/*
 * 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.beam.runners.dataflow.internal;

import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkArgument;
import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkNotNull;
import static org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkState;

import com.google.auto.value.AutoValue;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
import org.apache.beam.runners.dataflow.util.RandomAccessData;
import org.apache.beam.sdk.coders.AtomicCoder;
import org.apache.beam.sdk.coders.ByteArrayCoder;
import org.apache.beam.sdk.coders.Coder;
import org.apache.beam.sdk.coders.Coder.NonDeterministicException;
import org.apache.beam.sdk.coders.CoderException;
import org.apache.beam.sdk.coders.CustomCoder;
import org.apache.beam.sdk.coders.ListCoder;
import org.apache.beam.sdk.coders.StructuredCoder;
import org.apache.beam.sdk.coders.VarIntCoder;
import org.apache.beam.sdk.coders.VarLongCoder;
import org.apache.beam.sdk.util.VarInt;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableList;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.hash.HashFunction;
import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.hash.Hashing;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An Ism file is a prefix encoded composite key value file broken into shards. Each composite key
 * is composed of a fixed number of component keys. A fixed number of those sub keys represent the
 * shard key portion; see {@link IsmRecord} and {@link IsmRecordCoder} for further details around
 * the data format. In addition to the data, there is a bloom filter, and multiple indices to allow
 * for efficient retrieval.
 *
 * <p>An Ism file is composed of these high level sections (in order):
 *
 * <ul>
 *   <li>shard block
 *   <li>bloom filter (See {@code ScalableBloomFilter} for details on encoding format)
 *   <li>shard index
 *   <li>footer (See {@link Footer} for details on encoding format)
 * </ul>
 *
 * <p>The shard block is composed of multiple copies of the following:
 *
 * <ul>
 *   <li>data block
 *   <li>data index
 * </ul>
 *
 * <p>The data block is composed of multiple copies of the following:
 *
 * <ul>
 *   <li>key prefix (See {@link KeyPrefix} for details on encoding format)
 *   <li>unshared key bytes
 *   <li>value bytes
 *   <li>optional 0x00 0x00 bytes followed by metadata bytes (if the following 0x00 0x00 bytes are
 *       not present, then there are no metadata bytes)
 * </ul>
 *
 * Each key written into the data block must be in unsigned lexicographically increasing order and
 * also its shard portion of the key must hash to the same shard id as all other keys within the
 * same data block. The hashing function used is the <a
 * href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">32-bit murmur3 algorithm, x86
 * variant</a> (little-endian variant), using {@code 1225801234} as the seed value.
 *
 * <p>The data index is composed of {@code N} copies of the following:
 *
 * <ul>
 *   <li>key prefix (See {@link KeyPrefix} for details on encoding format)
 *   <li>unshared key bytes
 *   <li>byte offset to key prefix in data block (variable length long coding)
 * </ul>
 *
 * <p>The shard index is composed of a {@link VarInt variable length integer} encoding representing
 * the number of shard index records followed by that many shard index records. See {@link
 * IsmShardCoder} for further details as to its encoding scheme.
 */
@SuppressWarnings({
  "nullness" // TODO(https://github.com/apache/beam/issues/20497)
})
public class IsmFormat {
  private static final int HASH_SEED = 1225801234;
  private static final HashFunction HASH_FUNCTION = Hashing.murmur3_32(HASH_SEED);
  public static final int SHARD_BITS = 0x7F; // [0-127] shards + [128-255] metadata shards

  /**
   * A record containing a composite key and either a value or metadata. The composite key must not
   * contain the metadata key component place holder if producing a value record, and must contain
   * the metadata component key place holder if producing a metadata record.
   *
   * <p>The composite key is a fixed number of component keys where the first {@code N} component
   * keys are used to create a shard id via hashing. See {@link IsmRecordCoder#hash(List)} for
   * further details.
   */
  @AutoValue
  public abstract static class IsmRecord<V> {
    abstract List<?> keyComponents();

    abstract @Nullable V value();

