Package it.unimi.dsi.sux4j.mph

Class TwoStepsGOV3Function<T>

java.lang.Object

it.unimi.dsi.fastutil.objects.AbstractObject2LongFunction<K>

it.unimi.dsi.sux4j.mph.AbstractHashFunction<T>

it.unimi.dsi.sux4j.mph.TwoStepsGOV3Function<T>

All Implemented Interfaces:

Function<T,java.lang.Long>, Object2LongFunction<T>, Size64, java.io.Serializable, java.util.function.Function<T,java.lang.Long>, java.util.function.ToLongFunction<T>

public class TwoStepsGOV3Function<T>
extends AbstractHashFunction<T>
implements java.io.Serializable, Size64

A function stored using two GOV3Functions—one for frequent values, and one for infrequent values. This naive idea turns out to be very effective in reducing the function size when the distribution of values is skewed (e.g., as it happens in a TwoStepsLcpMonotoneMinimalPerfectHashFunction).

To create an instance, we perform a pre-scan of the values to be assigned. If possible, we finds the best possible r such that the 2^r − 1 most frequent values can be stored in a GOV3Function and suitably remapped when read. The function uses 2^r − 1 as an escape symbol for all other values, which are stored in a separate function.

Warning: during the construction phase, a filter will be set on the ChunkedHashStore used to store the keys. If you are passing a store, you will have to reset it to its previous state.

Since:

4.0

Author:

Sebastiano Vigna

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static class	TwoStepsGOV3Function.Builder<T>	A builder class for TwoStepsGOV3Function.

Field Summary

Fields

Modifier and Type	Field	Description
protected int	escape	The escape value returned by firstFunction to suggest that secondFunction should be queried instead, provided that there is a first function.
protected GOV3Function<T>	firstFunction	The first function, or null.
protected long	n	The number of keys.
protected double	rankMean	The mean of the rank distribution.
protected long[]	remap	A mapping from values of the first function to actual values, provided that there is a first function.
protected GOV3Function<T>	secondFunction	The second function.
protected long	seed	The seed to be used when converting keys to triples.
static long	serialVersionUID
protected TransformationStrategy<? super T>	transform	The transformation strategy to turn objects of type T into bit vectors.
protected int	width	The width of the output of this function, in bits.

Fields inherited from class it.unimi.dsi.fastutil.objects.AbstractObject2LongFunction

defRetValue

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	TwoStepsGOV3Function(java.lang.Iterable<? extends T> keys, TransformationStrategy<? super T> transform, LongBigList values, java.io.File tempDir, ChunkedHashStore<T> chunkedHashStore)	Creates a new two-step function for the given keys and values.

Method Summary

Modifier and Type	Method	Description
long	getLong(java.lang.Object o)
long	getLongByTriple(long[] triple)
static void	main(java.lang.String[] arg)
long	numBits()	Returns the number of bits used by this structure.
long	size64()

Methods inherited from class it.unimi.dsi.sux4j.mph.AbstractHashFunction

containsKey, size

Methods inherited from class it.unimi.dsi.fastutil.objects.AbstractObject2LongFunction

defaultReturnValue, defaultReturnValue

Methods inherited from interface it.unimi.dsi.fastutil.Function

apply, clear

Methods inherited from interface java.util.function.Function

andThen, compose

Methods inherited from class java.lang.Object

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

Methods inherited from interface it.unimi.dsi.fastutil.objects.Object2LongFunction

applyAsLong, get, put, put, remove, removeLong

Methods inherited from interface it.unimi.dsi.fastutil.Size64

size

Field Detail

serialVersionUID

public static final long serialVersionUID

See Also:

Constant Field Values

n

protected final long n

The number of keys.

transform

protected final TransformationStrategy<? super T> transform

The transformation strategy to turn objects of type T into bit vectors.

firstFunction

protected final GOV3Function<T> firstFunction

The first function, or null. The special output value escape denotes that secondFunction should be queried instead.

secondFunction

protected final GOV3Function<T> secondFunction

The second function. All queries for which firstFunction returns escape (or simply all queries, if firstFunction is null) will be rerouted here.

remap

protected final long[] remap

A mapping from values of the first function to actual values, provided that there is a first function.

escape

protected final int escape

The escape value returned by firstFunction to suggest that secondFunction should be queried instead, provided that there is a first function.

seed

protected long seed

The seed to be used when converting keys to triples.

width

protected final int width

The width of the output of this function, in bits.

rankMean

protected final double rankMean

The mean of the rank distribution.

Constructor Detail

TwoStepsGOV3Function

protected TwoStepsGOV3Function(java.lang.Iterable<? extends T> keys,
                               TransformationStrategy<? super T> transform,
                               LongBigList values,
                               java.io.File tempDir,
                               ChunkedHashStore<T> chunkedHashStore)
                        throws java.io.IOException

Creates a new two-step function for the given keys and values.

Parameters:

keys - the keys in the domain of the function.

transform - a transformation strategy for the keys.

values - values to be assigned to each key, in the same order of the iterator returned by keys; if null, the assigned value will the ordinal number of each key.

tempDir - a temporary directory for the store files, or null for the standard temporary directory.

chunkedHashStore - a chunked hash store containing the keys associated with their rank, or null; the store can be unchecked, but in this case keys and transform must be non-null.

Throws:

java.io.IOException

Method Detail

getLong

public long getLong(java.lang.Object o)

Specified by:

getLong in interface Object2LongFunction<T>

getLongByTriple

public long getLongByTriple(long[] triple)

size64

public long size64()

Specified by:

size64 in interface Size64

Overrides:

size64 in class AbstractHashFunction<T>

numBits

public long numBits()

Returns the number of bits used by this structure.

Returns:

the number of bits used by this structure.

main

public static void main(java.lang.String[] arg)
                 throws java.lang.NoSuchMethodException,
                        java.io.IOException,
                        JSAPException

Throws:

java.lang.NoSuchMethodException

java.io.IOException

JSAPException