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nickmcdonnough's solution

to Crypto Square in the Clojure Track

Published at Jul 13 2018 · 2 comments
Instructions
Test suite
Solution

Implement the classic method for composing secret messages called a square code.

Given an English text, output the encoded version of that text.

First, the input is normalized: the spaces and punctuation are removed from the English text and the message is downcased.

Then, the normalized characters are broken into rows. These rows can be regarded as forming a rectangle when printed with intervening newlines.

For example, the sentence

"If man was meant to stay on the ground, god would have given us roots."

is normalized to:

"ifmanwasmeanttostayonthegroundgodwouldhavegivenusroots"

The plaintext should be organized in to a rectangle. The size of the rectangle (r x c) should be decided by the length of the message, such that c >= r and c - r <= 1, where c is the number of columns and r is the number of rows.

Our normalized text is 54 characters long, dictating a rectangle with c = 8 and r = 7:

"ifmanwas"
"meanttos"
"tayonthe"
"groundgo"
"dwouldha"
"vegivenu"
"sroots  "

The coded message is obtained by reading down the columns going left to right.

The message above is coded as:

"imtgdvsfearwermayoogoanouuiontnnlvtwttddesaohghnsseoau"

Output the encoded text in chunks that fill perfect rectangles (r X c), with c chunks of r length, separated by spaces. For phrases that are n characters short of the perfect rectangle, pad each of the last n chunks with a single trailing space.

"imtgdvs fearwer mayoogo anouuio ntnnlvt wttddes aohghn  sseoau "

Notice that were we to stack these, we could visually decode the cyphertext back in to the original message:

"imtgdvs"
"fearwer"
"mayoogo"
"anouuio"
"ntnnlvt"
"wttddes"
"aohghn "
"sseoau "

Source

J Dalbey's Programming Practice problems http://users.csc.calpoly.edu/~jdalbey/103/Projects/ProgrammingPractice.html

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

crypto_square_test.clj

(ns crypto-square-test
  (:require [clojure.test :refer [deftest is]]
            crypto-square))

(deftest normalize-splunk
  (is (= "splunk" (crypto-square/normalize-plaintext "s#!@$%plunk"))))
(deftest normalize-with-punctuation
  (is (= "123go" (crypto-square/normalize-plaintext "1, 2, 3 GO!"))))

(deftest square-2
  (is (= 2 (crypto-square/square-size "1234"))))
(deftest square-3
  (is (= 3 (crypto-square/square-size "123456789"))))
(deftest square-4
  (is (= 4 (crypto-square/square-size "123456789abc"))))

(deftest segments
  (is (= ["neverv", "exthin", "eheart", "withid", "lewoes"]
         (crypto-square/plaintext-segments "Never vex thine heart with idle woes."))))
(deftest segments-2
  (is (= ["zomg", "zomb", "ies"]
         (crypto-square/plaintext-segments "ZOMG! ZOMBIES!!!"))))

(deftest cipher-1
  (is (= "tasneyinicdsmiohooelntuillibsuuml"
         (crypto-square/ciphertext "Time is an illusion. Lunchtime doubly so."))))
(deftest cipher-2
  (is (= "wneiaweoreneawssciliprerlneoidktcms"
         (crypto-square/ciphertext "We all know interspecies romance is weird."))))
(deftest cipher-3
  (is (= "msemo aanin dnin ndla etlt shui"
         (crypto-square/normalize-ciphertext "Madness, and then illumination."))))
(deftest cipher-4
  (is (= "vrel aepe mset paoo irpo"
         (crypto-square/normalize-ciphertext "Vampires are people too!"))))
(deftest cipher-5
  (is (= (str "ageihdsednsh lsagtoonaepe lannswnccair hrditeaetnrh "
              "ueethdnatoio mbqyewdnotto aouayicdwhod nranatosaef "
              "bnldrhnhrrb efirersodir irnieecusno nedgnailoat")
         (let [plaintext (str "All human beings are born free "
                              "and equal in dignity and rights. "
                              "They are endowed with reason and conscience "
                              "and should act towards one another "
                              "in a spirit of brotherhood.")]
           (crypto-square/normalize-ciphertext plaintext)))))
(ns crypto-square
  (:require [clojure.string :as str]))

(defn transpose [coll]
  (apply mapv vector coll))

(defn squarish-root [n]
  (-> n (Math/pow 0.5) Math/ceil int))

(defn normalize-plaintext [phrase]
  (->> phrase
       str/lower-case
       (re-seq #"\w")
       str/join))

(defn square-size [phrase]
  (squarish-root (count phrase)))

(defn plaintext-segments [phrase]
  (let [normal (normalize-plaintext phrase)
        size (square-size normal)]
    (->> normal
         str/lower-case
         (partition-all size)
         (map str/join)
         vec)))

(defn ciphertext [phrase]
  (let [normal (normalize-plaintext phrase)
        size (square-size normal)
        pts (plaintext-segments normal)
        diff (Math/abs (- size (count (last pts))))
        padding (take diff (cycle "@"))
        padded (if (empty? padding)
                 pts
                 (assoc pts (dec size) (flatten (concat (last pts) padding))))
        ciphered (transpose padded)]
    (->> ciphered
         normalize-plaintext
         (partition-all size)
         (map str/join)
         str/join)))

(defn normalize-ciphertext [phrase]
  (let [ciphered (ciphertext phrase)]
    (do
      (println ciphered))
    (->> ciphered
         (partition-all (-> ciphered square-size dec))
         (map str/join)
         (str/join " "))))

Community comments

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Avatar of alexkalderimis

I like the use of mapv. Most of the complexity in ciphertext can be avoided by using partition to produce nil padded groups, since when stringified with apply str the nulls coalesce to empty strings.

Avatar of nickmcdonnough
nickmcdonnough
Solution Author
commented over 6 years ago

@alexkalderimis awesome. thanks for the input!

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