. Setting this parameter to
   false creates a single suffix path for all input sequences. This will result in a larger
   FST, but requires substantially less memory and CPU during building.
doShareNonSingletonNodes – 
   Only used if doShareSuffix is true.  Set this to
   true to ensure FST is fully minimal, at cost of more
   CPU and more RAM during building.
shareMaxTailLength – 
   Only used if doShareSuffix is true.  Set this to
   Integer.MAX_VALUE to ensure FST is fully minimal, at cost of more
   CPU and more RAM during building.
outputs – The output type for each input sequence. Applies only if building an FST. For FSA, use NoOutputs.getSingleton() and NoOutputs.getNoOutput() as the singleton output object.
allowArrayArcs – Pass false to disable the array arc optimization
   while building the FST; this will make the resulting
   FST smaller but slower to traverse.
bytesPageBits – How many bits wide to make each
   byte[] block in the BytesStore; if you know the FST
   will be large then make this larger.  For example 15
   bits = 32768 byte pages./**
   * Instantiates an FST/FSA builder with all the possible tuning and construction
   * tweaks. Read parameter documentation carefully.
   * 
   * @param inputType 
   *    The input type (transition labels). Can be anything from {@link INPUT_TYPE}
   *    enumeration. Shorter types will consume less memory. Strings (character sequences) are 
   *    represented as {@link INPUT_TYPE#BYTE4} (full unicode codepoints). 
   *     
   * @param minSuffixCount1
   *    If pruning the input graph during construction, this threshold is used for telling
   *    if a node is kept or pruned. If transition_count(node) &gt;= minSuffixCount1, the node
   *    is kept. 
   *    
   * @param minSuffixCount2
   *    (Note: only Mike McCandless knows what this one is really doing...) 
   * 
   * @param doShareSuffix 
   *    If <code>true</code>, the shared suffixes will be compacted into unique paths.
   *    This requires an additional RAM-intensive hash map for lookups in memory. Setting this parameter to
   *    <code>false</code> creates a single suffix path for all input sequences. This will result in a larger
   *    FST, but requires substantially less memory and CPU during building.  
   *
   * @param doShareNonSingletonNodes
   *    Only used if doShareSuffix is true.  Set this to
   *    true to ensure FST is fully minimal, at cost of more
   *    CPU and more RAM during building.
   *
   * @param shareMaxTailLength
   *    Only used if doShareSuffix is true.  Set this to
   *    Integer.MAX_VALUE to ensure FST is fully minimal, at cost of more
   *    CPU and more RAM during building.
   *
   * @param outputs The output type for each input sequence. Applies only if building an FST. For
   *    FSA, use {@link NoOutputs#getSingleton()} and {@link NoOutputs#getNoOutput()} as the
   *    singleton output object.
   *
   * @param allowArrayArcs Pass false to disable the array arc optimization
   *    while building the FST; this will make the resulting
   *    FST smaller but slower to traverse.
   *
   * @param bytesPageBits How many bits wide to make each
   *    byte[] block in the BytesStore; if you know the FST
   *    will be large then make this larger.  For example 15
   *    bits = 32768 byte pages.
   */
  public Builder(FST.INPUT_TYPE inputType, int minSuffixCount1, int minSuffixCount2, boolean doShareSuffix,
                 boolean doShareNonSingletonNodes, int shareMaxTailLength, Outputs<T> outputs,
                 boolean allowArrayArcs, int bytesPageBits) {
    this.minSuffixCount1 = minSuffixCount1;
    this.minSuffixCount2 = minSuffixCount2;
    this.doShareNonSingletonNodes = doShareNonSingletonNodes;
    this.shareMaxTailLength = shareMaxTailLength;
    this.allowArrayArcs = allowArrayArcs;
    fst = new FST<>(inputType, outputs, bytesPageBits);
    bytes = fst.bytes;
    assert bytes != null;
    if (doShareSuffix) {
      dedupHash = new NodeHash<>(fst, bytes.getReverseReader(false));
    } else {
      dedupHash = null;
    }
    NO_OUTPUT = outputs.getNoOutput();

