package org.apache.lucene.search.suggest.analyzing;
/*
* 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.
*/
// TODO
// - test w/ syns
// - add pruning of low-freq ngrams?
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
//import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Random;
import java.util.Set;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.AnalyzerWrapper;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.shingle.ShingleFilter;
import org.apache.lucene.analysis.tokenattributes.OffsetAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionLengthAttribute;
import org.apache.lucene.analysis.tokenattributes.TermToBytesRefAttribute;
import org.apache.lucene.codecs.CodecUtil;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.FieldType;
import org.apache.lucene.document.TextField;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.FieldInfo.IndexOptions;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.MultiFields;
import org.apache.lucene.index.Terms;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.search.suggest.InputIterator;
import org.apache.lucene.search.suggest.Lookup;
import org.apache.lucene.search.suggest.Sort;
import org.apache.lucene.store.ByteArrayDataInput;
import org.apache.lucene.store.DataInput;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.store.InputStreamDataInput;
import org.apache.lucene.store.OutputStreamDataOutput;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.CharsRef;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.IntsRef;
import org.apache.lucene.util.UnicodeUtil;
import org.apache.lucene.util.Version;
import org.apache.lucene.util.fst.Builder;
import org.apache.lucene.util.fst.FST.Arc;
import org.apache.lucene.util.fst.FST.BytesReader;
import org.apache.lucene.util.fst.FST;
import org.apache.lucene.util.fst.Outputs;
import org.apache.lucene.util.fst.PositiveIntOutputs;
import org.apache.lucene.util.fst.Util.MinResult;
import org.apache.lucene.util.fst.Util;
/**
* Builds an ngram model from the text sent to {@link
* #build} and predicts based on the last grams-1 tokens in
* the request sent to {@link #lookup}. This tries to
* handle the "long tail" of suggestions for when the
* incoming query is a never before seen query string.
*
* <p>Likely this suggester would only be used as a
* fallback, when the primary suggester fails to find
* any suggestions.
*
* <p>Note that the weight for each suggestion is unused,
* and the suggestions are the analyzed forms (so your
* analysis process should normally be very "light").
*
* <p>This uses the stupid backoff language model to smooth
* scores across ngram models; see
* "Large language models in machine translation",
* http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.76.1126
* for details.
*
* <p> From {@link #lookup}, the key of each result is the
* ngram token; the value is Long.MAX_VALUE * score (fixed
* point, cast to long). Divide by Long.MAX_VALUE to get
* the score back, which ranges from 0.0 to 1.0.
*
* onlyMorePopular is unused.
*
* @lucene.experimental
*/
public class FreeTextSuggester extends Lookup {
/** Codec name used in the header for the saved model. */
public final static String CODEC_NAME = "freetextsuggest";
/** Initial version of the the saved model file format. */
public final static int VERSION_START = 0;
/** Current version of the the saved model file format. */
public final static int VERSION_CURRENT = VERSION_START;
/** By default we use a bigram model. */
public static final int DEFAULT_GRAMS = 2;
// In general this could vary with gram, but the
// original paper seems to use this constant:
/** The constant used for backoff smoothing; during
* lookup, this means that if a given trigram did not
* occur, and we backoff to the bigram, the overall score
* will be 0.4 times what the bigram model would have
* assigned. */
public final static double ALPHA = 0.4;
/** Holds 1gram, 2gram, 3gram models as a single FST. */
private FST<Long> fst;
/**
* Analyzer that will be used for analyzing suggestions at
* index time.
*/
private final Analyzer indexAnalyzer;
private long totTokens;
/**
* Analyzer that will be used for analyzing suggestions at
* query time.