    @SuppressWarnings("mutable")
    abstract byte @Nullable [] metadata();

    IsmRecord() {} // Prevent public constructor

    /** Returns an IsmRecord with the specified key components and value. */
    public static <V> IsmRecord<V> of(List<?> keyComponents, V value) {
      checkArgument(!keyComponents.isEmpty(), "Expected non-empty list of key components.");
      checkArgument(
          !isMetadataKey(keyComponents), "Expected key components to not contain metadata key.");
      return new AutoValue_IsmFormat_IsmRecord<>(keyComponents, value, null);
    }

    public static <V> IsmRecord<V> meta(List<?> keyComponents, byte[] metadata) {
      checkNotNull(metadata);
      checkArgument(!keyComponents.isEmpty(), "Expected non-empty list of key components.");
      checkArgument(
          isMetadataKey(keyComponents), "Expected key components to contain metadata key.");
      return new AutoValue_IsmFormat_IsmRecord<>(keyComponents, null, metadata);
    }

    /** Returns the list of key components. */
    public List<?> getKeyComponents() {
      return keyComponents();
    }

    /** Returns the key component at the specified index. */
    public Object getKeyComponent(int index) {
      return keyComponents().get(index);
    }

    /** Returns the value. Throws {@link IllegalStateException} if this is not a value record. */
    public V getValue() {
      checkState(
          !isMetadataKey(keyComponents()), "This is a metadata record and not a value record.");
      return value();
    }

    /**
     * Returns the metadata. Throws {@link IllegalStateException} if this is not a metadata record.
     */
    public byte[] getMetadata() {
      checkState(
          isMetadataKey(keyComponents()), "This is a value record and not a metadata record.");
      return metadata();
    }
  }

  /**
   * A {@link Coder} for {@link IsmRecord}s.
   *
   * <p>Note that this coder standalone will not produce an Ism file. This coder can be used to
   * materialize a {@link PCollection} of {@link IsmRecord}s. Only when this coder is combined with
   * an {@code IsmSink} will one produce an Ism file.
   *
   * <p>The {@link IsmRecord} encoded format is:
   *
   * <ul>
   *   <li>encoded key component 1 using key component coder 1
   *   <li>...
   *   <li>encoded key component N using key component coder N
   *   <li>encoded value using value coder
   * </ul>
   */
  public static class IsmRecordCoder<V> extends CustomCoder<IsmRecord<V>> {
    /** Returns an IsmRecordCoder with the specified key component coders, value coder. */
    public static <V> IsmRecordCoder<V> of(
        int numberOfShardKeyCoders,
        int numberOfMetadataShardKeyCoders,
        List<Coder<?>> keyComponentCoders,
        Coder<V> valueCoder) {
      checkNotNull(keyComponentCoders);
      checkArgument(keyComponentCoders.size() > 0);
      checkArgument(numberOfShardKeyCoders > 0);
      checkArgument(numberOfShardKeyCoders <= keyComponentCoders.size());
      checkArgument(numberOfMetadataShardKeyCoders <= keyComponentCoders.size());
      return new IsmRecordCoder<>(
          numberOfShardKeyCoders, numberOfMetadataShardKeyCoders, keyComponentCoders, valueCoder);
    }

    private final int numberOfShardKeyCoders;
    private final int numberOfMetadataShardKeyCoders;
    private final List<Coder<?>> keyComponentCoders;
    private final Coder<V> valueCoder;

    private IsmRecordCoder(
        int numberOfShardKeyCoders,
        int numberOfMetadataShardKeyCoders,
        List<Coder<?>> keyComponentCoders,
        Coder<V> valueCoder) {
      this.numberOfShardKeyCoders = numberOfShardKeyCoders;
      this.numberOfMetadataShardKeyCoders = numberOfMetadataShardKeyCoders;
      this.keyComponentCoders = keyComponentCoders;
      this.valueCoder = valueCoder;
    }

