guava/src/com/google/common/hash/BloomFilter.java

/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed 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 com.google.common.hash;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Objects;
import com.google.common.base.Predicate;
import com.google.common.hash.BloomFilterStrategies.LockFreeBitArray;
import com.google.common.math.DoubleMath;
import com.google.common.primitives.SignedBytes;
import com.google.common.primitives.UnsignedBytes;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.Serializable;
import java.math.RoundingMode;
import java.util.stream.Collector;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A Bloom filter for instances of {@code T}. A Bloom filter offers an approximate containment test
 * with one-sided error: if it claims that an element is contained in it, this might be in error,
 * but if it claims that an element is <i>not</i> contained in it, then this is definitely true.
 *
 * <p>If you are unfamiliar with Bloom filters, this nice <a
 * href="http://llimllib.github.com/bloomfilter-tutorial/">tutorial</a> may help you understand how
 * they work.
 *
 * <p>The false positive probability ({@code FPP}) of a Bloom filter is defined as the probability
 * that {@linkplain #mightContain(Object)} will erroneously return {@code true} for an object that
 * has not actually been put in the {@code BloomFilter}.
 *
 * <p>Bloom filters are serializable. They also support a more compact serial representation via the
 * {@link #writeTo} and {@link #readFrom} methods. Both serialized forms will continue to be
 * supported by future versions of this library. However, serial forms generated by newer versions
 * of the code may not be readable by older versions of the code (e.g., a serialized Bloom filter
 * generated today may <i>not</i> be readable by a binary that was compiled 6 months ago).
 *
 * <p>As of Guava 23.0, this class is thread-safe and lock-free. It internally uses atomics and
 * compare-and-swap to ensure correctness when multiple threads are used to access it.
 *
 * @param <T> the type of instances that the {@code BloomFilter} accepts
 * @author Dimitris Andreou
 * @author Kevin Bourrillion
 * @since 11.0 (thread-safe since 23.0)
 */
@Beta
public final class BloomFilter<T> implements Predicate<T>, Serializable {
  /**
   * A strategy to translate T instances, to {@code numHashFunctions} bit indexes.
   *
   * <p>Implementations should be collections of pure functions (i.e. stateless).
   */
  interface Strategy extends java.io.Serializable {

    /**
     * Sets {@code numHashFunctions} bits of the given bit array, by hashing a user element.
     *
     * <p>Returns whether any bits changed as a result of this operation.
     */
    <T> boolean put(
        T object, Funnel<? super T> funnel, int numHashFunctions, LockFreeBitArray bits);

    /**
     * Queries {@code numHashFunctions} bits of the given bit array, by hashing a user element;
     * returns {@code true} if and only if all selected bits are set.
     */
    <T> boolean mightContain(
        T object, Funnel<? super T> funnel, int numHashFunctions, LockFreeBitArray bits);

    /**
     * Identifier used to encode this strategy, when marshalled as part of a BloomFilter. Only
     * values in the [-128, 127] range are valid for the compact serial form. Non-negative values
     * are reserved for enums defined in BloomFilterStrategies; negative values are reserved for any
     * custom, stateful strategy we may define (e.g. any kind of strategy that would depend on user
     * input).
     */
    int ordinal();
  }

  /** The bit set of the BloomFilter (not necessarily power of 2!) */
  private final LockFreeBitArray bits;

  /** Number of hashes per element */
  private final int numHashFunctions;

  /** The funnel to translate Ts to bytes */
  private final Funnel<? super T> funnel;

  /** The strategy we employ to map an element T to {@code numHashFunctions} bit indexes. */
  private final Strategy strategy;

  /** Creates a BloomFilter. */
  private BloomFilter(
      LockFreeBitArray bits, int numHashFunctions, Funnel<? super T> funnel, Strategy strategy) {
    checkArgument(numHashFunctions > 0, "numHashFunctions (%s) must be > 0", numHashFunctions);
    checkArgument(
        numHashFunctions <= 255, "numHashFunctions (%s) must be <= 255", numHashFunctions);
    this.bits = checkNotNull(bits);
    this.numHashFunctions = numHashFunctions;
    this.funnel = checkNotNull(funnel);
    this.strategy = checkNotNull(strategy);
  }