    @SuppressWarnings({"rawtypes","unchecked"}) final UnCompiledNode<T>[] f =
        (UnCompiledNode<T>[]) new UnCompiledNode[10];
    frontier = f;
    for(int idx=0;idx<frontier.length;idx++) {
      frontier[idx] = new UnCompiledNode<>(this, idx);
    }
  }

  public long getTermCount() {
    return frontier[0].inputCount;
  }

  public long getNodeCount() {
    // 1+ in order to count the -1 implicit final node
    return 1+nodeCount;
  }
  
  public long getArcCount() {
    return arcCount;
  }

  public long getMappedStateCount() {
    return dedupHash == null ? 0 : nodeCount;
  }

  private CompiledNode compileNode(UnCompiledNode<T> nodeIn, int tailLength) throws IOException {
    final long node;
    long bytesPosStart = bytes.getPosition();
    if (dedupHash != null && (doShareNonSingletonNodes || nodeIn.numArcs <= 1) && tailLength <= shareMaxTailLength) {
      if (nodeIn.numArcs == 0) {
        node = fst.addNode(this, nodeIn);
        lastFrozenNode = node;
      } else {
        node = dedupHash.add(this, nodeIn);
      }
    } else {
      node = fst.addNode(this, nodeIn);
    }
    assert node != -2;

    long bytesPosEnd = bytes.getPosition();
    if (bytesPosEnd != bytesPosStart) {
      // The FST added a new node:
      assert bytesPosEnd > bytesPosStart;
      lastFrozenNode = node;
    }

    nodeIn.clear();

    final CompiledNode fn = new CompiledNode();
    fn.node = node;
    return fn;
  }

  private void freezeTail(int prefixLenPlus1) throws IOException {
    //System.out.println("  compileTail " + prefixLenPlus1);
    final int downTo = Math.max(1, prefixLenPlus1);
    for(int idx=lastInput.length(); idx >= downTo; idx--) {

      boolean doPrune = false;
      boolean doCompile = false;

      final UnCompiledNode<T> node = frontier[idx];
      final UnCompiledNode<T> parent = frontier[idx-1];

      if (node.inputCount < minSuffixCount1) {
        doPrune = true;
        doCompile = true;
      } else if (idx > prefixLenPlus1) {
        // prune if parent's inputCount is less than suffixMinCount2
        if (parent.inputCount < minSuffixCount2 || (minSuffixCount2 == 1 && parent.inputCount == 1 && idx > 1)) {
          // my parent, about to be compiled, doesn't make the cut, so
          // I'm definitely pruned 

          // if minSuffixCount2 is 1, we keep only up
          // until the 'distinguished edge', ie we keep only the
          // 'divergent' part of the FST. if my parent, about to be
          // compiled, has inputCount 1 then we are already past the
          // distinguished edge.  NOTE: this only works if
          // the FST outputs are not "compressible" (simple
          // ords ARE compressible).
          doPrune = true;
        } else {
          // my parent, about to be compiled, does make the cut, so
          // I'm definitely not pruned 
          doPrune = false;
        }
        doCompile = true;
      } else {
        // if pruning is disabled (count is 0) we can always
        // compile current node
        doCompile = minSuffixCount2 == 0;
      }

      //System.out.println("    label=" + ((char) lastInput.ints[lastInput.offset+idx-1]) + " idx=" + idx + " inputCount=" + frontier[idx].inputCount + " doCompile=" + doCompile + " doPrune=" + doPrune);

      if (node.inputCount < minSuffixCount2 || (minSuffixCount2 == 1 && node.inputCount == 1 && idx > 1)) {
        // drop all arcs
        for(int arcIdx=0;arcIdx<node.numArcs;arcIdx++) {
          @SuppressWarnings({"rawtypes","unchecked"}) final UnCompiledNode<T> target =
          (UnCompiledNode<T>) node.arcs[arcIdx].target;
          target.clear();
        }
        node.numArcs = 0;
      }

      if (doPrune) {
        // this node doesn't make it -- deref it
        node.clear();
        parent.deleteLast(lastInput.intAt(idx-1), node);
      } else {

        if (minSuffixCount2 != 0) {
          compileAllTargets(node, lastInput.length()-idx);
        }
        final T nextFinalOutput = node.output;

