CMSMonoid
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package algebird
One-sided error bound on the error of each point query, i.e. frequency estimate.
A bound on the probability that a query estimate does not lie within some small interval (an interval
that depends on eps) around the truth.
A seed to initialize the random number generator used to create the pairwise independent hash functions.
An Option parameter about how many exact counts a sparse CMS wants to keep.
These are from algebra.Monoid
Creates a sketch out of multiple items.
Creates a sketch out of a single item.
Combines the two sketches.
Combines the two sketches.
The sketches must use the same hash functions.
Returns an instance of
T calculated by summing all instances in iter in one pass.
Returns an instance of
T calculated by summing all instances in iter in one pass. Returns None if
iter is empty, else Some[
T].
None if iter is empty, else an option value containing the summed
T
Monoid for adding CMS sketches.
Usage
epsanddeltaare parameters that bound the error of each query estimate. For example, errors in answering point queries (e.g., how often has element x appeared in the stream described by the sketch?) are often of the form: "with probability p >= 1 - delta, the estimate is close to the truth by some factor depending on eps."The type
Kis the type of items you want to count. You must provide an implicitCMSHasher[K]forK, and Algebird ships with several such implicits for commonly used types such asLongandBigInt.If your type
Kis not supported out of the box, you have two options: 1) You provide a "translation" function to convert items of your (unsupported) typeKto a supported type such as Double, and then use thecontramapfunction of CMSHasher to create the requiredCMSHasher[K]for your type (see the documentation of CMSHasher for an example); 2) You implement aCMSHasher[K]from scratch, using the existing CMSHasher implementations as a starting point.Note: Because Arrays in Scala/Java not have sane
equalsandhashCodeimplementations, you cannot safely use types such asArray[Byte]. Extra work is required for Arrays. For example, you may opt to convertArray[T]to aSeq[T]viatoSeq, or you can provide appropriate wrapper classes. Algebird provides one such wrapper class, Bytes, to safely wrap anArray[Byte]for use with CMS.The type used to identify the elements to be counted. For example, if you want to count the occurrence of user names, you could map each username to a unique numeric ID expressed as a
Long, and then count the occurrences of thoseLongs with a CMS of typeK=Long. Note that this mapping between the elements of your problem domain and their identifiers used for counting via CMS should be bijective. We require a CMSHasher context bound forK, see CMSHasherImplicits for available implicits that can be imported. Which type K should you pick in practice? For domains that have less than2^64unique elements, you'd typically useLong. For larger domains you can tryBigInt, for example. Other possibilities include Spire'sSafeLongandNumericaldata types (https://github.com/non/spire), though Algebird does not include the required implicits for CMS-hashing (cf. CMSHasherImplicits.