*/
private final Analyzer queryAnalyzer;
// 2 = bigram, 3 = trigram
private final int grams;
private final byte separator;
/** The default character used to join multiple tokens
* into a single ngram token. The input tokens produced
* by the analyzer must not contain this character. */
public static final byte DEFAULT_SEPARATOR = 0x1e;
/** Instantiate, using the provided analyzer for both
* indexing and lookup, using bigram model by default. */
public FreeTextSuggester(Analyzer analyzer) {
this(analyzer, analyzer, DEFAULT_GRAMS);
}
/** Instantiate, using the provided indexing and lookup
* analyzers, using bigram model by default. */
public FreeTextSuggester(Analyzer indexAnalyzer, Analyzer queryAnalyzer) {
this(indexAnalyzer, queryAnalyzer, DEFAULT_GRAMS);
}
/** Instantiate, using the provided indexing and lookup
* analyzers, with the specified model (2
* = bigram, 3 = trigram, etc.). */
public FreeTextSuggester(Analyzer indexAnalyzer, Analyzer queryAnalyzer, int grams) {
this(indexAnalyzer, queryAnalyzer, grams, DEFAULT_SEPARATOR);
}
/** Instantiate, using the provided indexing and lookup
* analyzers, and specified model (2 = bigram, 3 =
* trigram ,etc.). The separator is passed to {@link
* ShingleFilter#setTokenSeparator} to join multiple
* tokens into a single ngram token; it must be an ascii
* (7-bit-clean) byte. No input tokens should have this
* byte, otherwise {@code IllegalArgumentException} is
* thrown. */
public FreeTextSuggester(Analyzer indexAnalyzer, Analyzer queryAnalyzer, int grams, byte separator) {
this.grams = grams;
this.indexAnalyzer = addShingles(indexAnalyzer);
this.queryAnalyzer = addShingles(queryAnalyzer);
if (grams < 1) {
throw new IllegalArgumentException("grams must be >= 1");
}
if ((separator & 0x80) != 0) {
throw new IllegalArgumentException("separator must be simple ascii character");
}
this.separator = separator;
}
/** Returns byte size of the underlying FST. */
@Override
public long sizeInBytes() {
if (fst == null) {
return 0;
}
return fst.sizeInBytes();
}
private static class AnalyzingComparator implements Comparator<BytesRef> {
private final ByteArrayDataInput readerA = new ByteArrayDataInput();
private final ByteArrayDataInput readerB = new ByteArrayDataInput();
private final BytesRef scratchA = new BytesRef();
private final BytesRef scratchB = new BytesRef();
@Override
public int compare(BytesRef a, BytesRef b) {
readerA.reset(a.bytes, a.offset, a.length);
readerB.reset(b.bytes, b.offset, b.length);
// By token:
scratchA.length = readerA.readShort();
scratchA.bytes = a.bytes;
scratchA.offset = readerA.getPosition();
scratchB.bytes = b.bytes;
scratchB.length = readerB.readShort();
scratchB.offset = readerB.getPosition();
int cmp = scratchA.compareTo(scratchB);
if (cmp != 0) {
return cmp;
}
readerA.skipBytes(scratchA.length);
readerB.skipBytes(scratchB.length);
// By length (smaller surface forms sorted first):
cmp = a.length - b.length;
if (cmp != 0) {
return cmp;
}
// By surface form:
scratchA.offset = readerA.getPosition();
scratchA.length = a.length - scratchA.offset;
scratchB.offset = readerB.getPosition();
scratchB.length = b.length - scratchB.offset;
return scratchA.compareTo(scratchB);
}
}
private Analyzer addShingles(final Analyzer other) {
if (grams == 1) {
return other;
} else {
// TODO: use ShingleAnalyzerWrapper?
