com.yahoo.sketches.sampling

Class ReservoirSize

java.lang.Object

com.yahoo.sketches.sampling.ReservoirSize

public final class ReservoirSize
extends Object

This class provides a compact representation of reservoir size by encoding it into a fixed-point 16-bit value.

The value itself is a fractional power of 2, with 5 bits of exponent and 11 bits of mantissa. The exponent allows a choice of anywhere from 0-30, and there are 2048 possible reservoir size values within each octave. Because reservoir size must be an integer, this means that some sizes below 2048 may have multiple valid encodings.

Reservoir sizes can be specified exactly to 4096, and may be off by up to 0.03% thereafter. The value returned is always at least as large as the requested size, but may be larger.

NOTE: Numerical instability may cause an off-by-one error on reservoir size, causing a slight increase in storage over the optimal value.

Author:

Jon Malkin

Method Summary

Modifier and Type	Method and Description
static short	computeSize(int k) Given target reservoir size k, computes the smallest representable reservoir size that can hold k entries and returns it in a 16-bit fixed-point format as a short.
static int	decodeValue(short encodedSize) Decodes the 16-bit reservoir size value into an int.

Methods inherited from class java.lang.Object

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

Method Detail

computeSize

public static short computeSize(int k)

Given target reservoir size k, computes the smallest representable reservoir size that can hold k entries and returns it in a 16-bit fixed-point format as a short.

Parameters:

k - target reservoir size

Returns:

reservoir size as 16-bit encoded value

decodeValue

public static int decodeValue(short encodedSize)

Decodes the 16-bit reservoir size value into an int.

Parameters:

encodedSize - Encoded 16-bit value

Returns:

int represented by encodedSize