  /**
   * Creates a new {@code BloomFilter} that's a copy of this instance. The new instance is equal to
   * this instance but shares no mutable state.
   *
   * @since 12.0
   */
  public BloomFilter<T> copy() {
    return new BloomFilter<T>(bits.copy(), numHashFunctions, funnel, strategy);
  }

  /**
   * Returns {@code true} if the element <i>might</i> have been put in this Bloom filter, {@code
   * false} if this is <i>definitely</i> not the case.
   */
  public boolean mightContain(T object) {
    return strategy.mightContain(object, funnel, numHashFunctions, bits);
  }

  /**
   * @deprecated Provided only to satisfy the {@link Predicate} interface; use {@link #mightContain}
   *     instead.
   */
  @Deprecated
  @Override
  public boolean apply(T input) {
    return mightContain(input);
  }

  /**
   * Puts an element into this {@code BloomFilter}. Ensures that subsequent invocations of {@link
   * #mightContain(Object)} with the same element will always return {@code true}.
   *
   * @return true if the Bloom filter's bits changed as a result of this operation. If the bits
   *     changed, this is <i>definitely</i> the first time {@code object} has been added to the
   *     filter. If the bits haven't changed, this <i>might</i> be the first time {@code object} has
   *     been added to the filter. Note that {@code put(t)} always returns the <i>opposite</i>
   *     result to what {@code mightContain(t)} would have returned at the time it is called.
   * @since 12.0 (present in 11.0 with {@code void} return type})
   */
  @CanIgnoreReturnValue
  public boolean put(T object) {
    return strategy.put(object, funnel, numHashFunctions, bits);
  }

  /**
   * Returns the probability that {@linkplain #mightContain(Object)} will erroneously return {@code
   * true} for an object that has not actually been put in the {@code BloomFilter}.
   *
   * <p>Ideally, this number should be close to the {@code fpp} parameter passed in {@linkplain
   * #create(Funnel, int, double)}, or smaller. If it is significantly higher, it is usually the
   * case that too many elements (more than expected) have been put in the {@code BloomFilter},
   * degenerating it.
   *
   * @since 14.0 (since 11.0 as expectedFalsePositiveProbability())
   */
  public double expectedFpp() {
    return Math.pow((double) bits.bitCount() / bitSize(), numHashFunctions);
  }

  /**
   * Returns an estimate for the total number of distinct elements that have been added to this
   * Bloom filter. This approximation is reasonably accurate if it does not exceed the value of
   * {@code expectedInsertions} that was used when constructing the filter.
   *
   * @since 22.0
   */
  public long approximateElementCount() {
    long bitSize = bits.bitSize();
    long bitCount = bits.bitCount();

    /**
     * Each insertion is expected to reduce the # of clear bits by a factor of
     * `numHashFunctions/bitSize`. So, after n insertions, expected bitCount is `bitSize * (1 - (1 -
     * numHashFunctions/bitSize)^n)`. Solving that for n, and approximating `ln x` as `x - 1` when x
     * is close to 1 (why?), gives the following formula.
     */
    double fractionOfBitsSet = (double) bitCount / bitSize;
    return DoubleMath.roundToLong(
        -Math.log1p(-fractionOfBitsSet) * bitSize / numHashFunctions, RoundingMode.HALF_UP);
  }

  /** Returns the number of bits in the underlying bit array. */
  @VisibleForTesting
  long bitSize() {
    return bits.bitSize();
  }

  /**
   * Determines whether a given Bloom filter is compatible with this Bloom filter. For two Bloom
   * filters to be compatible, they must:
   *
   * <ul>
   *   <li>not be the same instance
   *   <li>have the same number of hash functions
   *   <li>have the same bit size
   *   <li>have the same strategy
   *   <li>have equal funnels
   * </ul>
   *
   * @param that The Bloom filter to check for compatibility.
   * @since 15.0
   */
  public boolean isCompatible(BloomFilter<T> that) {
    checkNotNull(that);
    return this != that
        && this.numHashFunctions == that.numHashFunctions
        && this.bitSize() == that.bitSize()
        && this.strategy.equals(that.strategy)
        && this.funnel.equals(that.funnel);
  }

