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tvirolai's solution

to ETL in the Common Lisp Track

Published at Mar 11 2020 · 0 comments
Test suite

We are going to do the Transform step of an Extract-Transform-Load.


Extract-Transform-Load (ETL) is a fancy way of saying, "We have some crufty, legacy data over in this system, and now we need it in this shiny new system over here, so we're going to migrate this."

(Typically, this is followed by, "We're only going to need to run this once." That's then typically followed by much forehead slapping and moaning about how stupid we could possibly be.)

The goal

We're going to extract some scrabble scores from a legacy system.

The old system stored a list of letters per score:

  • 1 point: "A", "E", "I", "O", "U", "L", "N", "R", "S", "T",
  • 2 points: "D", "G",
  • 3 points: "B", "C", "M", "P",
  • 4 points: "F", "H", "V", "W", "Y",
  • 5 points: "K",
  • 8 points: "J", "X",
  • 10 points: "Q", "Z",

The shiny new scrabble system instead stores the score per letter, which makes it much faster and easier to calculate the score for a word. It also stores the letters in lower-case regardless of the case of the input letters:

  • "a" is worth 1 point.
  • "b" is worth 3 points.
  • "c" is worth 3 points.
  • "d" is worth 2 points.
  • Etc.

Your mission, should you choose to accept it, is to transform the legacy data format to the shiny new format.


A final note about scoring, Scrabble is played around the world in a variety of languages, each with its own unique scoring table. For example, an "E" is scored at 2 in the Māori-language version of the game while being scored at 4 in the Hawaiian-language version.


Check out Installing Common Lisp for instructions to get started or take a look at the guides available in the track's side bar.


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Submitting Incomplete Solutions

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(ql:quickload "lisp-unit")
#-xlisp-test (load "etl")

(defpackage #:etl-test
  (:use #:common-lisp #:lisp-unit))

(in-package #:etl-test)

(defun make-hash (kvs)
   #'(lambda (h kv) (setf (gethash (first kv) h) (second kv)) h)
   :initial-value (make-hash-table :test 'equalp)))

(define-test transform-one-value
  (assert-equalp (make-hash '(("world" 1)))
      (etl:transform (make-hash '((1 ("WORLD")))))))

(define-test transform-more-values
  (assert-equalp (make-hash '(("world" 1) ("gschoolers" 1)))
      (etl:transform (make-hash '((1 ("WORLD" "GSCHOOLERS")))))))

(define-test more-keys
  (assert-equalp (make-hash '(("apple" 1) ("artichoke" 1) ("boat" 2) ("ballerina" 2)))
      (etl:transform (make-hash '((1 ("APPLE" "ARTICHOKE")) (2 ("BOAT" "BALLERINA")))))))

(define-test full-dataset
  (let ((input-data '((1 ("A" "E" "I" "O" "U" "L" "N" "R" "S" "T"))
                      (2 ("D" "G"))
                      (3 ("B" "C" "M" "P"))
                      (4 ("F" "H" "V" "W" "Y"))
                      (5 ("K"))
                      (8 ("J" "X"))
                      (10 ("Q" "Z"))))
        (expected-output '(("a" 1) ("b" 3) ("c" 3) ("d" 2) ("e" 1)
                           ("f" 4) ("g" 2) ("h" 4) ("i" 1) ("j" 8)
                           ("k" 5) ("l" 1) ("m" 3) ("n" 1) ("o" 1)
                           ("p" 3) ("q" 10) ("r" 1) ("s" 1) ("t" 1)
                           ("u" 1) ("v" 4) ("w" 4) ("x" 8) ("y" 4)
                           ("z" 10))))
    (assert-equalp (make-hash expected-output)
        (etl:transform (make-hash input-data)))))

(let ((*print-errors* t)
      (*print-failures* t))
  (run-tests :all :etl-test))
(in-package #:cl-user)
(defpackage #:etl
  (:use #:common-lisp)
  (:export #:transform))

(in-package #:etl)

(defun transform (data)
  (let ((new-data (make-hash-table :test #'equalp)))
    (maphash #'(lambda (k v)
                 (map 'list #'(lambda (char)
                                (setf (gethash (string-downcase char) new-data) k)) v))

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