/*
 * 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.codecs.blocktree;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import org.apache.lucene.codecs.CodecUtil;
import org.apache.lucene.codecs.FieldsProducer;
import org.apache.lucene.codecs.PostingsReaderBase;
import org.apache.lucene.index.CorruptIndexException;
import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.index.IndexFileNames;
import org.apache.lucene.index.IndexOptions;
import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.Terms;
import org.apache.lucene.store.ChecksumIndexInput;
import org.apache.lucene.store.IndexInput;
import org.apache.lucene.util.Accountable;
import org.apache.lucene.util.Accountables;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.fst.ByteSequenceOutputs;
import org.apache.lucene.util.fst.Outputs;

A block-based terms index and dictionary that assigns
 terms to variable length blocks according to how they
 share prefixes.  The terms index is a prefix trie
 whose leaves are term blocks.  The advantage of this
 approach is that seekExact is often able to
 determine a term cannot exist without doing any IO, and
 intersection with Automata is very fast.  Note that this
 terms dictionary has its own fixed terms index (ie, it
 does not support a pluggable terms index
 implementation).
 
NOTE: this terms dictionary supports
 min/maxItemsPerBlock during indexing to control how
 much memory the terms index uses.
 
The data structure used by this implementation is very
 similar to a burst trie
 (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.3499),
 but with added logic to break up too-large blocks of all
 terms sharing a given prefix into smaller ones.
 
Use CheckIndex with the -verbose option to see summary statistics on the blocks in the dictionary. See BlockTreeTermsWriter. 
@lucene.experimental
/** A block-based terms index and dictionary that assigns
 *  terms to variable length blocks according to how they
 *  share prefixes.  The terms index is a prefix trie
 *  whose leaves are term blocks.  The advantage of this
 *  approach is that seekExact is often able to
 *  determine a term cannot exist without doing any IO, and
 *  intersection with Automata is very fast.  Note that this
 *  terms dictionary has its own fixed terms index (ie, it
 *  does not support a pluggable terms index
 *  implementation).
 *
 *  <p><b>NOTE</b>: this terms dictionary supports
 *  min/maxItemsPerBlock during indexing to control how
 *  much memory the terms index uses.</p>
 *
 *  <p>The data structure used by this implementation is very
 *  similar to a burst trie
 *  (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.18.3499),
 *  but with added logic to break up too-large blocks of all
 *  terms sharing a given prefix into smaller ones.</p>
 *
 *  <p>Use {@link org.apache.lucene.index.CheckIndex} with the <code>-verbose</code>
 *  option to see summary statistics on the blocks in the
 *  dictionary.
 *
 *  See {@link BlockTreeTermsWriter}.
 *
 * @lucene.experimental
 */

public final class BlockTreeTermsReader extends FieldsProducer {

  static final Outputs<BytesRef> FST_OUTPUTS = ByteSequenceOutputs.getSingleton();
  
  static final BytesRef NO_OUTPUT = FST_OUTPUTS.getNoOutput();

  static final int OUTPUT_FLAGS_NUM_BITS = 2;
  static final int OUTPUT_FLAGS_MASK = 0x3;
  static final int OUTPUT_FLAG_IS_FLOOR = 0x1;
  static final int OUTPUT_FLAG_HAS_TERMS = 0x2;

  
Extension of terms file /** Extension of terms file */
  static final String TERMS_EXTENSION = "tim";
  final static String TERMS_CODEC_NAME = "BlockTreeTermsDict";

  
Initial terms format. /** Initial terms format. */
  public static final int VERSION_START = 3;

  
The long[] + byte[] metadata has been replaced with a single byte[]. /** The long[] + byte[] metadata has been replaced with a single byte[]. */
  public static final int VERSION_META_LONGS_REMOVED = 4;

  
Suffixes are compressed to save space. /** Suffixes are compressed to save space. */
  public static final int VERSION_COMPRESSED_SUFFIXES = 5;

  
Metadata is written to its own file. /** Metadata is written to its own file. */
  public static final int VERSION_META_FILE = 6;

  
Current terms format. /** Current terms format. */
  public static final int VERSION_CURRENT = VERSION_META_FILE;

  
Extension of terms index file /** Extension of terms index file */
  static final String TERMS_INDEX_EXTENSION = "tip";
  final static String TERMS_INDEX_CODEC_NAME = "BlockTreeTermsIndex";

  
Extension of terms meta file /** Extension of terms meta file */
  static final String TERMS_META_EXTENSION = "tmd";
  final static String TERMS_META_CODEC_NAME = "BlockTreeTermsMeta";

  // Open input to the main terms dict file (_X.tib)
  final IndexInput termsIn;
  // Open input to the terms index file (_X.tip)
  final IndexInput indexIn;