    /** Returns the list of key component coders. */
    public List<Coder<?>> getKeyComponentCoders() {
      return keyComponentCoders;
    }

    /** Returns the key coder at the specified index. */
    @SuppressWarnings("unchecked")
    public <T> Coder<T> getKeyComponentCoder(int index) {
      return (Coder<T>) keyComponentCoders.get(index);
    }

    /** Returns the value coder. */
    public Coder<V> getValueCoder() {
      return valueCoder;
    }

    @Override
    public void encode(IsmRecord<V> value, OutputStream outStream)
        throws CoderException, IOException {
      if (value.getKeyComponents().size() != keyComponentCoders.size()) {
        throw new CoderException(
            String.format(
                "Expected %s key component(s) but received key component(s) %s.",
                keyComponentCoders.size(), value.getKeyComponents()));
      }
      for (int i = 0; i < keyComponentCoders.size(); ++i) {
        getKeyComponentCoder(i).encode(value.getKeyComponent(i), outStream);
      }
      if (isMetadataKey(value.getKeyComponents())) {
        ByteArrayCoder.of().encode(value.getMetadata(), outStream);
      } else {
        valueCoder.encode(value.getValue(), outStream);
      }
    }

    @Override
    public IsmRecord<V> decode(InputStream inStream) throws CoderException, IOException {
      List<Object> keyComponents = new ArrayList<>(keyComponentCoders.size());
      for (Coder<?> keyCoder : keyComponentCoders) {
        keyComponents.add(keyCoder.decode(inStream));
      }
      if (isMetadataKey(keyComponents)) {
        return IsmRecord.meta(keyComponents, ByteArrayCoder.of().decode(inStream));
      } else {
        return IsmRecord.of(keyComponents, valueCoder.decode(inStream));
      }
    }

    public int getNumberOfShardKeyCoders(List<?> keyComponents) {
      if (isMetadataKey(keyComponents)) {
        return numberOfMetadataShardKeyCoders;
      } else {
        return numberOfShardKeyCoders;
      }
    }

    /**
     * Computes the shard id for the given key component(s).
     *
     * <p>The shard keys are encoded into their byte representations and hashed using the <a
     * href="http://smhasher.googlecode.com/svn/trunk/MurmurHash3.cpp">32-bit murmur3 algorithm, x86
     * variant</a> (little-endian variant), using {@code 1225801234} as the seed value. We ensure
     * that shard ids for metadata keys and normal keys do not overlap.
     */
    public int hash(List<?> keyComponents) {
      return encodeAndHash(keyComponents, new RandomAccessData(), new ArrayList<>());
    }

    /**
     * Computes the shard id for the given key component(s).
     *
     * <p>Mutates {@code keyBytes} such that when returned, contains the encoded version of the key
     * components.
     */
    public int encodeAndHash(List<?> keyComponents, RandomAccessData keyBytesToMutate) {
      return encodeAndHash(keyComponents, keyBytesToMutate, new ArrayList<>());
    }

    /**
     * Computes the shard id for the given key component(s).
     *
     * <p>Mutates {@code keyBytes} such that when returned, contains the encoded version of the key
     * components. Also, mutates {@code keyComponentByteOffsetsToMutate} to store the location where
     * each key component's encoded byte representation ends within {@code keyBytes}.
     */
    public int encodeAndHash(
        List<?> keyComponents,
        RandomAccessData keyBytesToMutate,
        List<Integer> keyComponentByteOffsetsToMutate) {
      checkNotNull(keyComponents);
      checkArgument(
          keyComponents.size() <= keyComponentCoders.size(),
          "Expected at most %s key component(s) but received %s.",
          keyComponentCoders.size(),
          keyComponents);

