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neenjaw's solution

to Crypto Square in the Elixir Track

Published at Apr 24 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

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 "

Running tests

Execute the tests with:

$ elixir crypto_square_test.exs

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!"
end

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.

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.exs

if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("crypto_square.exs", __DIR__)
end

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)

defmodule CryptoSquareTest do
  use ExUnit.Case

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

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

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

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

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

  @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
  end
end
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 =
      str
      |> String.downcase()
      |> String.replace(~r/[^[:alnum:]]/u, "")
      |> String.graphemes()

    str_length = length(normalized_str)

    rows =
      str_length
      |> :math.sqrt()
      |> Float.round()
      |> Kernel.trunc()

    columns =
      if rows * rows < str_length do
        rows + 1
      else
        rows
      end

    normalized_str
    |> Kernel.++(List.duplicate("", ((rows*columns) - str_length)))
    |> Enum.chunk_every(columns)
    |> Enum.zip()
    |> Enum.map(fn column -> column |> Tuple.to_list() |> Enum.join() end)
    |> Enum.join(" ")
  end
end

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