Class IntParallelCounterArray
- java.lang.Object
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- it.unimi.dsi.util.IntParallelCounterArray
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- All Implemented Interfaces:
java.io.Serializable
public class IntParallelCounterArray extends java.lang.Object implements java.io.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 ofregisterSize
bits. The first number depends on the desired relative standard deviation, and its logarithm can be computed usinglog2NumberOfRegisters(double)
, whereas the second number depends on an upper bound on the number of distinct elements to be counted, and it can be computed usingregisterSize(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
LongArrayBitVector
s, each containingCHUNK_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
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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 it.unimi.dsi.fastutil.longs.LongBigList[]
registers
registerSize
-bit views ofbitVector
.protected int
registerSize
The size in bits of each register.
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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.
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods 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()
it.unimi.dsi.fastutil.longs.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.
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Field Detail
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MAX_EXPONENT
public static final int MAX_EXPONENT
- See Also:
- Constant Field Values
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CHUNK_SHIFT
public static final int CHUNK_SHIFT
The logarithm of the maximum size in registers of a bit vector.- See Also:
- Constant Field Values
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CHUNK_SIZE
public static final long CHUNK_SIZE
The maximum size in registers of a bit vector.- See Also:
- Constant Field Values
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CHUNK_MASK
public static final long CHUNK_MASK
- See Also:
- Constant Field Values
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bitVector
protected final LongArrayBitVector[] bitVector
A an array of bit vectors containing all registers.
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registers
protected final it.unimi.dsi.fastutil.longs.LongBigList[] registers
registerSize
-bit views ofbitVector
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m
protected final int m
The number of registers.
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log2m
protected final int log2m
The number of registers.
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mMinus1
protected final int mMinus1
The number of registers minus one.
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registerSize
protected final int registerSize
The size in bits of each register.
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registerMask
protected final int registerMask
The mask corresponding to a register.
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nodeShift
protected final int nodeShift
The shift that selects the chunk corresponding to a node.
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base
protected double base
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Constructor Detail
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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.
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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.
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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
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Method Detail
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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
.
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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.
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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.
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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.
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printMins
public void printMins()
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registers
public it.unimi.dsi.fastutil.longs.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.
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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.
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