https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * 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 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 not contained in it, then this is definitely true.

If you are unfamiliar with Bloom filters, this nice tutorial may help you understand how
they work.

The false positive probability (FPP) of a Bloom filter is defined as the probability that mightContain(Object) will erroneously return true for an object that has not actually been put in the BloomFilter. 
Bloom filters are serializable. They also support a more compact serial representation via the writeTo and 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 not be readable by a binary that was compiled 6 months ago).

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.

Author:Dimitris Andreou, Kevin Bourrillion
Type parameters:
  • <T> – the type of instances that the BloomFilter accepts
Since:11.0 (thread-safe since 23.0)
/**
 * 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 numHashFunctions bit indexes. 
Implementations should be collections of pure functions (i.e. stateless).

/**
   * 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 numHashFunctions bits of the given bit array, by hashing a user element. 
Returns whether any bits changed as a result of this operation.

/**
     * 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 numHashFunctions bits of the given bit array, by hashing a user element; returns true if and only if all selected bits are set. 
/**
     * 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).

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

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

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

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

  
Creates a BloomFilter. 
/** 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 BloomFilter that's a copy of this instance. The new instance is equal to this instance but shares no mutable state. 
Since:12.0
/**
   * 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 true if the element might have been put in this Bloom filter, 
false if this is definitely not the case.

/**
   * 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 Predicate interface; use mightContain instead.
/**
   * @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 BloomFilter. Ensures that subsequent invocations of mightContain(Object) with the same element will always return true. 
Returns:true if the Bloom filter's bits changed as a result of this operation. If the bits
    changed, this is definitely the first time object has been added to the filter. If the bits haven't changed, this might be the first time object has been added to the filter. Note that put(t) always returns the opposite result to what mightContain(t) would have returned at the time it is called.
Since:12.0 (present in 11.0 with void return type})
/**
   * 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 mightContain(Object) will erroneously return 
true for an object that has not actually been put in the BloomFilter. 
Ideally, this number should be close to the fpp parameter passed in create(Funnel<? super Object>, 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 BloomFilter, degenerating it. 
Since:14.0 (since 11.0 as expectedFalsePositiveProbability())
/**
   * 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() {
    // You down with FPP? (Yeah you know me!) Who's down with FPP? (Every last homie!)
    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 expectedInsertions that was used when constructing the filter. 
Since:22.0
/**
   * 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. 
/** 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:

    
  
  • not be the same instance
  
  • have the same number of hash functions
  
  • have the same bit size
  
  • have the same strategy
  
  • have equal funnels



Params:
  • that – The Bloom filter to check for compatibility.
Since:15.0
/**
   * 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 this instance. Callers must ensure the Bloom
filters are appropriately sized to avoid saturating them.

Params:
  • that – The Bloom filter to combine this Bloom filter with. It is not mutated.
Throws:
Since:15.0
/**
   * 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 Collector expecting the specified number of insertions, and yielding a BloomFilter with false positive probability 3%. 
Note that if the Collector receives significantly more elements than specified, the resulting BloomFilter will suffer a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
Returns:a Collector generating a BloomFilter of the received elements
Since:23.0
/**
   * 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 Collector expecting the specified number of insertions, and yielding a BloomFilter with the specified expected false positive probability. 
Note that if the Collector receives significantly more elements than specified, the resulting BloomFilter will suffer a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
  • fpp – the desired false positive probability (must be positive and less than 1.0)
Returns:a Collector generating a BloomFilter of the received elements
Since:23.0
/**
   * 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 BloomFilter with the expected number of insertions and expected false positive probability. 
Note that overflowing a BloomFilter with significantly more elements than specified, will result in its saturation, and a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
  • fpp – the desired false positive probability (must be positive and less than 1.0)
Returns:a BloomFilter
/**
   * 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 BloomFilter with the expected number of insertions and expected false positive probability. 
Note that overflowing a BloomFilter with significantly more elements than specified, will result in its saturation, and a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
  • fpp – the desired false positive probability (must be positive and less than 1.0)
Returns:a BloomFilter
Since:19.0
/**
   * 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 BloomFilter with the expected number of insertions and a default expected false positive probability of 3%. 
Note that overflowing a BloomFilter with significantly more elements than specified, will result in its saturation, and a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
Returns:a BloomFilter
/**
   * 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 BloomFilter with the expected number of insertions and a default expected false positive probability of 3%. 
Note that overflowing a BloomFilter with significantly more elements than specified, will result in its saturation, and a sharp deterioration of its false positive probability. 
The constructed BloomFilter will be serializable if the provided Funnel<T> is. 
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 equals also relies on object identity of funnels. 
Params:
  • funnel – the funnel of T's that the constructed BloomFilter will use
  • expectedInsertions – the number of expected insertions to the constructed 
    BloomFilter; must be positive
Returns:a BloomFilter
Since:19.0
/**
   * 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.

See http://en.wikipedia.org/wiki/File:Bloom_filter_fp_probability.svg for the formula.

Params:
  • n – expected insertions (must be positive)
  • m – total number of bits in Bloom filter (must be positive)
/**
   * 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.

See http://en.wikipedia.org/wiki/Bloom_filter#Probability_of_false_positives for the
formula.

Params:
  • n – expected insertions (must be positive)
  • p – false positive rate (must be 0 < p < 1)
/**
   * 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 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. 
Use readFrom(InputStream, Funnel<? super Object>) to reconstruct the written BloomFilter. 
/**
   * 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 writeTo(OutputStream), into a 
BloomFilter. 
The Funnel to be used is not encoded in the stream, so it must be provided here. Warning: the funnel provided must behave identically to the one used to populate
the original Bloom filter!

Throws:
/**
   * 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);
    }
  }
}