  //private static final boolean DEBUG = BlockTreeTermsWriter.DEBUG;

  // Reads the terms dict entries, to gather state to
  // produce DocsEnum on demand
  final PostingsReaderBase postingsReader;

  private final Map<String,FieldReader> fieldMap;
  private final List<String> fieldList;

  final String segment;
  
  final int version;

  
Sole constructor. /** Sole constructor. */
  public BlockTreeTermsReader(PostingsReaderBase postingsReader, SegmentReadState state) throws IOException {
    boolean success = false;
    
    this.postingsReader = postingsReader;
    this.segment = state.segmentInfo.name;

    try {
      String termsName = IndexFileNames.segmentFileName(segment, state.segmentSuffix, TERMS_EXTENSION);
      termsIn = state.directory.openInput(termsName, state.context);
      version = CodecUtil.checkIndexHeader(termsIn, TERMS_CODEC_NAME, VERSION_START, VERSION_CURRENT, state.segmentInfo.getId(), state.segmentSuffix);

      String indexName = IndexFileNames.segmentFileName(segment, state.segmentSuffix, TERMS_INDEX_EXTENSION);
      indexIn = state.directory.openInput(indexName, state.context);
      CodecUtil.checkIndexHeader(indexIn, TERMS_INDEX_CODEC_NAME, version, version, state.segmentInfo.getId(), state.segmentSuffix);

      if (version < VERSION_META_FILE) {
        // Have PostingsReader init itself
        postingsReader.init(termsIn, state);

        // Verifying the checksum against all bytes would be too costly, but for now we at least
        // verify proper structure of the checksum footer. This is cheap and can detect some forms
        // of corruption such as file truncation.
        CodecUtil.retrieveChecksum(indexIn);
        CodecUtil.retrieveChecksum(termsIn);
      }

      // Read per-field details
      String metaName = IndexFileNames.segmentFileName(segment, state.segmentSuffix, TERMS_META_EXTENSION);
      Map<String, FieldReader> fieldMap = null;
      Throwable priorE = null;
      long indexLength = -1, termsLength = -1;
      try (ChecksumIndexInput metaIn = version >= VERSION_META_FILE ? state.directory.openChecksumInput(metaName, state.context) : null) {
        try {
          final IndexInput indexMetaIn, termsMetaIn;
          if (version >= VERSION_META_FILE) {
            CodecUtil.checkIndexHeader(metaIn, TERMS_META_CODEC_NAME, version, version, state.segmentInfo.getId(), state.segmentSuffix);
            indexMetaIn = termsMetaIn = metaIn;
            postingsReader.init(metaIn, state);
          } else {
            seekDir(termsIn);
            seekDir(indexIn);
            indexMetaIn = indexIn;
            termsMetaIn = termsIn;
          }

          final int numFields = termsMetaIn.readVInt();
          if (numFields < 0) {
            throw new CorruptIndexException("invalid numFields: " + numFields, termsMetaIn);
          }
          fieldMap = new HashMap<>((int) (numFields / 0.75f) + 1);
          for (int i = 0; i < numFields; ++i) {
            final int field = termsMetaIn.readVInt();
            final long numTerms = termsMetaIn.readVLong();
            if (numTerms <= 0) {
              throw new CorruptIndexException("Illegal numTerms for field number: " + field, termsMetaIn);
            }
            final BytesRef rootCode = readBytesRef(termsMetaIn);
            final FieldInfo fieldInfo = state.fieldInfos.fieldInfo(field);
            if (fieldInfo == null) {
              throw new CorruptIndexException("invalid field number: " + field, termsMetaIn);
            }
            final long sumTotalTermFreq = termsMetaIn.readVLong();
            // when frequencies are omitted, sumDocFreq=sumTotalTermFreq and only one value is written.
            final long sumDocFreq = fieldInfo.getIndexOptions() == IndexOptions.DOCS ? sumTotalTermFreq : termsMetaIn.readVLong();
            final int docCount = termsMetaIn.readVInt();
            if (version < VERSION_META_LONGS_REMOVED) {
              final int longsSize = termsMetaIn.readVInt();
              if (longsSize < 0) {
                throw new CorruptIndexException("invalid longsSize for field: " + fieldInfo.name + ", longsSize=" + longsSize, termsMetaIn);
              }
            }
            BytesRef minTerm = readBytesRef(termsMetaIn);
            BytesRef maxTerm = readBytesRef(termsMetaIn);
            if (docCount < 0 || docCount > state.segmentInfo.maxDoc()) { // #docs with field must be <= #docs
              throw new CorruptIndexException("invalid docCount: " + docCount + " maxDoc: " + state.segmentInfo.maxDoc(), termsMetaIn);
            }
            if (sumDocFreq < docCount) {  // #postings must be >= #docs with field
              throw new CorruptIndexException("invalid sumDocFreq: " + sumDocFreq + " docCount: " + docCount, termsMetaIn);
            }
            if (sumTotalTermFreq < sumDocFreq) { // #positions must be >= #postings
              throw new CorruptIndexException("invalid sumTotalTermFreq: " + sumTotalTermFreq + " sumDocFreq: " + sumDocFreq, termsMetaIn);
            }
            final long indexStartFP = indexMetaIn.readVLong();
            FieldReader previous = fieldMap.put(fieldInfo.name,
                new FieldReader(this, fieldInfo, numTerms, rootCode, sumTotalTermFreq, sumDocFreq, docCount,
                    indexStartFP, indexMetaIn, indexIn, minTerm, maxTerm));
            if (previous != null) {
              throw new CorruptIndexException("duplicate field: " + fieldInfo.name, termsMetaIn);
            }
          }
          if (version >= VERSION_META_FILE) {
            indexLength = metaIn.readLong();
            termsLength = metaIn.readLong();
          }
        } catch (Throwable exception) {
          priorE = exception;
        } finally {
          if (metaIn != null) {
            CodecUtil.checkFooter(metaIn, priorE);
          } else if (priorE != null) {
            IOUtils.rethrowAlways(priorE);
          }
        }
      }
      if (version >= VERSION_META_FILE) {
        // At this point the checksum of the meta file has been verified so the lengths are likely correct
        CodecUtil.retrieveChecksum(indexIn, indexLength);
        CodecUtil.retrieveChecksum(termsIn, termsLength);
      } else {
        assert indexLength == -1 : indexLength;
        assert termsLength == -1 : termsLength;
      }
      List<String> fieldList = new ArrayList<>(fieldMap.keySet());
      fieldList.sort(null);
      this.fieldMap = fieldMap;
      this.fieldList = Collections.unmodifiableList(fieldList);
      success = true;
    } finally {
      if (!success) {
        // this.close() will close in:
        IOUtils.closeWhileHandlingException(this);
      }
    }
  }

