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zorrus's solution

to Crypto Square in the Elixir Track

Published at Mar 02 2020 · 0 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 "

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!"
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

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

test_helper.exs

ExUnit.start()
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
    norm = normalize(str)
    norm_len = String.length(norm)
    {cols, rows} = get_size(norm_len)

    norm
    |> String.pad_trailing(cols * rows)
    |> split_string(cols)
    |> Enum.map(&String.split(&1, "", trim: true))
    |> Enum.zip
    |> Enum.map(fn x -> x |> Tuple.to_list |> Enum.join |> String.trim_trailing end)
    |> Enum.join(" ")

  end

  @spec normalize(String.t()) :: String.t()
  defp normalize(str) do
    str
    |> String.replace(~r/[[:punct:]]|[[:space:]]/u, "")
    |> String.downcase()
  end

  @spec split_string(String.t(), pos_integer())
  def split_string("", _len), do: []
  def split_string(str, len) do
    slen = str |> String.length
    [String.slice(str, 0, len) | split_string(String.slice(str, len, slen), len)]
  end

  @spec get_size(pos_integer()) :: {pos_integer(), pos_integer()}
  defp get_size(len) do

    make_pair = fn r ->
      case rem(len, r) do
        0 -> {div(len, r), r}
        _ -> {div(len, r) + 1, r}
      end
    end

    Range.new(1, len + 1)
    |> Enum.find(fn r ->
      c = div(len, r)
      c >= r and c - r <= 1
    end)
    |> make_pair.()
  end

end

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