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Introduction

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pold (partition and fold) is a Clojure library for

efficiently dividing data into any number of partitions and

accumulating them into a result.

Installation

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This library can be installed from Clojars[1].

Leiningen/Boot

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[de.dixieflatline/pold "0.2.0-SNAPSHOT"]

Usage

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Basic example

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pold applies a stateful partition and accumulation closure

to each value of a collection. The closure can be built with

partitioner which takes a set of partition functions.

Assume you want to partition numbers based on whether they

are even or odd and return the sum of each partition. In

this case, you could use odd? as key function. Whenever the

key changes, a new partition is created. The first number of

the partition is used as accumulator, which is initialized

with identity. Each subsequent number is added to the

accumulator, updating it.

  (require '[pold.core :refer [pold partitioner part]])

  (pold (partitioner (part odd? identity +))

        [1 1 2 4 3 9])

        ; => (2 6 12)

When no collection is provided pold returns a stateful transducer.

  (eduction (comp (pold (partitioner (part odd? identity +)))

                  (filter #(> % 2)))

            [1 1 2 4 3 9])

            ; => (6 12)

Nested partitions

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As described in the example above, partitioner accepts a set

of partition functions. This makes it possible to divide and

accumulate data into any number of partitions.

In the example below, an ordered list of artists, albums,

and songs is divided into three partitions, which are then

transformed into a list of nested maps.

  (def songs

    [{:artist "Aphex Twin"

      :album "On"

      :track 1

      :title "On"}

     {:artist "Aphex Twin"

      :album "On"

      :track 2

      :title "73-Yips"}

     {:artist "Aphex Twin"

      :album "On"

      :track 3

      :title "D-Scape"}

     {:artist "Aphex Twin"

      :album "On"

      :track 4

      :title "Xepha"}

     {:artist "Aphex Twin"

      :album "Blackbox Life Recorder 21f / In a Room7 F760"

      :track 1

      :title "Blackbox Life Recorder 21f"}

     {:artist "Aphex Twin"

      :album "Blackbox Life Recorder 21f / In a Room7 F760"

      :track 2

      :title "Zin2 Test5"}

     {:artist "Aphex Twin"

      :album "Blackbox Life Recorder 21f / In a Room7 F760"

      :track 3

      :title "In a Room7 F760"}

     {:artist "Aphex Twin"

      :album "Blackbox Life Recorder 21f / In a Room7 F760"

      :track 4

      :title "Blackbox Life Recorder 22 (Parallax mix)"}

     {:artist "Squarepusher"

      :album "Welcome to Europe"

      :track 1

      :title "Welcome to Europe"}

     {:artist "Squarepusher"

      :album "Welcome to Europe"

      :track 

      :title "Hanningfield Window"}

     {:artist "Squarepusher"

      :album "Welcome to Europe"

      :track 3

      :title "Exciton"}

     {:artist "Autechre"

      :album "Anti"

      :track 1

      :title

      "Lost"}

     {:artist"Autechre"

      :album "Anti"

      :track 2

      :title "Djarum"}

     {:artist "Autechre"

      :album "Anti"

      :track 3

      :title "Flutter"}])

(defn- make-partitioner

  []

  (partitioner

   (part :artist

         (fn [{:keys [artist]}]

           {:artist artist

            :albums []})

         #(update-in %1 [:albums] conj %2))

   (part :album

         (fn [{:keys [album]}]

           {:title album

            :songs []})

         #(update-in %1 [:songs] conj %2))

   (part :track

         (fn [{:keys [title]}]

           title))))

  (pold (make-partitioner) songs)

  ; => [{:artist "Aphex Twin", :albums [{:title "On"

  ;                                      :songs ["On"...

References

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Links

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