org.apache.lucene.search.NGramPhraseQuery.java Source code

Introduction

Here is the source code for org.apache.lucene.search.NGramPhraseQuery.java

Source

/*
 * 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.search;

import java.io.IOException;
import java.util.Objects;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;

/**
 * This is a {@link PhraseQuery} which is optimized for n-gram phrase query.
 * For example, when you query "ABCD" on a 2-gram field, you may want to use
 * NGramPhraseQuery rather than {@link PhraseQuery}, because NGramPhraseQuery
 * will {@link #rewrite(IndexReader)} the query to "AB/0 CD/2", while {@link PhraseQuery}
 * will query "AB/0 BC/1 CD/2" (where term/position).
 *
 */
public class NGramPhraseQuery extends Query {

    private final int n;
    private final PhraseQuery phraseQuery;

    /**
     * Constructor that takes gram size.
     * @param n n-gram size
     */
    public NGramPhraseQuery(int n, PhraseQuery query) {
        super();
        this.n = n;
        this.phraseQuery = Objects.requireNonNull(query);
    }

    @Override
    public Query rewrite(IndexReader reader) throws IOException {
        final Term[] terms = phraseQuery.getTerms();
        final int[] positions = phraseQuery.getPositions();

        boolean isOptimizable = phraseQuery.getSlop() == 0 && n >= 2 // non-overlap n-gram cannot be optimized
                && terms.length >= 3; // short ones can't be optimized

        if (isOptimizable) {
            for (int i = 1; i < positions.length; ++i) {
                if (positions[i] != positions[i - 1] + 1) {
                    isOptimizable = false;
                    break;
                }
            }
        }

        if (isOptimizable == false) {
            return phraseQuery.rewrite(reader);
        }

        PhraseQuery.Builder builder = new PhraseQuery.Builder();
        for (int i = 0; i < terms.length; ++i) {
            if (i % n == 0 || i == terms.length - 1) {
                builder.add(terms[i], i);
            }
        }
        return builder.build();
    }

    @Override
    public void visit(QueryVisitor visitor) {
        phraseQuery.visit(visitor.getSubVisitor(BooleanClause.Occur.MUST, this));
    }

    @Override
    public boolean equals(Object other) {
        return sameClassAs(other) && equalsTo(getClass().cast(other));
    }

    private boolean equalsTo(NGramPhraseQuery other) {
        return n == other.n && phraseQuery.equals(other.phraseQuery);
    }

    @Override
    public int hashCode() {
        int h = classHash();
        h = 31 * h + phraseQuery.hashCode();
        h = 31 * h + n;
        return h;
    }

    /** Return the n in n-gram */
    public int getN() {
        return n;
    }

    /** Return the list of terms. */
    public Term[] getTerms() {
        return phraseQuery.getTerms();
    }

    /** Return the list of relative positions that each term should appear at. */
    public int[] getPositions() {
        return phraseQuery.getPositions();
    }

    @Override
    public String toString(String field) {
        return phraseQuery.toString(field);
    }
}