  /**
   * Combines this Bloom filter with another Bloom filter by performing a bitwise OR of the
   * underlying data. The mutations happen to <b>this</b> instance. Callers must ensure the Bloom
   * filters are appropriately sized to avoid saturating them.
   *
   * @param that The Bloom filter to combine this Bloom filter with. It is not mutated.
   * @throws IllegalArgumentException if {@code isCompatible(that) == false}
   * @since 15.0
   */
  public void putAll(BloomFilter<T> that) {
    checkNotNull(that);
    checkArgument(this != that, "Cannot combine a BloomFilter with itself.");
    checkArgument(
        this.numHashFunctions == that.numHashFunctions,
        "BloomFilters must have the same number of hash functions (%s != %s)",
        this.numHashFunctions,
        that.numHashFunctions);
    checkArgument(
        this.bitSize() == that.bitSize(),
        "BloomFilters must have the same size underlying bit arrays (%s != %s)",
        this.bitSize(),
        that.bitSize());
    checkArgument(
        this.strategy.equals(that.strategy),
        "BloomFilters must have equal strategies (%s != %s)",
        this.strategy,
        that.strategy);
    checkArgument(
        this.funnel.equals(that.funnel),
        "BloomFilters must have equal funnels (%s != %s)",
        this.funnel,
        that.funnel);
    this.bits.putAll(that.bits);
  }

  @Override
  public boolean equals(@Nullable Object object) {
    if (object == this) {
      return true;
    }
    if (object instanceof BloomFilter) {
      BloomFilter<?> that = (BloomFilter<?>) object;
      return this.numHashFunctions == that.numHashFunctions
          && this.funnel.equals(that.funnel)
          && this.bits.equals(that.bits)
          && this.strategy.equals(that.strategy);
    }
    return false;
  }

  @Override
  public int hashCode() {
    return Objects.hashCode(numHashFunctions, funnel, strategy, bits);
  }

  /**
   * Returns a {@code Collector} expecting the specified number of insertions, and yielding a {@link
   * BloomFilter} with false positive probability 3%.
   *
   * <p>Note that if the {@code Collector} receives significantly more elements than specified, the
   * resulting {@code BloomFilter} will suffer a sharp deterioration of its false positive
   * probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @return a {@code Collector} generating a {@code BloomFilter} of the received elements
   * @since 23.0
   */
  public static <T> Collector<T, ?, BloomFilter<T>> toBloomFilter(
      Funnel<? super T> funnel, long expectedInsertions) {
    return toBloomFilter(funnel, expectedInsertions, 0.03);
  }

  /**
   * Returns a {@code Collector} expecting the specified number of insertions, and yielding a {@link
   * BloomFilter} with the specified expected false positive probability.
   *
   * <p>Note that if the {@code Collector} receives significantly more elements than specified, the
   * resulting {@code BloomFilter} will suffer a sharp deterioration of its false positive
   * probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @param fpp the desired false positive probability (must be positive and less than 1.0)
   * @return a {@code Collector} generating a {@code BloomFilter} of the received elements
   * @since 23.0
   */
  public static <T> Collector<T, ?, BloomFilter<T>> toBloomFilter(
      Funnel<? super T> funnel, long expectedInsertions, double fpp) {
    checkNotNull(funnel);
    checkArgument(
        expectedInsertions >= 0, "Expected insertions (%s) must be >= 0", expectedInsertions);
    checkArgument(fpp > 0.0, "False positive probability (%s) must be > 0.0", fpp);
    checkArgument(fpp < 1.0, "False positive probability (%s) must be < 1.0", fpp);
    return Collector.of(
        () -> BloomFilter.create(funnel, expectedInsertions, fpp),
        BloomFilter::put,
        (bf1, bf2) -> {
          bf1.putAll(bf2);
          return bf1;
        },
        Collector.Characteristics.UNORDERED,
        Collector.Characteristics.CONCURRENT);
  }

  /**
   * Creates a {@link BloomFilter} with the expected number of insertions and expected false
   * positive probability.
   *
   * <p>Note that overflowing a {@code BloomFilter} with significantly more elements than specified,
   * will result in its saturation, and a sharp deterioration of its false positive probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @param fpp the desired false positive probability (must be positive and less than 1.0)
   * @return a {@code BloomFilter}
   */
  public static <T> BloomFilter<T> create(
      Funnel<? super T> funnel, int expectedInsertions, double fpp) {
    return create(funnel, (long) expectedInsertions, fpp);
  }