      final int numberOfKeyCodersToUse;
      final int shardOffset;
      if (isMetadataKey(keyComponents)) {
        numberOfKeyCodersToUse = numberOfMetadataShardKeyCoders;
        shardOffset = SHARD_BITS + 1;
      } else {
        numberOfKeyCodersToUse = numberOfShardKeyCoders;
        shardOffset = 0;
      }

      checkArgument(
          numberOfKeyCodersToUse <= keyComponents.size(),
          "Expected at least %s key component(s) but received %s.",
          numberOfShardKeyCoders,
          keyComponents);

      try {
        // Encode the shard portion
        for (int i = 0; i < numberOfKeyCodersToUse; ++i) {
          getKeyComponentCoder(i)
              .encode(keyComponents.get(i), keyBytesToMutate.asOutputStream(), Context.NESTED);
          keyComponentByteOffsetsToMutate.add(keyBytesToMutate.size());
        }
        int rval =
            HASH_FUNCTION.hashBytes(keyBytesToMutate.array(), 0, keyBytesToMutate.size()).asInt()
                & SHARD_BITS;
        rval += shardOffset;

        // Encode the remainder
        for (int i = numberOfKeyCodersToUse; i < keyComponents.size(); ++i) {
          getKeyComponentCoder(i)
              .encode(keyComponents.get(i), keyBytesToMutate.asOutputStream(), Context.NESTED);
          keyComponentByteOffsetsToMutate.add(keyBytesToMutate.size());
        }
        return rval;
      } catch (IOException e) {
        throw new IllegalStateException(
            String.format("Failed to hash %s with coder %s", keyComponents, this), e);
      }
    }

    @Override
    public List<Coder<?>> getCoderArguments() {
      return ImmutableList.<Coder<?>>builder().addAll(keyComponentCoders).add(valueCoder).build();
    }

    @Override
    public void verifyDeterministic() throws Coder.NonDeterministicException {
      verifyDeterministic(
          this, "Key component coders expected to be deterministic.", keyComponentCoders);
      verifyDeterministic(this, "Value coder expected to be deterministic.", valueCoder);
    }

    @Override
    public boolean consistentWithEquals() {
      for (Coder<?> keyComponentCoder : keyComponentCoders) {
        if (!keyComponentCoder.consistentWithEquals()) {
          return false;
        }
      }
      return valueCoder.consistentWithEquals();
    }

    @Override
    public Object structuralValue(IsmRecord<V> record) {
      checkNotNull(record);
      checkState(
          record.getKeyComponents().size() == keyComponentCoders.size(),
          "Expected the number of key component coders %s "
              + "to match the number of key components %s.",
          keyComponentCoders.size(),
          record.getKeyComponents());

      if (consistentWithEquals()) {
        ArrayList<Object> keyComponentStructuralValues = new ArrayList<>();
        for (int i = 0; i < keyComponentCoders.size(); ++i) {
          keyComponentStructuralValues.add(
              getKeyComponentCoder(i).structuralValue(record.getKeyComponent(i)));
        }
        if (isMetadataKey(record.getKeyComponents())) {
          return IsmRecord.meta(keyComponentStructuralValues, record.getMetadata());
        } else {
          return IsmRecord.of(
              keyComponentStructuralValues, valueCoder.structuralValue(record.getValue()));
        }
      }
      return super.structuralValue(record);
    }

    @Override
    public boolean equals(@Nullable Object other) {
      if (other == this) {
        return true;
      }
      if (!(other instanceof IsmRecordCoder)) {
        return false;
      }
      IsmRecordCoder<?> that = (IsmRecordCoder<?>) other;
      return this.numberOfShardKeyCoders == that.numberOfShardKeyCoders
          && this.numberOfMetadataShardKeyCoders == that.numberOfMetadataShardKeyCoders
          && Objects.equals(this.keyComponentCoders, that.keyComponentCoders)
          && Objects.equals(this.valueCoder, that.valueCoder);
    }