// Tack on ShingleFilter to the end, to generate token ngrams:
return new AnalyzerWrapper(other.getReuseStrategy()) {
@Override
protected Analyzer getWrappedAnalyzer(String fieldName) {
return other;
}
@Override
protected TokenStreamComponents wrapComponents(String fieldName, TokenStreamComponents components) {
ShingleFilter shingles = new ShingleFilter(components.getTokenStream(), 2, grams);
shingles.setTokenSeparator(Character.toString((char) separator));
return new TokenStreamComponents(components.getTokenizer(), shingles);
}
};
}
}
@Override
public void build(InputIterator iterator) throws IOException {
build(iterator, IndexWriterConfig.DEFAULT_RAM_BUFFER_SIZE_MB);
}
/** Build the suggest index, using up to the specified
* amount of temporary RAM while building. Note that
* the weights for the suggestions are ignored. */
public void build(InputIterator iterator, double ramBufferSizeMB) throws IOException {
if (iterator.hasPayloads()) {
throw new IllegalArgumentException("payloads are not supported");
}
String prefix = getClass().getSimpleName();
File directory = Sort.defaultTempDir();
// TODO: messy ... java7 has Files.createTempDirectory
// ... but 4.x is java6:
File tempIndexPath = null;
Random random = new Random();
while (true) {
tempIndexPath = new File(directory, prefix + ".index." + random.nextInt(Integer.MAX_VALUE));
if (tempIndexPath.mkdir()) {
break;
}
}
Directory dir = FSDirectory.open(tempIndexPath);
IndexWriterConfig iwc = new IndexWriterConfig(Version.LUCENE_46, indexAnalyzer);
iwc.setOpenMode(IndexWriterConfig.OpenMode.CREATE);
iwc.setRAMBufferSizeMB(ramBufferSizeMB);
IndexWriter writer = new IndexWriter(dir, iwc);
FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);
// TODO: if only we had IndexOptions.TERMS_ONLY...
ft.setIndexOptions(IndexOptions.DOCS_AND_FREQS);
ft.setOmitNorms(true);
ft.freeze();
Document doc = new Document();
Field field = new Field("body", "", ft);
doc.add(field);
totTokens = 0;
IndexReader reader = null;
boolean success = false;
try {
while (true) {
BytesRef surfaceForm = iterator.next();
if (surfaceForm == null) {
break;
}
field.setStringValue(surfaceForm.utf8ToString());
writer.addDocument(doc);
}
reader = DirectoryReader.open(writer, false);
Terms terms = MultiFields.getTerms(reader, "body");
if (terms == null) {
throw new IllegalArgumentException("need at least one suggestion");
}
// Move all ngrams into an FST:
TermsEnum termsEnum = terms.iterator(null);
Outputs<Long> outputs = PositiveIntOutputs.getSingleton();
Builder<Long> builder = new Builder<Long>(FST.INPUT_TYPE.BYTE1, outputs);
IntsRef scratchInts = new IntsRef();
while (true) {
BytesRef term = termsEnum.next();
if (term == null) {
break;
}
int ngramCount = countGrams(term);
if (ngramCount > grams) {
throw new IllegalArgumentException("tokens must not contain separator byte; got token=" + term + " but gramCount=" + ngramCount + ", which is greater than expected max ngram size=" + grams);
}
if (ngramCount == 1) {
totTokens += termsEnum.totalTermFreq();
}
builder.add(Util.toIntsRef(term, scratchInts), encodeWeight(termsEnum.totalTermFreq()));
}
fst = builder.finish();
if (fst == null) {
throw new IllegalArgumentException("need at least one suggestion");
}
//System.out.println("FST: " + fst.getNodeCount() + " nodes");
/*
PrintWriter pw = new PrintWriter("/x/tmp/out.dot");
Util.toDot(fst, pw, true, true);
pw.close();
*/
success = true;
} finally {
try {
if (success) {
IOUtils.close(writer, reader);
} else {
IOUtils.closeWhileHandlingException(writer, reader);
}
} finally {
for(String file : dir.listAll()) {
File path = new File(tempIndexPath, file);
if (path.delete() == false) {
throw new IllegalStateException("failed to remove " + path);
}
}
if (tempIndexPath.delete() == false) {
