/*
 * 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.backward_codecs.lucene80;

import java.io.IOException;
import java.util.Objects;
import org.apache.lucene.backward_codecs.packed.LegacyDirectWriter;
import org.apache.lucene.codecs.DocValuesConsumer;
import org.apache.lucene.codecs.DocValuesFormat;
import org.apache.lucene.codecs.DocValuesProducer;
import org.apache.lucene.index.DocValuesType;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.SegmentWriteState;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.util.SmallFloat;

/**
 * Lucene 8.0 DocValues format.
 *
 * <p>Documents that have a value for the field are encoded in a way that it is always possible to
 * know the ordinal of the current document in the set of documents that have a value. For instance,
 * say the set of documents that have a value for the field is <code>{1, 5, 6, 11}</code>. When the
 * iterator is on <code>6</code>, it knows that this is the 3rd item of the set. This way, values
 * can be stored densely and accessed based on their index at search time. If all documents in a
 * segment have a value for the field, the index is the same as the doc ID, so this case is encoded
 * implicitly and is very fast at query time. On the other hand if some documents are missing a
 * value for the field then the set of documents that have a value is encoded into blocks. All doc
 * IDs that share the same upper 16 bits are encoded into the same block with the following
 * strategies:
 *
 * <ul>
 *   <li>SPARSE: This strategy is used when a block contains at most 4095 documents. The lower 16
 *       bits of doc IDs are stored as {@link DataOutput#writeShort(short) shorts} while the upper
 *       16 bits are given by the block ID.
 *   <li>DENSE: This strategy is used when a block contains between 4096 and 65535 documents. The
 *       lower bits of doc IDs are stored in a bit set. Advancing &lt; 512 documents is performed
 *       using {@link Long#numberOfTrailingZeros(long) ntz} operations while the index is computed
 *       by accumulating the {@link Long#bitCount(long) bit counts} of the visited longs. Advancing
 *       &gt;= 512 documents is performed by skipping to the start of the needed 512 document
 *       sub-block and iterating to the specific document within that block. The index for the
 *       sub-block that is skipped to is retrieved from a rank-table positioned beforethe bit set.
 *       The rank-table holds the origo index numbers for all 512 documents sub-blocks, represented
 *       as an unsigned short for each 128 blocks.
 *   <li>ALL: This strategy is used when a block contains exactly 65536 documents, meaning that the
 *       block is full. In that case doc IDs do not need to be stored explicitly. This is typically
 *       faster than both SPARSE and DENSE which is a reason why it is preferable to have all
 *       documents that have a value for a field using contiguous doc IDs, for instance by using
 *       {@link IndexWriterConfig#setIndexSort(org.apache.lucene.search.Sort) index sorting}.
 * </ul>
 *
 * <p>Skipping blocks to arrive at a wanted document is either done on an iterative basis or by
 * using the jump-table stored at the end of the chain of blocks. The jump-table holds the offset as
 * well as the index for all blocks, packed in a single long per block.
 *
 * <p>Then the five per-document value types (Numeric,Binary,Sorted,SortedSet,SortedNumeric) are
 * encoded using the following strategies:
 *
 * <p>{@link DocValuesType#NUMERIC NUMERIC}:
 *
 * <ul>
 *   <li>Delta-compressed: per-document integers written as deltas from the minimum value,
 *       compressed with bitpacking. For more information, see {@link LegacyDirectWriter}.
 *   <li>Table-compressed: when the number of unique values is very small (&lt; 256), and when there
 *       are unused "gaps" in the range of values used (such as {@link SmallFloat}), a lookup table
 *       is written instead. Each per-document entry is instead the ordinal to this table, and those
 *       ordinals are compressed with bitpacking ({@link LegacyDirectWriter}).
 *   <li>GCD-compressed: when all numbers share a common divisor, such as dates, the greatest common
 *       denominator (GCD) is computed, and quotients are stored using Delta-compressed Numerics.
 *   <li>Monotonic-compressed: when all numbers are monotonically increasing offsets, they are
 *       written as blocks of bitpacked integers, encoding the deviation from the expected delta.
 *   <li>Const-compressed: when there is only one possible value, no per-document data is needed and
 *       this value is encoded alone.
 * </ul>
 *
 * <p>Depending on calculated gains, the numbers might be split into blocks of 16384 values. In that
 * case, a jump-table with block offsets is appended to the blocks for O(1) access to the needed
 * block.
 *
 * <p>{@link DocValuesType#BINARY BINARY}:
 *
 * <ul>
 *   <li>Fixed-width Binary: one large concatenated byte[] is written, along with the fixed length.
 *       Each document's value can be addressed directly with multiplication ({@code docID *
 *       length}).
 *   <li>Variable-width Binary: one large concatenated byte[] is written, along with end addresses
 *       for each document. The addresses are written as Monotonic-compressed numerics.
 *   <li>Prefix-compressed Binary: values are written in chunks of 16, with the first value written
 *       completely and other values sharing prefixes. chunk addresses are written as
 *       Monotonic-compressed numerics. A reverse lookup index is written from a portion of every
 *       1024th term.
 * </ul>
 *
 * <p>{@link DocValuesType#SORTED SORTED}:
 *
 * <ul>
 *   <li>Sorted: a mapping of ordinals to deduplicated terms is written as Prefix-compressed Binary,
 *       along with the per-document ordinals written using one of the numeric strategies above.
 * </ul>
 *
 * <p>{@link DocValuesType#SORTED_SET SORTED_SET}:
 *
 * <ul>
 *   <li>Single: if all documents have 0 or 1 value, then data are written like SORTED.
 *   <li>SortedSet: a mapping of ordinals to deduplicated terms is written as Binary, an ordinal
 *       list and per-document index into this list are written using the numeric strategies above.
 * </ul>
 *
 * <p>{@link DocValuesType#SORTED_NUMERIC SORTED_NUMERIC}:
 *
 * <ul>
 *   <li>Single: if all documents have 0 or 1 value, then data are written like NUMERIC.
 *   <li>SortedNumeric: a value list and per-document index into this list are written using the
 *       numeric strategies above.
 * </ul>
 *
 * <p>Files:
 *
 * <ol>
 *   <li><code>.dvd</code>: DocValues data
 *   <li><code>.dvm</code>: DocValues metadata
 * </ol>
 *
 * @lucene.experimental
 */
public final class Lucene80DocValuesFormat extends DocValuesFormat {