    @Override
    public int hashCode() {
      return Objects.hash(
          numberOfShardKeyCoders, numberOfMetadataShardKeyCoders, keyComponentCoders, valueCoder);
    }
  }

  /** Validates that the key portion of the given coder is deterministic. */
  public static void validateCoderIsCompatible(IsmRecordCoder<?> coder) {
    for (Coder<?> keyComponentCoder : coder.getKeyComponentCoders()) {
      try {
        keyComponentCoder.verifyDeterministic();
      } catch (NonDeterministicException e) {
        throw new IllegalArgumentException(
            String.format(
                "Key component coder %s is expected to be deterministic.", keyComponentCoder),
            e);
      }
    }
  }

  /** Returns true if and only if any of the passed in key components represent a metadata key. */
  public static boolean isMetadataKey(List<?> keyComponents) {
    for (Object keyComponent : keyComponents) {
      if (keyComponent == METADATA_KEY) {
        return true;
      }
    }
    return false;
  }

  /** A marker object representing the wildcard metadata key component. */
  private static final Object METADATA_KEY =
      new Object() {
        @Override
        public String toString() {
          return "META";
        }

        @Override
        public boolean equals(@Nullable Object obj) {
          return this == obj;
        }

        @Override
        public int hashCode() {
          return -1248902349;
        }
      };

  /**
   * An object representing a wild card for a key component. Encoded using {@link MetadataKeyCoder}.
   */
  public static Object getMetadataKey() {
    return METADATA_KEY;
  }

  /**
   * A coder for metadata key component. Can be used to wrap key component coder allowing for the
   * metadata key component to be used as a place holder instead of an actual key.
   */
  public static class MetadataKeyCoder<K> extends StructuredCoder<K> {
    public static <K> MetadataKeyCoder<K> of(Coder<K> keyCoder) {
      checkNotNull(keyCoder);
      return new MetadataKeyCoder<>(keyCoder);
    }

    private final Coder<K> keyCoder;

    private MetadataKeyCoder(Coder<K> keyCoder) {
      this.keyCoder = keyCoder;
    }

    public Coder<K> getKeyCoder() {
      return keyCoder;
    }

    @Override
    public void encode(K value, OutputStream outStream) throws CoderException, IOException {
      if (value == METADATA_KEY) {
        outStream.write(0);
      } else {
        outStream.write(1);
        keyCoder.encode(value, outStream);
      }
    }

    @Override
    public K decode(InputStream inStream) throws CoderException, IOException {
      int marker = inStream.read();
      if (marker == 0) {
        return (K) getMetadataKey();
      } else if (marker == 1) {
        return keyCoder.decode(inStream);
      } else {
        throw new CoderException(String.format("Expected marker but got %s.", marker));
      }
    }

    @Override
    public List<Coder<?>> getCoderArguments() {
      return ImmutableList.of(keyCoder);
    }

    @Override
    public void verifyDeterministic() throws NonDeterministicException {
      verifyDeterministic(this, "Expected key coder to be deterministic", keyCoder);
    }
  }

  /**
   * A shard descriptor containing shard id, the data block offset, and the index offset for the
   * given shard.
   */
  @AutoValue
  public abstract static class IsmShard {
    abstract int id();

    abstract long blockOffset();

    abstract long indexOffset();

    IsmShard() {}

    /** Returns an IsmShard with the given id, block offset and no index offset. */
    public static IsmShard of(int id, long blockOffset) {
      IsmShard ismShard = new AutoValue_IsmFormat_IsmShard(id, blockOffset, -1);
      checkState(id >= 0, "%s attempting to be written with negative shard id.", ismShard);
      checkState(
          blockOffset >= 0, "%s attempting to be written with negative block offset.", ismShard);
      return ismShard;
    }