throw new IllegalStateException("failed to remove " + tempIndexPath);
}
dir.close();
}
}
}
@Override
public boolean store(OutputStream output) throws IOException {
DataOutput out = new OutputStreamDataOutput(output);
CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT);
out.writeByte(separator);
out.writeVInt(grams);
out.writeVLong(totTokens);
fst.save(out);
return true;
}
@Override
public boolean load(InputStream input) throws IOException {
DataInput in = new InputStreamDataInput(input);
CodecUtil.checkHeader(in, CODEC_NAME, VERSION_START, VERSION_START);
byte separatorOrig = in.readByte();
if (separatorOrig != separator) {
throw new IllegalStateException("separator=" + separator + " is incorrect: original model was built with separator=" + separatorOrig);
}
int gramsOrig = in.readVInt();
if (gramsOrig != grams) {
throw new IllegalStateException("grams=" + grams + " is incorrect: original model was built with grams=" + gramsOrig);
}
totTokens = in.readVLong();
fst = new FST<Long>(in, PositiveIntOutputs.getSingleton());
return true;
}
@Override
public List<LookupResult> lookup(final CharSequence key, /* ignored */ boolean onlyMorePopular, int num) {
try {
return lookup(key, num);
} catch (IOException ioe) {
// bogus:
throw new RuntimeException(ioe);
}
}
private int countGrams(BytesRef token) {
int count = 1;
for(int i=0;i<token.length;i++) {
if (token.bytes[token.offset + i] == separator) {
count++;
}
}
return count;
}
/** Retrieve suggestions. */
public List<LookupResult> lookup(final CharSequence key, int num) throws IOException {
TokenStream ts = queryAnalyzer.tokenStream("", key.toString());
try {
TermToBytesRefAttribute termBytesAtt = ts.addAttribute(TermToBytesRefAttribute.class);
OffsetAttribute offsetAtt = ts.addAttribute(OffsetAttribute.class);
PositionLengthAttribute posLenAtt = ts.addAttribute(PositionLengthAttribute.class);
PositionIncrementAttribute posIncAtt = ts.addAttribute(PositionIncrementAttribute.class);
ts.reset();
BytesRef[] lastTokens = new BytesRef[grams];
//System.out.println("lookup: key='" + key + "'");
// Run full analysis, but save only the
// last 1gram, last 2gram, etc.:
BytesRef tokenBytes = termBytesAtt.getBytesRef();
int maxEndOffset = -1;
boolean sawRealToken = false;
while(ts.incrementToken()) {
termBytesAtt.fillBytesRef();
sawRealToken |= tokenBytes.length > 0;
// TODO: this is somewhat iffy; today, ShingleFilter
// sets posLen to the gram count; maybe we should make
// a separate dedicated att for this?
int gramCount = posLenAtt.getPositionLength();
assert gramCount <= grams;
// Safety: make sure the recalculated count "agrees":
if (countGrams(tokenBytes) != gramCount) {
throw new IllegalArgumentException("tokens must not contain separator byte; got token=" + tokenBytes + " but gramCount=" + gramCount + " does not match recalculated count=" + countGrams(tokenBytes));
}
maxEndOffset = Math.max(maxEndOffset, offsetAtt.endOffset());
lastTokens[gramCount-1] = BytesRef.deepCopyOf(tokenBytes);
}
ts.end();
if (!sawRealToken) {
throw new IllegalArgumentException("no tokens produced by analyzer, or the only tokens were empty strings");
}
// Carefully fill last tokens with _ tokens;
// ShingleFilter appraently won't emit "only hole"
// tokens:
int endPosInc = posIncAtt.getPositionIncrement();
// Note this will also be true if input is the empty
// string (in which case we saw no tokens and
// maxEndOffset is still -1), which in fact works out OK
// because we fill the unigram with an empty BytesRef
// below:
boolean lastTokenEnded = offsetAtt.endOffset() > maxEndOffset || endPosInc > 0;
//System.out.println("maxEndOffset=" + maxEndOffset + " vs " + offsetAtt.endOffset());
if (lastTokenEnded) {
//System.out.println(" lastTokenEnded");