  /** Configuration option for doc values. */
  public enum Mode {
    /** Trade compression ratio for retrieval speed. */
    BEST_SPEED,
    /** Trade retrieval speed for compression ratio. */
    BEST_COMPRESSION
  }

  /** Attribute key for compression mode. */
  public static final String MODE_KEY = Lucene80DocValuesFormat.class.getSimpleName() + ".mode";

  private final Mode mode;

  /** Default constructor. */
  public Lucene80DocValuesFormat() {
    this(Mode.BEST_SPEED);
  }

  /** Constructor */
  public Lucene80DocValuesFormat(Mode mode) {
    super("Lucene80");
    this.mode = Objects.requireNonNull(mode);
  }

  /**
   * Note: although this format is only used on older versions, we need to keep the write logic in
   * addition to the read logic. It's possible for doc values on older segments to be written to
   * through doc values updates.
   */
  @Override
  public DocValuesConsumer fieldsConsumer(SegmentWriteState state) throws IOException {
    return new Lucene80DocValuesConsumer(
        state, DATA_CODEC, DATA_EXTENSION, META_CODEC, META_EXTENSION, mode);
  }

  @Override
  public DocValuesProducer fieldsProducer(SegmentReadState state) throws IOException {
    return new Lucene80DocValuesProducer(
        state, DATA_CODEC, DATA_EXTENSION, META_CODEC, META_EXTENSION);
  }

  static final String DATA_CODEC = "Lucene80DocValuesData";
  static final String DATA_EXTENSION = "dvd";
  static final String META_CODEC = "Lucene80DocValuesMetadata";
  static final String META_EXTENSION = "dvm";
  static final int VERSION_START = 0;
  static final int VERSION_BIN_COMPRESSED = 1;
  static final int VERSION_CONFIGURABLE_COMPRESSION = 2;
  static final int VERSION_CURRENT = VERSION_CONFIGURABLE_COMPRESSION;

  // indicates docvalues type
  static final byte NUMERIC = 0;
  static final byte BINARY = 1;
  static final byte SORTED = 2;
  static final byte SORTED_SET = 3;
  static final byte SORTED_NUMERIC = 4;

  static final int DIRECT_MONOTONIC_BLOCK_SHIFT = 16;

  static final int NUMERIC_BLOCK_SHIFT = 14;
  static final int NUMERIC_BLOCK_SIZE = 1 << NUMERIC_BLOCK_SHIFT;

  static final int BINARY_BLOCK_SHIFT = 5;
  static final int BINARY_DOCS_PER_COMPRESSED_BLOCK = 1 << BINARY_BLOCK_SHIFT;

  static final int TERMS_DICT_BLOCK_SHIFT = 4;
  static final int TERMS_DICT_BLOCK_SIZE = 1 << TERMS_DICT_BLOCK_SHIFT;
  static final int TERMS_DICT_BLOCK_MASK = TERMS_DICT_BLOCK_SIZE - 1;

  static final int TERMS_DICT_BLOCK_COMPRESSION_THRESHOLD = 32;
  static final int TERMS_DICT_BLOCK_LZ4_SHIFT = 6;
  static final int TERMS_DICT_BLOCK_LZ4_SIZE = 1 << TERMS_DICT_BLOCK_LZ4_SHIFT;
  static final int TERMS_DICT_BLOCK_LZ4_MASK = TERMS_DICT_BLOCK_LZ4_SIZE - 1;
  static final int TERMS_DICT_COMPRESSOR_LZ4_CODE = 1;
  // Writing a special code so we know this is a LZ4-compressed block.
  static final int TERMS_DICT_BLOCK_LZ4_CODE =
      TERMS_DICT_BLOCK_LZ4_SHIFT << 16 | TERMS_DICT_COMPRESSOR_LZ4_CODE;

  static final int TERMS_DICT_REVERSE_INDEX_SHIFT = 10;
  static final int TERMS_DICT_REVERSE_INDEX_SIZE = 1 << TERMS_DICT_REVERSE_INDEX_SHIFT;
  static final int TERMS_DICT_REVERSE_INDEX_MASK = TERMS_DICT_REVERSE_INDEX_SIZE - 1;
}
