/*
 * 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.queries.function.valuesource;

import java.io.IOException;
import java.util.Map;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.PostingsEnum;
import org.apache.lucene.index.Terms;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.queries.function.FunctionValues;
import org.apache.lucene.queries.function.docvalues.IntDocValues;
import org.apache.lucene.search.DocIdSetIterator;
import org.apache.lucene.util.BytesRef;

/**
 * Function that returns {@link org.apache.lucene.index.PostingsEnum#freq()} for the
 * supplied term in every document.
 * <p>
 * If the term does not exist in the document, returns 0.
 * If frequencies are omitted, returns 1.
 */
public class TermFreqValueSource extends DocFreqValueSource {
  public TermFreqValueSource(String field, String val, String indexedField, BytesRef indexedBytes) {
    super(field, val, indexedField, indexedBytes);
  }

  @Override
  public String name() {
    return "termfreq";
  }

  @Override
  public FunctionValues getValues(Map context, LeafReaderContext readerContext) throws IOException {
    final Terms terms = readerContext.reader().terms(indexedField);

    return new IntDocValues(this) {
      PostingsEnum docs ;
      int atDoc;
      int lastDocRequested = -1;

      { reset(); }

      public void reset() throws IOException {
        // no one should call us for deleted docs?
        
        if (terms != null) {
          final TermsEnum termsEnum = terms.iterator();
          if (termsEnum.seekExact(indexedBytes)) {
            docs = termsEnum.postings(null);
          } else {
            docs = null;
          }
        } else {
          docs = null;
        }

        if (docs == null) {
          docs = new PostingsEnum() {
            @Override
            public int freq() {
              return 0;
            }

            @Override
            public int nextPosition() throws IOException {
              return -1;
            }

            @Override
            public int startOffset() throws IOException {
              return -1;
            }

            @Override
            public int endOffset() throws IOException {
              return -1;
            }

            @Override
            public BytesRef getPayload() throws IOException {
              throw new UnsupportedOperationException();
            }

            @Override
            public int docID() {
              return DocIdSetIterator.NO_MORE_DOCS;
            }

            @Override
            public int nextDoc() {
              return DocIdSetIterator.NO_MORE_DOCS;
            }

            @Override
            public int advance(int target) {
              return DocIdSetIterator.NO_MORE_DOCS;
            }

            @Override
            public long cost() {
              return 0;
            }
          };
        }
        atDoc = -1;
      }

      @Override
      public int intVal(int doc) {
        try {
          if (doc < lastDocRequested) {
            // out-of-order access.... reset
            reset();
          }
          lastDocRequested = doc;

          if (atDoc < doc) {
            atDoc = docs.advance(doc);
          }

          if (atDoc > doc) {
            // term doesn't match this document... either because we hit the
            // end, or because the next doc is after this doc.
            return 0;
          }

          // a match!
          return docs.freq();
        } catch (IOException e) {
          throw new RuntimeException("caught exception in function "+description()+" : doc="+doc, e);
        }
      }
    };
  }
}