        // We "fake" the node as being final if it has no
        // outgoing arcs; in theory we could leave it
        // as non-final (the FST can represent this), but
        // FSTEnum, Util, etc., have trouble w/ non-final
        // dead-end states:
        final boolean isFinal = node.isFinal || node.numArcs == 0;

        if (doCompile) {
          // this node makes it and we now compile it.  first,
          // compile any targets that were previously
          // undecided:
          parent.replaceLast(lastInput.intAt(idx-1),
                             compileNode(node, 1+lastInput.length()-idx),
                             nextFinalOutput,
                             isFinal);
        } else {
          // replaceLast just to install
          // nextFinalOutput/isFinal onto the arc
          parent.replaceLast(lastInput.intAt(idx-1),
                             node,
                             nextFinalOutput,
                             isFinal);
          // this node will stay in play for now, since we are
          // undecided on whether to prune it.  later, it
          // will be either compiled or pruned, so we must
          // allocate a new node:
          frontier[idx] = new UnCompiledNode<>(this, idx);
        }
      }
    }
  }

  // for debugging
  /*
  private String toString(BytesRef b) {
    try {
      return b.utf8ToString() + " " + b;
    } catch (Throwable t) {
      return b.toString();
    }
  }
  */

  
Add the next input/output pair. The provided input must be sorted after the previous one according to IntsRef.compareTo. It's also OK to add the same input twice in a row with different outputs, as long as Outputs implements the Outputs.merge method. Note that input is fully consumed after this method is returned (so caller is free to reuse), but output is not. So if your outputs are changeable (eg ByteSequenceOutputs or IntSequenceOutputs) then you cannot reuse across calls. /** Add the next input/output pair.  The provided input
   *  must be sorted after the previous one according to
   *  {@link IntsRef#compareTo}.  It's also OK to add the same
   *  input twice in a row with different outputs, as long
   *  as {@link Outputs} implements the {@link Outputs#merge}
   *  method. Note that input is fully consumed after this
   *  method is returned (so caller is free to reuse), but
   *  output is not.  So if your outputs are changeable (eg
   *  {@link ByteSequenceOutputs} or {@link
   *  IntSequenceOutputs}) then you cannot reuse across
   *  calls. */
  public void add(IntsRef input, T output) throws IOException {
    /*
    if (DEBUG) {
      BytesRef b = new BytesRef(input.length);
      for(int x=0;x<input.length;x++) {
        b.bytes[x] = (byte) input.ints[x];
      }
      b.length = input.length;
      if (output == NO_OUTPUT) {
        System.out.println("\nFST ADD: input=" + toString(b) + " " + b);
      } else {
        System.out.println("\nFST ADD: input=" + toString(b) + " " + b + " output=" + fst.outputs.outputToString(output));
      }
    }
    */

    // De-dup NO_OUTPUT since it must be a singleton:
    if (output.equals(NO_OUTPUT)) {
      output = NO_OUTPUT;
    }

    assert lastInput.length() == 0 || input.compareTo(lastInput.get()) >= 0: "inputs are added out of order lastInput=" + lastInput.get() + " vs input=" + input;
    assert validOutput(output);

    //System.out.println("\nadd: " + input);
    if (input.length == 0) {
      // empty input: only allowed as first input.  we have
      // to special case this because the packed FST
      // format cannot represent the empty input since
      // 'finalness' is stored on the incoming arc, not on
      // the node
      frontier[0].inputCount++;
      frontier[0].isFinal = true;
      fst.setEmptyOutput(output);
      return;
    }

    // compare shared prefix length
    int pos1 = 0;
    int pos2 = input.offset;
    final int pos1Stop = Math.min(lastInput.length(), input.length);
    while(true) {
      frontier[pos1].inputCount++;
      //System.out.println("  incr " + pos1 + " ct=" + frontier[pos1].inputCount + " n=" + frontier[pos1]);
      if (pos1 >= pos1Stop || lastInput.intAt(pos1) != input.ints[pos2]) {
        break;
      }
      pos1++;
      pos2++;
    }
    final int prefixLenPlus1 = pos1+1;
      
    if (frontier.length < input.length+1) {
      final UnCompiledNode<T>[] next = ArrayUtil.grow(frontier, input.length+1);
      for(int idx=frontier.length;idx<next.length;idx++) {
        next[idx] = new UnCompiledNode<>(this, idx);
      }
      frontier = next;
    }

    // minimize/compile states from previous input's
    // orphan'd suffix
    freezeTail(prefixLenPlus1);

