Package org.apache.datasketches.sampling

Class ReservoirLongsSketch

java.lang.Object
org.apache.datasketches.sampling.ReservoirLongsSketch
public final class ReservoirLongsSketch extends Object
This sketch provides a reservoir sample over an input stream of longs. The sketch contains a uniform random sample of items from the stream.
Author:
Jon Malkin, Kevin Lang

  • Method Summary

    Modifier and Type
    Method
    Description
    SampleSubsetSummary
    estimateSubsetSum(Predicate<Long> predicate)
    Computes an estimated subset sum from the entire stream for objects matching a given predicate.
    int
    getK()
    Returns the sketch's value of k, the maximum number of samples stored in the reservoir.
    long
    getN()
    Returns the number of items processed from the input stream
    int
    getNumSamples()
    Returns the current number of items in the reservoir, which may be smaller than the reservoir capacity.
    long[]
    getSamples()
    Returns a copy of the items in the reservoir.
    static ReservoirLongsSketch
    heapify(org.apache.datasketches.memory.Memory srcMem)
    Returns a sketch instance of this class from the given srcMem, which must be a Memory representation of this sketch class.
    static ReservoirLongsSketch
    newInstance(int k)
    Construct a mergeable reservoir sampling sketch with up to k samples using the default resize factor (8).
    static ReservoirLongsSketch
    newInstance(int k, ResizeFactor rf)
    Construct a mergeable reservoir sampling sketch with up to k samples using the default resize factor (8).
    void
    reset()
    Resets this sketch to the empty state, but retains the original value of k.
    byte[]
    toByteArray()
    Returns a byte array representation of this sketch
    String
    toString()
    Returns a human-readable summary of the sketch, without items.
    static String
    toString(byte[] byteArr)
    Returns a human readable string of the preamble of a byte array image of a ReservoirLongsSketch.
    static String
    toString(org.apache.datasketches.memory.Memory mem)
    Returns a human readable string of the preamble of a Memory image of a ReservoirLongsSketch.
    void
    update(long item)
    Randomly decide whether or not to include an item in the sample set.

    Methods inherited from class java.lang.Object

    equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

  • Method Details

    • newInstance

      public static ReservoirLongsSketch newInstance(int k)
      Construct a mergeable reservoir sampling sketch with up to k samples using the default resize factor (8).
      Parameters:
      k - Maximum size of sampling. Allocated size may be smaller until sampling fills. Unlike many sketches in this package, this value does not need to be a power of 2.
      Returns:
      A ReservoirLongsSketch initialized with maximum size k and the default resize factor.

    • newInstance

      public static ReservoirLongsSketch newInstance(int k, ResizeFactor rf)
      Construct a mergeable reservoir sampling sketch with up to k samples using the default resize factor (8).
      Parameters:
      k - Maximum size of sampling. Allocated size may be smaller until sampling fills. Unlike many sketches in this package, this value does not need to be a power of 2.
      rf - See Resize Factor
      Returns:
      A ReservoirLongsSketch initialized with maximum size k and ResizeFactor rf.

    • heapify

      public static ReservoirLongsSketch heapify(org.apache.datasketches.memory.Memory srcMem)
      Returns a sketch instance of this class from the given srcMem, which must be a Memory representation of this sketch class.
      Parameters:
      srcMem - a Memory representation of a sketch of this class. See Memory
      Returns:
      a sketch instance of this class

    • getK

      public int getK()
      Returns the sketch's value of k, the maximum number of samples stored in the reservoir. The current number of items in the sketch may be lower.
      Returns:
      k, the maximum number of samples in the reservoir

    • getN

      public long getN()
      Returns the number of items processed from the input stream
      Returns:
      n, the number of stream items the sketch has seen

    • getNumSamples

      public int getNumSamples()
      Returns the current number of items in the reservoir, which may be smaller than the reservoir capacity.
      Returns:
      the number of items currently in the reservoir

    • getSamples

      public long[] getSamples()
      Returns a copy of the items in the reservoir. The returned array length may be smaller than the reservoir capacity.
      Returns:
      A copy of the reservoir array

    • update

      public void update(long item)
      Randomly decide whether or not to include an item in the sample set.
      Parameters:
      item - a unit-weight (equivalently, unweighted) item of the set being sampled from

    • reset

      public void reset()
      Resets this sketch to the empty state, but retains the original value of k.

    • toString

      public String toString()
      Returns a human-readable summary of the sketch, without items.
      Overrides:
      toString in class Object
      Returns:
      A string version of the sketch summary

    • toString

      public static String toString(byte[] byteArr)
      Returns a human readable string of the preamble of a byte array image of a ReservoirLongsSketch.
      Parameters:
      byteArr - the given byte array
      Returns:
      a human readable string of the preamble of a byte array image of a ReservoirLongsSketch.

    • toString

      public static String toString(org.apache.datasketches.memory.Memory mem)
      Returns a human readable string of the preamble of a Memory image of a ReservoirLongsSketch.
      Parameters:
      mem - the given Memory
      Returns:
      a human readable string of the preamble of a Memory image of a ReservoirLongsSketch.

    • toByteArray

      public byte[] toByteArray()
      Returns a byte array representation of this sketch
      Returns:
      a byte array representation of this sketch

    • estimateSubsetSum

      public SampleSubsetSummary estimateSubsetSum(Predicate<Long> predicate)
      Computes an estimated subset sum from the entire stream for objects matching a given predicate. Provides a lower bound, estimate, and upper bound using a target of 2 standard deviations.

      This is technically a heuristic method, and tries to err on the conservative side.

      Parameters:
      predicate - A predicate to use when identifying items.
      Returns:
      A summary object containing the estimate, upper and lower bounds, and the total sketch weight.