    /** Returns an IsmShard with the given id, block offset, and index offset. */
    public static IsmShard of(int id, long blockOffset, long indexOffset) {
      IsmShard ismShard = new AutoValue_IsmFormat_IsmShard(id, blockOffset, indexOffset);
      checkState(id >= 0, "%s attempting to be written with negative shard id.", ismShard);
      checkState(
          blockOffset >= 0, "%s attempting to be written with negative block offset.", ismShard);
      checkState(
          indexOffset >= 0, "%s attempting to be written with negative index offset.", ismShard);
      return ismShard;
    }

    /** Return the shard id. */
    public int getId() {
      return id();
    }

    /** Return the absolute position within the Ism file where the data block begins. */
    public long getBlockOffset() {
      return blockOffset();
    }

    /**
     * Return the absolute position within the Ism file where the index block begins. Throws {@link
     * IllegalStateException} if the index offset was never specified.
     */
    public long getIndexOffset() {
      checkState(
          indexOffset() >= 0, "Unable to fetch index offset because it was never specified.");
      return indexOffset();
    }

    /** Returns a new IsmShard like this one with the specified index offset. */
    public IsmShard withIndexOffset(long indexOffset) {
      return of(id(), blockOffset(), indexOffset);
    }
  }

  /**
   * A {@link ListCoder} wrapping a {@link IsmShardCoder} used to encode the shard index. See {@link
   * ListCoder} for its encoding specification and {@link IsmShardCoder} for its encoding
   * specification.
   */
  public static final Coder<List<IsmShard>> ISM_SHARD_INDEX_CODER =
      ListCoder.of(IsmShardCoder.of());

  /**
   * A coder for {@link IsmShard}s.
   *
   * <p>The shard descriptor is encoded as:
   *
   * <ul>
   *   <li>id (variable length integer encoding)
   *   <li>blockOffset (variable length long encoding)
   *   <li>indexOffset (variable length long encoding)
   * </ul>
   */
  public static class IsmShardCoder extends AtomicCoder<IsmShard> {
    private static final IsmShardCoder INSTANCE = new IsmShardCoder();

    /** Returns an IsmShardCoder. */
    public static IsmShardCoder of() {
      return INSTANCE;
    }

    private IsmShardCoder() {}

    @Override
    public void encode(IsmShard value, OutputStream outStream) throws CoderException, IOException {
      checkState(
          value.getIndexOffset() >= 0, "%s attempting to be written without index offset.", value);
      VarIntCoder.of().encode(value.getId(), outStream);
      VarLongCoder.of().encode(value.getBlockOffset(), outStream);
      VarLongCoder.of().encode(value.getIndexOffset(), outStream);
    }

    @Override
    public IsmShard decode(InputStream inStream) throws CoderException, IOException {
      return IsmShard.of(
          VarIntCoder.of().decode(inStream),
          VarLongCoder.of().decode(inStream),
          VarLongCoder.of().decode(inStream));
    }

    @Override
    public void verifyDeterministic() {
      VarIntCoder.of().verifyDeterministic();
      VarLongCoder.of().verifyDeterministic();
    }

    @Override
    public boolean consistentWithEquals() {
      return true;
    }
  }

  /**
   * The prefix used before each key which contains the number of shared and unshared bytes from the
   * previous key that was read. The key prefix along with the previous key and the unshared key
   * bytes allows one to construct the current key by doing the following {@code currentKey =
   * previousKey[0 : sharedBytes] + read(unsharedBytes)}.
   *
   * <p>The key prefix is encoded as:
   *
   * <ul>
   *   <li>number of shared key bytes (variable length integer coding)
   *   <li>number of unshared key bytes (variable length integer coding)
   * </ul>
   */
  @AutoValue
  public abstract static class KeyPrefix {
    public abstract int getSharedKeySize();

    public abstract int getUnsharedKeySize();

    public static KeyPrefix of(int sharedKeySize, int unsharedKeySize) {
      return new AutoValue_IsmFormat_KeyPrefix(sharedKeySize, unsharedKeySize);
    }
  }