  /**
   * Creates a {@link BloomFilter} with the expected number of insertions and expected false
   * positive probability.
   *
   * <p>Note that overflowing a {@code BloomFilter} with significantly more elements than specified,
   * will result in its saturation, and a sharp deterioration of its false positive probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @param fpp the desired false positive probability (must be positive and less than 1.0)
   * @return a {@code BloomFilter}
   * @since 19.0
   */
  public static <T> BloomFilter<T> create(
      Funnel<? super T> funnel, long expectedInsertions, double fpp) {
    return create(funnel, expectedInsertions, fpp, BloomFilterStrategies.MURMUR128_MITZ_64);
  }

  @VisibleForTesting
  static <T> BloomFilter<T> create(
      Funnel<? super T> funnel, long expectedInsertions, double fpp, Strategy strategy) {
    checkNotNull(funnel);
    checkArgument(
        expectedInsertions >= 0, "Expected insertions (%s) must be >= 0", expectedInsertions);
    checkArgument(fpp > 0.0, "False positive probability (%s) must be > 0.0", fpp);
    checkArgument(fpp < 1.0, "False positive probability (%s) must be < 1.0", fpp);
    checkNotNull(strategy);

    if (expectedInsertions == 0) {
      expectedInsertions = 1;
    }
    /*
     * TODO(user): Put a warning in the javadoc about tiny fpp values, since the resulting size
     * is proportional to -log(p), but there is not much of a point after all, e.g.
     * optimalM(1000, 0.0000000000000001) = 76680 which is less than 10kb. Who cares!
     */
    long numBits = optimalNumOfBits(expectedInsertions, fpp);
    int numHashFunctions = optimalNumOfHashFunctions(expectedInsertions, numBits);
    try {
      return new BloomFilter<T>(new LockFreeBitArray(numBits), numHashFunctions, funnel, strategy);
    } catch (IllegalArgumentException e) {
      throw new IllegalArgumentException("Could not create BloomFilter of " + numBits + " bits", e);
    }
  }

  /**
   * Creates a {@link BloomFilter} with the expected number of insertions and a default expected
   * false positive probability of 3%.
   *
   * <p>Note that overflowing a {@code BloomFilter} with significantly more elements than specified,
   * will result in its saturation, and a sharp deterioration of its false positive probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @return a {@code BloomFilter}
   */
  public static <T> BloomFilter<T> create(Funnel<? super T> funnel, int expectedInsertions) {
    return create(funnel, (long) expectedInsertions);
  }

  /**
   * Creates a {@link BloomFilter} with the expected number of insertions and a default expected
   * false positive probability of 3%.
   *
   * <p>Note that overflowing a {@code BloomFilter} with significantly more elements than specified,
   * will result in its saturation, and a sharp deterioration of its false positive probability.
   *
   * <p>The constructed {@code BloomFilter} will be serializable if the provided {@code Funnel<T>}
   * is.
   *
   * <p>It is recommended that the funnel be implemented as a Java enum. This has the benefit of
   * ensuring proper serialization and deserialization, which is important since {@link #equals}
   * also relies on object identity of funnels.
   *
   * @param funnel the funnel of T's that the constructed {@code BloomFilter} will use
   * @param expectedInsertions the number of expected insertions to the constructed {@code
   *     BloomFilter}; must be positive
   * @return a {@code BloomFilter}
   * @since 19.0
   */
  public static <T> BloomFilter<T> create(Funnel<? super T> funnel, long expectedInsertions) {
    return create(funnel, expectedInsertions, 0.03); // FYI, for 3%, we always get 5 hash functions
  }

  // Cheat sheet:
  //
  // m: total bits
  // n: expected insertions
  // b: m/n, bits per insertion
  // p: expected false positive probability
  //
  // 1) Optimal k = b * ln2
  // 2) p = (1 - e ^ (-kn/m))^k
  // 3) For optimal k: p = 2 ^ (-k) ~= 0.6185^b
  // 4) For optimal k: m = -nlnp / ((ln2) ^ 2)

  /**
   * Computes the optimal k (number of hashes per element inserted in Bloom filter), given the
   * expected insertions and total number of bits in the Bloom filter.
   *
   * <p>See http://en.wikipedia.org/wiki/File:Bloom_filter_fp_probability.svg for the formula.
   *
   * @param n expected insertions (must be positive)
   * @param m total number of bits in Bloom filter (must be positive)
   */
  @VisibleForTesting
  static int optimalNumOfHashFunctions(long n, long m) {
    // (m / n) * log(2), but avoid truncation due to division!
    return Math.max(1, (int) Math.round((double) m / n * Math.log(2)));
  }