    // init tail states for current input
    for(int idx=prefixLenPlus1;idx<=input.length;idx++) {
      frontier[idx-1].addArc(input.ints[input.offset + idx - 1],
                             frontier[idx]);
      frontier[idx].inputCount++;
    }

    final UnCompiledNode<T> lastNode = frontier[input.length];
    if (lastInput.length() != input.length || prefixLenPlus1 != input.length + 1) {
      lastNode.isFinal = true;
      lastNode.output = NO_OUTPUT;
    }

    // push conflicting outputs forward, only as far as
    // needed
    for(int idx=1;idx<prefixLenPlus1;idx++) {
      final UnCompiledNode<T> node = frontier[idx];
      final UnCompiledNode<T> parentNode = frontier[idx-1];

      final T lastOutput = parentNode.getLastOutput(input.ints[input.offset + idx - 1]);
      assert validOutput(lastOutput);

      final T commonOutputPrefix;
      final T wordSuffix;

      if (lastOutput != NO_OUTPUT) {
        commonOutputPrefix = fst.outputs.common(output, lastOutput);
        assert validOutput(commonOutputPrefix);
        wordSuffix = fst.outputs.subtract(lastOutput, commonOutputPrefix);
        assert validOutput(wordSuffix);
        parentNode.setLastOutput(input.ints[input.offset + idx - 1], commonOutputPrefix);
        node.prependOutput(wordSuffix);
      } else {
        commonOutputPrefix = wordSuffix = NO_OUTPUT;
      }

      output = fst.outputs.subtract(output, commonOutputPrefix);
      assert validOutput(output);
    }

    if (lastInput.length() == input.length && prefixLenPlus1 == 1+input.length) {
      // same input more than 1 time in a row, mapping to
      // multiple outputs
      lastNode.output = fst.outputs.merge(lastNode.output, output);
    } else {
      // this new arc is private to this new input; set its
      // arc output to the leftover output:
      frontier[prefixLenPlus1-1].setLastOutput(input.ints[input.offset + prefixLenPlus1-1], output);
    }

    // save last input
    lastInput.copyInts(input);

    //System.out.println("  count[0]=" + frontier[0].inputCount);
  }

  private boolean validOutput(T output) {
    return output == NO_OUTPUT || !output.equals(NO_OUTPUT);
  }

  
Returns final FST.  NOTE: this will return null if
 nothing is accepted by the FST. /** Returns final FST.  NOTE: this will return null if
   *  nothing is accepted by the FST. */
  public FST<T> finish() throws IOException {

    final UnCompiledNode<T> root = frontier[0];

    // minimize nodes in the last word's suffix
    freezeTail(0);
    if (root.inputCount < minSuffixCount1 || root.inputCount < minSuffixCount2 || root.numArcs == 0) {
      if (fst.emptyOutput == null) {
        return null;
      } else if (minSuffixCount1 > 0 || minSuffixCount2 > 0) {
        // empty string got pruned
        return null;
      }
    } else {
      if (minSuffixCount2 != 0) {
        compileAllTargets(root, lastInput.length());
      }
    }
    //if (DEBUG) System.out.println("  builder.finish root.isFinal=" + root.isFinal + " root.output=" + root.output);
    fst.finish(compileNode(root, lastInput.length()).node);

    return fst;
  }

  private void compileAllTargets(UnCompiledNode<T> node, int tailLength) throws IOException {
    for(int arcIdx=0;arcIdx<node.numArcs;arcIdx++) {
      final Arc<T> arc = node.arcs[arcIdx];
      if (!arc.target.isCompiled()) {
        // not yet compiled
        @SuppressWarnings({"rawtypes","unchecked"}) final UnCompiledNode<T> n = (UnCompiledNode<T>) arc.target;
        if (n.numArcs == 0) {
          //System.out.println("seg=" + segment + "        FORCE final arc=" + (char) arc.label);
          arc.isFinal = n.isFinal = true;
        }
        arc.target = compileNode(n, tailLength-1);
      }
    }
  }

  
Expert: holds a pending (seen but not yet serialized) arc. /** Expert: holds a pending (seen but not yet serialized) arc. */
  public static class Arc<T> {
    public int label;                             // really an "unsigned" byte
    public Node target;
    public boolean isFinal;
    public T output;
    public T nextFinalOutput;
  }

