Class CHDMinimalPerfectHashFunction<T>

  • All Implemented Interfaces:
    it.unimi.dsi.fastutil.Function<T,​java.lang.Long>, it.unimi.dsi.fastutil.objects.Object2LongFunction<T>, it.unimi.dsi.fastutil.Size64, java.io.Serializable, java.util.function.Function<T,​java.lang.Long>, java.util.function.ToLongFunction<T>

    public class CHDMinimalPerfectHashFunction<T>
    extends AbstractHashFunction<T>
    implements java.io.Serializable
    A minimal perfect hash function implemented using the “hash, displace and compress” technique.

    Given a list of keys without duplicates, the builder of this class finds a minimal perfect hash function for the list. Subsequent calls to the getLong(Object) method will return a distinct number for each key in the list. For keys out of the list, the resulting number is not specified. In some (rare) cases it might be possible to establish that a key was not in the original list, and in that case -1 will be returned; by signing the function (see below), you can guarantee with a prescribed probability that -1 will be returned on keys not in the original list. The class can then be saved by serialisation and reused later.

    This class uses a chunked hash store to provide highly scalable construction. Note that at construction time you can pass a it.unimi.dsi.sux4j.io.ChunkedHashStore containing the keys (associated with any value); however, if the store is rebuilt because of a DuplicateException it will be rebuilt associating with each key its ordinal position.

    The memory requirements for the algorithm we use are ≈2 bits per key for load factor equal to one and λ = 5. Thus, this class can use ≈10% less memory than a GOVMinimalPerfectHashFunction.

    However, its construction time is an order of magnitude larger, and query time is about 50% slower. Different tradeoffs between construction time, query time and space can be obtained by tweaking the load factor and the parameter λ (see the paper below for their exact meaning).

    For convenience, this class provides a main method that reads from standard input a (possibly gzip'd) sequence of newline-separated strings, and writes a serialised minimal perfect hash function for the given list.

    Signing

    Optionally, it is possible to sign the minimal perfect hash function. A w-bit signature will be associated with each key, so that getLong(Object) will return -1 on strings that are not in the original key set. As usual, false positives are possible with probability 2-w.

    How it Works

    The technique used is described by Djamal Belazzougui, Fabiano C. Botelho and Martin Dietzfelbinger in “Hash, displace and compress”, Algorithms - ESA 2009, LNCS 5757, pages 682−693, 2009. However, with respect to the algorithm described in the paper, this implementation is much more scalable, as it uses a ChunkedHashStore to split the generation of large key sets into generation of smaller functions for each chunk (of size approximately 216).

    Since:
    3.2.0
    Author:
    Sebastiano Vigna
    See Also:
    Serialized Form
    • Field Summary

      Fields 
      Modifier and Type Field Description
      protected EliasFanoLongBigList coefficients
      The displacement coefficients.
      protected long globalSeed
      The seed used to generate the initial hash triple.
      static int LOG2_CHUNK_SIZE
      The logarithm of the desired chunk size.
      protected long n
      The number of keys.
      protected SparseRank rank
      The sparse ranking structure containing the unused entries.
      static long serialVersionUID  
      protected long signatureMask
      The mask to compare signatures, or zero for no signatures.
      protected it.unimi.dsi.fastutil.longs.LongBigList signatures
      The signatures.
      protected it.unimi.dsi.bits.TransformationStrategy<? super T> transform
      The transformation strategy.
      • Fields inherited from class it.unimi.dsi.fastutil.objects.AbstractObject2LongFunction

        defRetValue
    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected CHDMinimalPerfectHashFunction​(java.lang.Iterable<? extends T> keys, it.unimi.dsi.bits.TransformationStrategy<? super T> transform, int lambda, double loadFactor, int signatureWidth, java.io.File tempDir, ChunkedHashStore<T> chunkedHashStore)
      Creates a new CHD minimal perfect hash function for the given keys.
    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      long getLong​(java.lang.Object key)  
      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.fastutil.objects.AbstractObject2LongFunction

        defaultReturnValue, defaultReturnValue
      • 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.Function

        apply, clear
      • Methods inherited from interface java.util.function.Function

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

        andThen, andThenByte, andThenChar, andThenDouble, andThenFloat, andThenInt, andThenLong, andThenObject, andThenReference, andThenShort, applyAsLong, composeByte, composeChar, composeDouble, composeFloat, composeInt, composeLong, composeObject, composeReference, composeShort, get, getOrDefault, getOrDefault, put, put, remove, removeLong
    • Field Detail

      • LOG2_CHUNK_SIZE

        public static final int LOG2_CHUNK_SIZE
        The logarithm of the desired chunk size.
        See Also:
        Constant Field Values
      • n

        protected final long n
        The number of keys.
      • globalSeed

        protected final long globalSeed
        The seed used to generate the initial hash triple.
      • transform

        protected final it.unimi.dsi.bits.TransformationStrategy<? super T> transform
        The transformation strategy.
      • rank

        protected final SparseRank rank
        The sparse ranking structure containing the unused entries.
      • signatureMask

        protected final long signatureMask
        The mask to compare signatures, or zero for no signatures.
      • signatures

        protected final it.unimi.dsi.fastutil.longs.LongBigList signatures
        The signatures.
    • Constructor Detail

      • CHDMinimalPerfectHashFunction

        protected CHDMinimalPerfectHashFunction​(java.lang.Iterable<? extends T> keys,
                                                it.unimi.dsi.bits.TransformationStrategy<? super T> transform,
                                                int lambda,
                                                double loadFactor,
                                                int signatureWidth,
                                                java.io.File tempDir,
                                                ChunkedHashStore<T> chunkedHashStore)
                                         throws java.io.IOException
        Creates a new CHD minimal perfect hash function for the given keys.
        Parameters:
        keys - the keys to hash, or null.
        transform - a transformation strategy for the keys.
        lambda - the average bucket size.
        loadFactor - the load factor.
        signatureWidth - a signature width, or 0 for no signature.
        tempDir - a temporary directory for the store files, or null for the standard temporary directory.
        chunkedHashStore - a chunked hash store containing the keys, or null; the store can be unchecked, but in this case keys and transform must be non-null.
        Throws:
        java.io.IOException
    • Method Detail

      • numBits

        public long numBits()
        Returns the number of bits used by this structure.
        Returns:
        the number of bits used by this structure.
      • getLong

        public long getLong​(java.lang.Object key)
        Specified by:
        getLong in interface it.unimi.dsi.fastutil.objects.Object2LongFunction<T>
      • size64

        public long size64()
        Specified by:
        size64 in interface it.unimi.dsi.fastutil.Size64
        Overrides:
        size64 in class AbstractHashFunction<T>
      • main

        public static void main​(java.lang.String[] arg)
                         throws java.lang.NoSuchMethodException,
                                java.io.IOException,
                                com.martiansoftware.jsap.JSAPException
        Throws:
        java.lang.NoSuchMethodException
        java.io.IOException
        com.martiansoftware.jsap.JSAPException