org.apache.lucene.codecs.bloom

Class FuzzySet

  • All Implemented Interfaces:
    Accountable


    public class FuzzySet
    extends Object
    implements Accountable

    A class used to represent a set of many, potentially large, values (e.g. many long strings such as URLs), using a significantly smaller amount of memory.

    The set is "lossy" in that it cannot definitively state that is does contain a value but it can definitively say if a value is not in the set. It can therefore be used as a Bloom Filter.

    Another application of the set is that it can be used to perform fuzzy counting because it can estimate reasonably accurately how many unique values are contained in the set.

    This class is NOT threadsafe.

    Internally a Bitset is used to record values and once a client has finished recording a stream of values the downsize(float) method can be used to create a suitably smaller set that is sized appropriately for the number of values recorded and desired saturation levels.

    • Method Detail

      • getNearestSetSize

        public static int getNearestSetSize(int maxNumberOfBits)
        Rounds down required maxNumberOfBits to the nearest number that is made up of all ones as a binary number. Use this method where controlling memory use is paramount.
      • getNearestSetSize

        public static int getNearestSetSize(int maxNumberOfValuesExpected,
                                            float desiredSaturation)
        Use this method to choose a set size where accuracy (low content saturation) is more important than deciding how much memory to throw at the problem.
        Parameters:
        desiredSaturation - A number between 0 and 1 expressing the % of bits set once all values have been recorded
        Returns:
        The size of the set nearest to the required size
      • contains

        public FuzzySet.ContainsResult contains(BytesRef value)
        The main method required for a Bloom filter which, given a value determines set membership. Unlike a conventional set, the fuzzy set returns NO or MAYBE rather than true or false.
        Returns:
        NO or MAYBE
      • serialize

        public void serialize(DataOutput out)
                       throws IOException
        Serializes the data set to file using the following format:
        • FuzzySet -->FuzzySetVersion,HashFunctionName,BloomSize, NumBitSetWords,BitSetWordNumBitSetWords
        • HashFunctionName --> String The name of a ServiceProvider registered HashFunction
        • FuzzySetVersion --> Uint32 The version number of the FuzzySet class
        • BloomSize --> Uint32 The modulo value used to project hashes into the field's Bitset
        • NumBitSetWords --> Uint32 The number of longs (as returned from FixedBitSet.getBits())
        • BitSetWord --> Long A long from the array returned by FixedBitSet.getBits()
        Parameters:
        out - Data output stream
        Throws:
        IOException - If there is a low-level I/O error
      • addValue

        public void addValue(BytesRef value)
                      throws IOException
        Records a value in the set. The referenced bytes are hashed and then modulo n'd where n is the chosen size of the internal bitset.
        Parameters:
        value - the key value to be hashed
        Throws:
        IOException - If there is a low-level I/O error
      • downsize

        public FuzzySet downsize(float targetMaxSaturation)
        Parameters:
        targetMaxSaturation - A number between 0 and 1 describing the % of bits that would ideally be set in the result. Lower values have better accuracy but require more space.
        Returns:
        a smaller FuzzySet or null if the current set is already over-saturated