Package it.unimi.dsi.util

Class IntParallelCounterArray

java.lang.Object
it.unimi.dsi.util.IntParallelCounterArray
All Implemented Interfaces:
Serializable
public class IntParallelCounterArray
extends Object
implements Serializable
An array of approximate sets each represented using a Parallel counter.

Parallel counters represent the number of elements of a set in an approximate way. They have been introduced by Philippe Flajolet, Éric Fusy, Olivier Gandouet, and Freédeéric Meunier in “Parallel: the analysis of a near-optimal cardinality estimation algorithm”, Proceedings of the 13th conference on analysis of algorithm (AofA 07), pages 127−146, 2007. They are an improvement over the basic idea of loglog counting, introduced by Marianne Durand and Philippe Flajolet in “Loglog counting of large cardinalities”, ESA 2003, 11th Annual European Symposium, volume 2832 of Lecture Notes in Computer Science, pages 605−617, Springer, 2003.

Each counter is composed by m registers, and each register is made of registerSize bits. The first number depends on the desired relative standard deviation, and its logarithm can be computed using log2NumberOfRegisters(double), whereas the second number depends on an upper bound on the number of distinct elements to be counted, and it can be computed using registerSize(long).

Actually, this class implements an array of counters. Each counter is completely independent, but they all use the same hash function. The reason for this design is that in our intended applications hundred of millions of counters are common, and the JVM overhead to create such a number of objects would be unbearable. This class allocates an array of LongArrayBitVectors, each containing CHUNK_SIZE registers, and can thus handle billions of billions of registers efficiently (in turn, this means being able to handle an array of millions of billions of high-precision counters).

When creating an instance, you can choose the size of the array (i.e., the number of counters) and the desired relative standard deviation (either explicitly or choosing the number of registers per counter). Then, you can add an element to a counter. At any time, you can count count (approximately) the number of distinct elements that have been added to a counter.

Author:
Paolo Boldi, Sebastiano Vigna
See Also:
Serialized Form

  • Field Summary

    Fields 
    Modifier and Type Field Description
    protected double base  
    protected LongArrayBitVector[] bitVector
    A an array of bit vectors containing all registers.
    static long CHUNK_MASK  
    static int CHUNK_SHIFT
    The logarithm of the maximum size in registers of a bit vector.
    static long CHUNK_SIZE
    The maximum size in registers of a bit vector.
    protected int log2m
    The number of registers.
    protected int m
    The number of registers.
    static int MAX_EXPONENT  
    protected int mMinus1
    The number of registers minus one.
    protected int nodeShift
    The shift that selects the chunk corresponding to a node.
    protected int registerMask
    The mask corresponding to a register.
    protected LongBigList[] registers
    registerSize-bit views of bitVector.
    protected int registerSize
    The size in bits of each register.

  • Constructor Summary

    Constructors 
    Constructor Description
    IntParallelCounterArray​(int arraySize, long n, double rsd, double floatingPointPrecision)
    Creates a new array of counters.
    IntParallelCounterArray​(int arraySize, long n, int log2m, double floatingPointPrecision)
    Creates a new array of counters.
    IntParallelCounterArray​(int arraySize, long n, int log2m, double floatingPointPrecision, long seed)
    Creates a new array of counters.

  • Method Summary

    Modifier and Type Method Description
    void add​(int k, int v)
    Adds an element to a counter.
    double count​(int k)
    Estimates the number of distinct elements that have been added to a given counter so far.
    static int log2NumberOfRegisters​(double rsd)
    Returns the logarithm of the number of registers per counter that are necessary to attain a given relative standard deviation.
    void printMins()  
    LongBigList[] registers()
    Returns the array of big lists of registers underlying this array of counters.
    static int registerSize​(long n)
    Returns the register size in bits, given an upper bound on the number of distinct elements.
    static double relativeStandardDeviation​(int log2m)
    Returns the relative standard deviation corresponding to a given logarithm of the number of registers per counter.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Field Details

  • Constructor Details

    • IntParallelCounterArray

      public IntParallelCounterArray​(int arraySize, long n, double rsd, double floatingPointPrecision)
      Creates a new array of counters.
      Parameters:
      arraySize - the number of counters.
      n - the expected number of elements.
      rsd - the relative standard deviation.
      floatingPointPrecision - the precision used for floating-point computations.

    • IntParallelCounterArray

      public IntParallelCounterArray​(int arraySize, long n, int log2m, double floatingPointPrecision)
      Creates a new array of counters.
      Parameters:
      arraySize - the number of counters.
      n - the expected number of elements.
      log2m - the logarithm of the number of registers per counter.
      floatingPointPrecision - the precision used for floating-point computations.

    • IntParallelCounterArray

      public IntParallelCounterArray​(int arraySize, long n, int log2m, double floatingPointPrecision, long seed)
      Creates a new array of counters.
      Parameters:
      arraySize - the number of counters.
      n - the expected number of elements.
      log2m - the logarithm of the number of registers per counter.
      floatingPointPrecision - the precision used for floating-point computations.
      seed - the seed used to compute the hash function

  • Method Details

    • log2NumberOfRegisters

      public static int log2NumberOfRegisters​(double rsd)
      Returns the logarithm of the number of registers per counter that are necessary to attain a given relative standard deviation.
      Parameters:
      rsd - the relative standard deviation to be attained.
      Returns:
      the logarithm of the number of registers that are necessary to attain relative standard deviation rsd.

    • relativeStandardDeviation

      public static double relativeStandardDeviation​(int log2m)
      Returns the relative standard deviation corresponding to a given logarithm of the number of registers per counter.
      Parameters:
      log2m - the logarithm of the number of registers.
      Returns:
      the resulting relative standard deviation.

    • registerSize

      public static int registerSize​(long n)
      Returns the register size in bits, given an upper bound on the number of distinct elements.
      Parameters:
      n - an upper bound on the number of distinct elements.
      Returns:
      the register size in bits.

    • add

      public void add​(int k, int v)
      Adds an element to a counter.
      Parameters:
      k - the index of the counter.
      v - the element to be added.

    • printMins

      public void printMins()

    • registers

      public LongBigList[] registers()
      Returns the array of big lists of registers underlying this array of counters.

      The main purpose of this method is debugging, as it makes comparing the evolution of the state of two implementations easy.

      Returns:
      the array of big lists of registers underlying this array of counters.

    • count

      public double count​(int k)
      Estimates the number of distinct elements that have been added to a given counter so far.
      Parameters:
      k - the index of the counter.
      Returns:
      an approximation of the number of distinct elements that have been added to counter k so far.