Package org.apache.datasketches.sampling

Class VarOptItemsSketch<T>

  • Type Parameters:
    T - The type of object held in the sketch.
    public final class VarOptItemsSketch<T>
extends Object
    This sketch provides a variance optimal sample over an input stream of weighted items. The sketch can be used to compute subset sums over predicates, producing estimates with optimal variance for a given sketch size.

    Using this sketch with uniformly constant item weights (e.g. 1.0) will produce a standard reservoir sample over the steam.

    Author:
    Jon Malkin, Kevin Lang

    • Method Detail

      • newInstance

        public static <T> VarOptItemsSketch<T> newInstance​(int k)
        Construct a varopt sampling sketch with up to k samples using the default resize factor (8).
        Type Parameters:
        T - The type of object held in the sketch.
        Parameters:
        k - Maximum size of sampling. Allocated size may be smaller until sketch fills. Unlike many sketches in this package, this value does not need to be a power of 2.
        Returns:
        A VarOptItemsSketch initialized with maximum size k and resize factor rf.

      • newInstance

        public static <T> VarOptItemsSketch<T> newInstance​(int k,
                                                   ResizeFactor rf)
        Construct a varopt sampling sketch with up to k samples using the specified resize factor.
        Type Parameters:
        T - The type of object held in the sketch.
        Parameters:
        k - Maximum size of sampling. Allocated size may be smaller until sketch fills. Unlike many sketches in this package, this value does not need to be a power of 2. The maximum size is Integer.MAX_VALUE-1.
        rf - See Resize Factor
        Returns:
        A VarOptItemsSketch initialized with maximum size k and resize factor rf.

      • heapify

        public static <T> VarOptItemsSketch<T> heapify​(org.apache.datasketches.memory.Memory srcMem,
                                               ArrayOfItemsSerDe<T> serDe)
        Returns a sketch instance of this class from the given srcMem, which must be a Memory representation of this sketch class.
        Type Parameters:
        T - The type of item this sketch contains
        Parameters:
        srcMem - a Memory representation of a sketch of this class. See Memory
        serDe - An instance of ArrayOfItemsSerDe
        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 sketch. The current number of items in the sketch may be lower.
        Returns:
        k, the maximum number of samples in the sketch

      • 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 sketch, which may be smaller than the sketch capacity.
        Returns:
        the number of items currently in the sketch

      • getSketchSamples

        public VarOptItemsSamples<T> getSketchSamples()
        Gets a result iterator object.
        Returns:
        An object with an iterator over the results

      • update

        public void update​(T item,
                   double weight)
        Randomly decide whether or not to include an item in the sample set.
        Parameters:
        item - an item of the set being sampled from
        weight - a strictly positive weight associated with the item

      • 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.
        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 VarOptItemsSketch.
        Parameters:
        byteArr - the given byte array
        Returns:
        a human readable string of the preamble of a byte array image of a VarOptItemsSketch.

      • 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 VarOptItemsSketch.
        Parameters:
        mem - the given Memory
        Returns:
        a human readable string of the preamble of a Memory image of a VarOptItemsSketch.

      • toByteArray

        public byte[] toByteArray​(ArrayOfItemsSerDe<? super T> serDe)
        Returns a byte array representation of this sketch. May fail for polymorphic item types.
        Parameters:
        serDe - An instance of ArrayOfItemsSerDe
        Returns:
        a byte array representation of this sketch

      • toByteArray

        public byte[] toByteArray​(ArrayOfItemsSerDe<? super T> serDe,
                          Class<?> clazz)
        Returns a byte array representation of this sketch. Copies contents into an array of the specified class for serialization to allow for polymorphic types.
        Parameters:
        serDe - An instance of ArrayOfItemsSerDe
        clazz - The class represented by <T>
        Returns:
        a byte array representation of this sketch

      • estimateSubsetSum

        public SampleSubsetSummary estimateSubsetSum​(Predicate<T> 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.