// If user hit space after the last token, then
// "upgrade" all tokens. This way "foo " will suggest
// all bigrams starting w/ foo, and not any unigrams
// starting with "foo":
for(int i=grams-1;i>0;i--) {
BytesRef token = lastTokens[i-1];
if (token == null) {
continue;
}
token.grow(token.length+1);
token.bytes[token.length] = separator;
token.length++;
lastTokens[i] = token;
}
lastTokens[0] = new BytesRef();
}
Arc<Long> arc = new Arc<Long>();
BytesReader bytesReader = fst.getBytesReader();
// Try highest order models first, and if they return
// results, return that; else, fallback:
double backoff = 1.0;
List<LookupResult> results = new ArrayList<LookupResult>(num);
// We only add a given suffix once, from the highest
// order model that saw it; for subsequent lower order
// models we skip it:
final Set<BytesRef> seen = new HashSet<BytesRef>();
for(int gram=grams-1;gram>=0;gram--) {
BytesRef token = lastTokens[gram];
// Don't make unigram predictions from empty string:
if (token == null || (token.length == 0 && key.length() > 0)) {
// Input didn't have enough tokens:
//System.out.println(" gram=" + gram + ": skip: not enough input");
continue;
}
if (endPosInc > 0 && gram <= endPosInc) {
// Skip hole-only predictions; in theory we
// shouldn't have to do this, but we'd need to fix
// ShingleFilter to produce only-hole tokens:
//System.out.println(" break: only holes now");
break;
}
//System.out.println("try " + (gram+1) + " gram token=" + token.utf8ToString());
// TODO: we could add fuzziness here
// match the prefix portion exactly
//Pair<Long,BytesRef> prefixOutput = null;
Long prefixOutput = null;
try {
prefixOutput = lookupPrefix(fst, bytesReader, token, arc);
} catch (IOException bogus) {
throw new RuntimeException(bogus);
}
//System.out.println(" prefixOutput=" + prefixOutput);
if (prefixOutput == null) {
// This model never saw this prefix, e.g. the
// trigram model never saw context "purple mushroom"
backoff *= ALPHA;
continue;
}
// TODO: we could do this division at build time, and
// bake it into the FST?
// Denominator for computing scores from current
// model's predictions:
long contextCount = totTokens;
BytesRef lastTokenFragment = null;
for(int i=token.length-1;i>=0;i--) {
if (token.bytes[token.offset+i] == separator) {
BytesRef context = new BytesRef(token.bytes, token.offset, i);
Long output = Util.get(fst, Util.toIntsRef(context, new IntsRef()));
assert output != null;
contextCount = decodeWeight(output);
lastTokenFragment = new BytesRef(token.bytes, token.offset + i + 1, token.length - i - 1);
break;
}
}
final BytesRef finalLastToken;
if (lastTokenFragment == null) {
finalLastToken = BytesRef.deepCopyOf(token);
} else {
finalLastToken = BytesRef.deepCopyOf(lastTokenFragment);
}
assert finalLastToken.offset == 0;
CharsRef spare = new CharsRef();
// complete top-N
MinResult<Long> completions[] = null;
try {
// Because we store multiple models in one FST
// (1gram, 2gram, 3gram), we must restrict the
// search so that it only considers the current
// model. For highest order model, this is not
// necessary since all completions in the FST
// must be from this model, but for lower order
// models we have to filter out the higher order
// ones:
// Must do num+seen.size() for queue depth because we may
// reject up to seen.size() paths in acceptResult():
Util.TopNSearcher<Long> searcher = new Util.TopNSearcher<Long>(fst, num, num+seen.size(), weightComparator) {
BytesRef scratchBytes = new BytesRef();
@Override
protected void addIfCompetitive(Util.FSTPath<Long> path) {
if (path.arc.label != separator) {
//System.out.println(" keep path: " + Util.toBytesRef(path.input, new BytesRef()).utf8ToString() + "; " + path + "; arc=" + path.arc);
super.addIfCompetitive(path);
} else {
