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davelively14's solution

to Crypto Square in the Elixir Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code 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:


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:


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. Phrases that fill perfect rectangles (r X c) should be output c chunks of r length, separated by spaces. Phrases that do not fill perfect rectangles will have n empty spaces. Those spaces should be distributed evenly, added to the end of the last n chunks.

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:


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!"

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

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

For more detailed information about the Elixir track, please see the help page.


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.


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

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

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
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
    str = str |> String.downcase |> String.graphemes |> Enum.filter(&(&1 =~ ~r/[0-9a-z]/))
    length = str |> length
    cond do
      rem((:math.sqrt(length) * 10) |> round, 10) == 0 ->
        square(str, length |> :math.sqrt |> round)
      true ->
        non_square(str, length |> :math.sqrt |> Float.ceil |> round)

  def non_square(str, c), do: non_square(str, c, rem(length(str), c), init_map(c))
  def non_square([], _c, _tracker, result), do: result |> to_square
  def non_square(str, c, tracker, result) do
    case tracker > 0 do
      true ->
        {next_str, new_map} = put_chunk(str, c, result)
        non_square(next_str, c, tracker - 1, new_map)
      false ->
        {next_str, new_map} = put_chunk(str, c - 1, result)
        non_square(next_str, c, tracker, new_map)

  def square(str, c), do: square(str, c, init_map(c))
  def square([], _c, result), do: result |> to_square
  def square(str, c, result) do
    {next_str, new_map} = put_chunk(str, c, result)
    square(next_str, c, new_map)

  def put_chunk(list, max, result), do: put_chunk(list, max, result, 1)
  def put_chunk([], _max, result, _counter), do: {[], result}
  def put_chunk(list, max, result, counter) when counter > max, do: {list, result}
  def put_chunk([head | tail], max, result, counter) do
    put_chunk(tail, max, Map.update!(result, counter, &(&1 <> head)), counter + 1)

  def init_map(counter), do: init_map(counter, %{})
  def init_map(0, map), do: map
  def init_map(counter, map), do: init_map(counter - 1, Map.put(map, counter, ""))

  def to_square(map), do: to_square(map, Map.keys(map), "")
  def to_square(_map, [], result), do: result |> String.lstrip
  def to_square(map, [head | tail], result) do
    to_square(map, tail, result <> " " <> Map.get(map, head))

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