  // NOTE: not many instances of Node or CompiledNode are in
  // memory while the FST is being built; it's only the
  // current "frontier":

  static interface Node {
    boolean isCompiled();
  }

  public long fstRamBytesUsed() {
    return fst.ramBytesUsed();
  }

  static final class CompiledNode implements Node {
    long node;
    @Override
    public boolean isCompiled() {
      return true;
    }
  }

  
Expert: holds a pending (seen but not yet serialized) Node. /** Expert: holds a pending (seen but not yet serialized) Node. */
  public static final class UnCompiledNode<T> implements Node {
    final Builder<T> owner;
    public int numArcs;
    public Arc<T>[] arcs;
    // TODO: instead of recording isFinal/output on the
    // node, maybe we should use -1 arc to mean "end" (like
    // we do when reading the FST).  Would simplify much
    // code here...
    public T output;
    public boolean isFinal;
    public long inputCount;

    
This node's depth, starting from the automaton root. /** This node's depth, starting from the automaton root. */
    public final int depth;

    
Params:
depth – 
         The node's depth starting from the automaton root. Needed for
         LUCENE-2934 (node expansion based on conditions other than the
         fanout size)./**
     * @param depth
     *          The node's depth starting from the automaton root. Needed for
     *          LUCENE-2934 (node expansion based on conditions other than the
     *          fanout size).
     */
    @SuppressWarnings({"rawtypes","unchecked"})
    public UnCompiledNode(Builder<T> owner, int depth) {
      this.owner = owner;
      arcs = (Arc<T>[]) new Arc[1];
      arcs[0] = new Arc<>();
      output = owner.NO_OUTPUT;
      this.depth = depth;
    }

    @Override
    public boolean isCompiled() {
      return false;
    }

    public void clear() {
      numArcs = 0;
      isFinal = false;
      output = owner.NO_OUTPUT;
      inputCount = 0;

      // We don't clear the depth here because it never changes 
      // for nodes on the frontier (even when reused).
    }

    public T getLastOutput(int labelToMatch) {
      assert numArcs > 0;
      assert arcs[numArcs-1].label == labelToMatch;
      return arcs[numArcs-1].output;
    }

    public void addArc(int label, Node target) {
      assert label >= 0;
      assert numArcs == 0 || label > arcs[numArcs-1].label: "arc[numArcs-1].label=" + arcs[numArcs-1].label + " new label=" + label + " numArcs=" + numArcs;
      if (numArcs == arcs.length) {
        final Arc<T>[] newArcs = ArrayUtil.grow(arcs, numArcs+1);
        for(int arcIdx=numArcs;arcIdx<newArcs.length;arcIdx++) {
          newArcs[arcIdx] = new Arc<>();
        }
        arcs = newArcs;
      }
      final Arc<T> arc = arcs[numArcs++];
      arc.label = label;
      arc.target = target;
      arc.output = arc.nextFinalOutput = owner.NO_OUTPUT;
      arc.isFinal = false;
    }

    public void replaceLast(int labelToMatch, Node target, T nextFinalOutput, boolean isFinal) {
      assert numArcs > 0;
      final Arc<T> arc = arcs[numArcs-1];
      assert arc.label == labelToMatch: "arc.label=" + arc.label + " vs " + labelToMatch;
      arc.target = target;
      //assert target.node != -2;
      arc.nextFinalOutput = nextFinalOutput;
      arc.isFinal = isFinal;
    }

    public void deleteLast(int label, Node target) {
      assert numArcs > 0;
      assert label == arcs[numArcs-1].label;
      assert target == arcs[numArcs-1].target;
      numArcs--;
    }

    public void setLastOutput(int labelToMatch, T newOutput) {
      assert owner.validOutput(newOutput);
      assert numArcs > 0;
      final Arc<T> arc = arcs[numArcs-1];
      assert arc.label == labelToMatch;
      arc.output = newOutput;
    }

    // pushes an output prefix forward onto all arcs
    public void prependOutput(T outputPrefix) {
      assert owner.validOutput(outputPrefix);

      for(int arcIdx=0;arcIdx<numArcs;arcIdx++) {
        arcs[arcIdx].output = owner.fst.outputs.add(outputPrefix, arcs[arcIdx].output);
        assert owner.validOutput(arcs[arcIdx].output);
      }

      if (isFinal) {
        output = owner.fst.outputs.add(outputPrefix, output);
        assert owner.validOutput(output);
      }
    }
  }
}