//System.out.println(" prevent path: " + Util.toBytesRef(path.input, new BytesRef()).utf8ToString() + "; " + path + "; arc=" + path.arc);
}
}
@Override
protected boolean acceptResult(IntsRef input, Long output) {
Util.toBytesRef(input, scratchBytes);
finalLastToken.grow(finalLastToken.length + scratchBytes.length);
int lenSav = finalLastToken.length;
finalLastToken.append(scratchBytes);
//System.out.println(" accept? input='" + scratchBytes.utf8ToString() + "'; lastToken='" + finalLastToken.utf8ToString() + "'; return " + (seen.contains(finalLastToken) == false));
boolean ret = seen.contains(finalLastToken) == false;
finalLastToken.length = lenSav;
return ret;
}
};
// since this search is initialized with a single start node
// it is okay to start with an empty input path here
searcher.addStartPaths(arc, prefixOutput, true, new IntsRef());
completions = searcher.search();
} catch (IOException bogus) {
throw new RuntimeException(bogus);
}
int prefixLength = token.length;
BytesRef suffix = new BytesRef(8);
//System.out.println(" " + completions.length + " completions");
nextCompletion:
for (MinResult<Long> completion : completions) {
token.length = prefixLength;
// append suffix
Util.toBytesRef(completion.input, suffix);
token.append(suffix);
//System.out.println(" completion " + token.utf8ToString());
// Skip this path if a higher-order model already
// saw/predicted its last token:
BytesRef lastToken = token;
for(int i=token.length-1;i>=0;i--) {
if (token.bytes[token.offset+i] == separator) {
assert token.length-i-1 > 0;
lastToken = new BytesRef(token.bytes, token.offset+i+1, token.length-i-1);
break;
}
}
if (seen.contains(lastToken)) {
//System.out.println(" skip dup " + lastToken.utf8ToString());
continue nextCompletion;
}
seen.add(BytesRef.deepCopyOf(lastToken));
spare.grow(token.length);
UnicodeUtil.UTF8toUTF16(token, spare);
LookupResult result = new LookupResult(spare.toString(), (long) (Long.MAX_VALUE * backoff * ((double) decodeWeight(completion.output)) / contextCount));
results.add(result);
assert results.size() == seen.size();
//System.out.println(" add result=" + result);
}
backoff *= ALPHA;
}
Collections.sort(results, new Comparator<LookupResult>() {
@Override
public int compare(LookupResult a, LookupResult b) {
if (a.value > b.value) {
return -1;
} else if (a.value < b.value) {
return 1;
} else {
// Tie break by UTF16 sort order:
return ((String) a.key).compareTo((String) b.key);
}
}
});
if (results.size() > num) {
results.subList(num, results.size()).clear();
}
return results;
} finally {
IOUtils.closeWhileHandlingException(ts);
}
}
/** weight -> cost */
private long encodeWeight(long ngramCount) {
return Long.MAX_VALUE - ngramCount;
}
/** cost -> weight */
//private long decodeWeight(Pair<Long,BytesRef> output) {
private long decodeWeight(Long output) {
assert output != null;
return (int)(Long.MAX_VALUE - output);
}
// NOTE: copied from WFSTCompletionLookup & tweaked
private Long lookupPrefix(FST<Long> fst, FST.BytesReader bytesReader,
BytesRef scratch, Arc<Long> arc) throws /*Bogus*/IOException {
Long output = fst.outputs.getNoOutput();
fst.getFirstArc(arc);
byte[] bytes = scratch.bytes;
int pos = scratch.offset;
int end = pos + scratch.length;
while (pos < end) {
if (fst.findTargetArc(bytes[pos++] & 0xff, arc, arc, bytesReader) == null) {
return null;
} else {
output = fst.outputs.add(output, arc.output);
}
}
return output;
}
static final Comparator<Long> weightComparator = new Comparator<Long> () {
@Override
public int compare(Long left, Long right) {
return left.compareTo(right);
}
};
/**
* Returns the weight associated with an input string,
* or null if it does not exist.
*/
public Object get(CharSequence key) {
throw new UnsupportedOperationException();
}
}