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michaelkpfeifer's solution

to Crypto Square in the Elixir Track

Published at Jul 12 2020 · 0 comments
Test suite

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:


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:

"sroots  "

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

The message above is coded as:


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:

"aohghn "
"sseoau "

Running tests

Execute the tests with:

$ mix test

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.


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.


defmodule CryptoSquareTest do
  use ExUnit.Case

  # @tag :pending
  test "empty string" do
    assert CryptoSquare.encode("") == ""

  @tag :pending
  test "perfect square" do
    assert CryptoSquare.encode("abcd") == "ac bd"

  @tag :pending
  test "uppercase string" do
    assert CryptoSquare.encode("ABCD") == "ac bd"

  @tag :pending
  test "small imperfect square" do
    assert CryptoSquare.encode("This is easy") == "tis hsy ie sa"

  @tag :pending
  test "punctuation and numbers" do
    assert CryptoSquare.encode("1, 2, 3, Go! Go, for God's sake!") == "1gga 2ook 3fde gos ors"

  @tag :pending
  test "long string" do
    msg = "If man was meant to stay on the ground, god would have given us roots."
    cipher = "imtgdvs fearwer mayoogo anouuio ntnnlvt wttddes aohghn sseoau"
    assert CryptoSquare.encode(msg) == cipher


ExUnit.configure(exclude: :pending, trace: true)
defmodule CryptoSquare do
  @doc """
  Encode string square methods
  ## Examples

    iex> CryptoSquare.encode("abcd")
    "ac bd"
  @spec encode(String.t()) :: String.t()
  def encode("") do

  def encode(str) do
    normalized_str = normalize(str)
    {nr_rows, nr_columns} = dimensions(normalized_str)

    String.pad_trailing(normalized_str, nr_rows * nr_columns)
    |> String.graphemes()
    |> Enum.chunk_every(nr_columns)
    |> Enum.zip()
    |> Enum.map(fn tuple ->
      |> Enum.join()
      |> String.replace(" ", "")
    |> Enum.join(" ")

  def normalize(str) do
    Regex.replace(~r/[^0-9a-z]/, String.downcase(str), "")

  def dimensions(str) do
    len = String.length(str)
    lower_bound = max(floor(:math.sqrt(len)) - 1, 1)
    upper_bound = ceil(:math.sqrt(len)) + 1

    for nr_rows <- lower_bound..upper_bound,
        nr_columns <- lower_bound..upper_bound,
        nr_rows * nr_columns >= len,
        nr_columns >= nr_rows,
        nr_columns - nr_rows <= 1 do
      {nr_rows, nr_columns}
    |> List.first()

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