Class IntParallelCounterArray
- All Implemented Interfaces:
Serializable
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 LongArrayBitVector
s, 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:
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Field Summary
FieldsModifier and TypeFieldDescriptionprotected double
protected final LongArrayBitVector[]
A an array of bit vectors containing all registers.static final long
static final int
The logarithm of the maximum size in registers of a bit vector.static final long
The maximum size in registers of a bit vector.protected final int
The number of registers.protected final int
The number of registers.static final int
protected final int
The number of registers minus one.protected final int
The shift that selects the chunk corresponding to a node.protected final int
The mask corresponding to a register.protected final it.unimi.dsi.fastutil.longs.LongBigList[]
registerSize
-bit views ofbitVector
.protected final int
The size in bits of each register. -
Constructor Summary
ConstructorsConstructorDescriptionIntParallelCounterArray
(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 TypeMethodDescriptionvoid
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
it.unimi.dsi.fastutil.longs.LongBigList[]
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 Details
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MAX_EXPONENT
public static final int MAX_EXPONENT- See Also:
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CHUNK_SHIFT
public static final int CHUNK_SHIFTThe logarithm of the maximum size in registers of a bit vector.- See Also:
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CHUNK_SIZE
public static final long CHUNK_SIZEThe maximum size in registers of a bit vector.- See Also:
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CHUNK_MASK
public static final long CHUNK_MASK- See Also:
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bitVector
A an array of bit vectors containing all registers. -
registers
protected final it.unimi.dsi.fastutil.longs.LongBigList[] registersregisterSize
-bit views ofbitVector
. -
m
protected final int mThe number of registers. -
log2m
protected final int log2mThe number of registers. -
mMinus1
protected final int mMinus1The number of registers minus one. -
registerSize
protected final int registerSizeThe size in bits of each register. -
registerMask
protected final int registerMaskThe mask corresponding to a register. -
nodeShift
protected final int nodeShiftThe shift that selects the chunk corresponding to a node. -
base
protected double base
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Constructor Details
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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 Details
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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() -
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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