  private static BytesRef readBytesRef(IndexInput in) throws IOException {
    int numBytes = in.readVInt();
    if (numBytes < 0) {
      throw new CorruptIndexException("invalid bytes length: " + numBytes, in);
    }
    
    BytesRef bytes = new BytesRef();
    bytes.length = numBytes;
    bytes.bytes = new byte[numBytes];
    in.readBytes(bytes.bytes, 0, numBytes);

    return bytes;
  }

  
Seek input to the directory offset. /** Seek {@code input} to the directory offset. */
  private static void seekDir(IndexInput input) throws IOException {
    input.seek(input.length() - CodecUtil.footerLength() - 8);
    long offset = input.readLong();
    input.seek(offset);
  }

  // for debugging
  // private static String toHex(int v) {
  //   return "0x" + Integer.toHexString(v);
  // }

  @Override
  public void close() throws IOException {
    try {
      IOUtils.close(indexIn, termsIn, postingsReader);
    } finally { 
      // Clear so refs to terms index is GCable even if
      // app hangs onto us:
      fieldMap.clear();
    }
  }

  @Override
  public Iterator<String> iterator() {
    return fieldList.iterator();
  }

  @Override
  public Terms terms(String field) throws IOException {
    assert field != null;
    return fieldMap.get(field);
  }

  @Override
  public int size() {
    return fieldMap.size();
  }

  // for debugging
  String brToString(BytesRef b) {
    if (b == null) {
      return "null";
    } else {
      try {
        return b.utf8ToString() + " " + b;
      } catch (Throwable t) {
        // If BytesRef isn't actually UTF8, or it's eg a
        // prefix of UTF8 that ends mid-unicode-char, we
        // fallback to hex:
        return b.toString();
      }
    }
  }

  @Override
  public long ramBytesUsed() {
    long sizeInBytes = postingsReader.ramBytesUsed();
    for(FieldReader reader : fieldMap.values()) {
      sizeInBytes += reader.ramBytesUsed();
    }
    return sizeInBytes;
  }

  @Override
  public Collection<Accountable> getChildResources() {
    List<Accountable> resources = new ArrayList<>(Accountables.namedAccountables("field", fieldMap));
    resources.add(Accountables.namedAccountable("delegate", postingsReader));
    return Collections.unmodifiableList(resources);
  }

  @Override
  public void checkIntegrity() throws IOException { 
    // terms index
    CodecUtil.checksumEntireFile(indexIn);

    // term dictionary
    CodecUtil.checksumEntireFile(termsIn);
      
    // postings
    postingsReader.checkIntegrity();
  }

  @Override
  public String toString() {
    return getClass().getSimpleName() + "(fields=" + fieldMap.size() + ",delegate=" + postingsReader + ")";
  }
}