  /**
   * Computes m (total bits of Bloom filter) which is expected to achieve, for the specified
   * expected insertions, the required false positive probability.
   *
   * <p>See http://en.wikipedia.org/wiki/Bloom_filter#Probability_of_false_positives for the
   * formula.
   *
   * @param n expected insertions (must be positive)
   * @param p false positive rate (must be 0 < p < 1)
   */
  @VisibleForTesting
  static long optimalNumOfBits(long n, double p) {
    if (p == 0) {
      p = Double.MIN_VALUE;
    }
    return (long) (-n * Math.log(p) / (Math.log(2) * Math.log(2)));
  }

  private Object writeReplace() {
    return new SerialForm<T>(this);
  }

  private static class SerialForm<T> implements Serializable {
    final long[] data;
    final int numHashFunctions;
    final Funnel<? super T> funnel;
    final Strategy strategy;

    SerialForm(BloomFilter<T> bf) {
      this.data = LockFreeBitArray.toPlainArray(bf.bits.data);
      this.numHashFunctions = bf.numHashFunctions;
      this.funnel = bf.funnel;
      this.strategy = bf.strategy;
    }

    Object readResolve() {
      return new BloomFilter<T>(new LockFreeBitArray(data), numHashFunctions, funnel, strategy);
    }

    private static final long serialVersionUID = 1;
  }

  /**
   * Writes this {@code BloomFilter} to an output stream, with a custom format (not Java
   * serialization). This has been measured to save at least 400 bytes compared to regular
   * serialization.
   *
   * <p>Use {@linkplain #readFrom(InputStream, Funnel)} to reconstruct the written BloomFilter.
   */
  public void writeTo(OutputStream out) throws IOException {
    // Serial form:
    // 1 signed byte for the strategy
    // 1 unsigned byte for the number of hash functions
    // 1 big endian int, the number of longs in our bitset
    // N big endian longs of our bitset
    DataOutputStream dout = new DataOutputStream(out);
    dout.writeByte(SignedBytes.checkedCast(strategy.ordinal()));
    dout.writeByte(UnsignedBytes.checkedCast(numHashFunctions)); // note: checked at the c'tor
    dout.writeInt(bits.data.length());
    for (int i = 0; i < bits.data.length(); i++) {
      dout.writeLong(bits.data.get(i));
    }
  }

  /**
   * Reads a byte stream, which was written by {@linkplain #writeTo(OutputStream)}, into a {@code
   * BloomFilter}.
   *
   * <p>The {@code Funnel} to be used is not encoded in the stream, so it must be provided here.
   * <b>Warning:</b> the funnel provided <b>must</b> behave identically to the one used to populate
   * the original Bloom filter!
   *
   * @throws IOException if the InputStream throws an {@code IOException}, or if its data does not
   *     appear to be a BloomFilter serialized using the {@linkplain #writeTo(OutputStream)} method.
   */
  public static <T> BloomFilter<T> readFrom(InputStream in, Funnel<? super T> funnel)
      throws IOException {
    checkNotNull(in, "InputStream");
    checkNotNull(funnel, "Funnel");
    int strategyOrdinal = -1;
    int numHashFunctions = -1;
    int dataLength = -1;
    try {
      DataInputStream din = new DataInputStream(in);
      // currently this assumes there is no negative ordinal; will have to be updated if we
      // add non-stateless strategies (for which we've reserved negative ordinals; see
      // Strategy.ordinal()).
      strategyOrdinal = din.readByte();
      numHashFunctions = UnsignedBytes.toInt(din.readByte());
      dataLength = din.readInt();

      Strategy strategy = BloomFilterStrategies.values()[strategyOrdinal];
      long[] data = new long[dataLength];
      for (int i = 0; i < data.length; i++) {
        data[i] = din.readLong();
      }
      return new BloomFilter<T>(new LockFreeBitArray(data), numHashFunctions, funnel, strategy);
    } catch (RuntimeException e) {
      String message =
          "Unable to deserialize BloomFilter from InputStream."
              + " strategyOrdinal: "
              + strategyOrdinal
              + " numHashFunctions: "
              + numHashFunctions
              + " dataLength: "
              + dataLength;
      throw new IOException(message, e);
    }
  }
}