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/**
 * 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.spell;

import java.io.IOException;
import java.util.Comparator;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.index.TermEnum;
import org.apache.lucene.util.BytesRefIterator;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.StringHelper;

/**
 * HighFrequencyDictionary: terms taken from the given field
 * of a Lucene index, which appear in a number of documents
 * above a given threshold.
 *
 * Threshold is a value in [0..1] representing the minimum
 * number of documents (of the total) where a term should appear.
 * 
 * Based on LuceneDictionary.
 */
public class HighFrequencyDictionary implements Dictionary {
  private IndexReader reader;
  private String field;
  private float thresh;

  public HighFrequencyDictionary(IndexReader reader, String field, float thresh) {
    this.reader = reader;
    this.field = StringHelper.intern(field);
    this.thresh = thresh;
  }

  public final BytesRefIterator getWordsIterator() throws IOException {
    return new HighFrequencyIterator();
  }

  final class HighFrequencyIterator implements TermFreqIterator {
    private final BytesRef spare = new BytesRef();
    private final TermEnum termsEnum;
    private int minNumDocs;
    private long freq;
    private final Comparator comp;

    HighFrequencyIterator() throws IOException {
      termsEnum = reader.terms(new Term(field, ""));
      minNumDocs = (int)(thresh * (float)reader.numDocs());
      Term term = termsEnum.term();
      if (term == null || term.field() != field) {
        comp = null;
      } else {
        comp = BytesRef.getUTF8SortedAsUnicodeComparator();
      }
    }

    private boolean isFrequent(int freq) {
      return freq >= minNumDocs;
    }
    
    public long weight() {
      return freq;
    }

    //@Override - not until Java 6
    public BytesRef next() throws IOException {
      if (termsEnum != null) {
        Term actualTerm;
        do {
          actualTerm = termsEnum.term();
          if (actualTerm == null || actualTerm.field() != field) {
            return null;
          }
          if (isFrequent(termsEnum.docFreq())) {
            freq = termsEnum.docFreq();
            spare.copyChars(actualTerm.text());
            termsEnum.next();
            return spare;
          }
        } while(termsEnum.next());
      }
      return  null;
    }

    //@Override - not until Java 6
    public Comparator getComparator() {
      return comp;
    }
  }
}