  /** A {@link Coder} for {@link KeyPrefix}. */
  public static final class KeyPrefixCoder extends AtomicCoder<KeyPrefix> {
    private static final KeyPrefixCoder INSTANCE = new KeyPrefixCoder();

    public static KeyPrefixCoder of() {
      return INSTANCE;
    }

    @Override
    public void encode(KeyPrefix value, OutputStream outStream) throws CoderException, IOException {
      VarInt.encode(value.getSharedKeySize(), outStream);
      VarInt.encode(value.getUnsharedKeySize(), outStream);
    }

    @Override
    public KeyPrefix decode(InputStream inStream) throws CoderException, IOException {
      return KeyPrefix.of(VarInt.decodeInt(inStream), VarInt.decodeInt(inStream));
    }

    @Override
    public void verifyDeterministic() {}

    @Override
    public boolean consistentWithEquals() {
      return true;
    }

    @Override
    public boolean isRegisterByteSizeObserverCheap(KeyPrefix value) {
      return true;
    }

    @Override
    public long getEncodedElementByteSize(KeyPrefix value) throws Exception {
      checkNotNull(value);
      return VarInt.getLength(value.getSharedKeySize())
          + (long) VarInt.getLength(value.getUnsharedKeySize());
    }
  }

  /**
   * The footer stores the relevant information required to locate the index and bloom filter. It
   * also stores a version byte and the number of keys stored.
   *
   * <p>The footer is encoded as the value containing:
   *
   * <ul>
   *   <li>start of bloom filter offset (big endian long coding)
   *   <li>start of shard index position offset (big endian long coding)
   *   <li>number of keys in file (big endian long coding)
   *   <li>0x01 (version key as a single byte)
   * </ul>
   */
  @AutoValue
  public abstract static class Footer {
    public static final int LONG_BYTES = 8;
    public static final int FIXED_LENGTH = 3 * LONG_BYTES + 1;
    public static final byte VERSION = 2;

    public abstract byte getVersion();

    public abstract long getIndexPosition();

    public abstract long getBloomFilterPosition();

    public abstract long getNumberOfKeys();

    public static Footer of(long indexPosition, long bloomFilterPosition, long numberOfKeys) {
      return new AutoValue_IsmFormat_Footer(
          VERSION, indexPosition, bloomFilterPosition, numberOfKeys);
    }
  }

  /** A {@link Coder} for {@link Footer}. */
  public static final class FooterCoder extends AtomicCoder<Footer> {
    private static final FooterCoder INSTANCE = new FooterCoder();

    public static FooterCoder of() {
      return INSTANCE;
    }

    @Override
    public void encode(Footer value, OutputStream outStream) throws CoderException, IOException {
      DataOutputStream dataOut = new DataOutputStream(outStream);
      dataOut.writeLong(value.getIndexPosition());
      dataOut.writeLong(value.getBloomFilterPosition());
      dataOut.writeLong(value.getNumberOfKeys());
      dataOut.write(Footer.VERSION);
    }

    @Override
    public Footer decode(InputStream inStream) throws CoderException, IOException {
      DataInputStream dataIn = new DataInputStream(inStream);
      Footer footer = Footer.of(dataIn.readLong(), dataIn.readLong(), dataIn.readLong());
      int version = dataIn.read();
      if (version != Footer.VERSION) {
        throw new IOException(
            "Unknown version " + version + ". " + "Only version 2 is currently supported.");
      }
      return footer;
    }

    @Override
    public void verifyDeterministic() {}

    @Override
    public boolean consistentWithEquals() {
      return true;
    }

    @Override
    public boolean isRegisterByteSizeObserverCheap(Footer value) {
      return true;
    }

    @Override
    public long getEncodedElementByteSize(Footer value) throws Exception {
      return Footer.FIXED_LENGTH;
    }
  }
}
