Rotational Cipher for a Technical Challenge

rotational_cipher.rb

class RotationalCipher
  
  def initialize(offset)
    while offset > 26
      offset -= 26
    end
    generate_cipher_map(offset)
  end

  class << self
    def rotx(offset, string, encrypt=true)
      instance = self.new(offset)
      encrypt ? instance.encrypt(string) : instance.decrypt(string)
    end
  end

  def encrypt(string)
    resolve_cipher(string, @cipher_map)
  end

  def decrypt(string)
    resolve_cipher(string, @cipher_map.invert)
  end

private
  def generate_cipher_map(offset)
    @cipher_map = {}
    letters = Array('a'..'z')
    cipher_letters = letters[offset..-1].concat(letters[0..offset-1])
    cipher_letters.each_with_index do |element, index|
      @cipher_map[letters[index]] = element
    end
  end
  
  def uppercase?(char)
    char != char.downcase ? true : false
  end

  def resolve_cipher(string, rot_map)
    result = ''
    string.each_char do |c|
      is_uppercase = uppercase?(c)
      c.downcase!
      if is_uppercase && rot_map.has_key?(c)
        result << rot_map[c].upcase
      elsif !is_uppercase && rot_map.has_key?(c)
        result << rot_map[c]
      else
        result << c
      end
    end
    result
  end

end

rotational_cipher_spec.rb

require 'rspec'
require File.expand_path('../rotational_cipher', __FILE__)

describe RotationalCipher do
  
  let(:rotx)   { RotationalCipher.new(5)  }
  let(:rotx_10){ RotationalCipher.new(10) }
  let(:rotx_36){ RotationalCipher.new(36) }

  describe 'can encrypt' do

    it 'a simple string' do
      code  = rotx.encrypt 'hi'
      code2 = rotx_10.encrypt 'hello'
      code3 = rotx_36.encrypt 'hello'
      
      code.should  eq('mn')
      code2.should eq('rovvy')
      code3.should eq('rovvy')
    end

    it 'a string with puncuation' do
      code  = rotx.encrypt 'hi!'
      code2 = rotx_10.encrypt 'hello,'
      code3 = rotx_36.encrypt 'hello,'

      code.should  eq('mn!')
      code2.should eq('rovvy,')
      code3.should eq('rovvy,')
    end

    it 'a string with whitespace' do
      code  = rotx.encrypt('hi dude')

      code.should eq('mn izij')
    end

    it 'a string with capitalization' do
      code  = rotx.encrypt('Hi Dude!')
      code2 = rotx_10.encrypt('Hello, World')

      code.should eq('Mn Izij!')
      code2.should eq('Rovvy, Gybvn')
    end

  end

  describe 'can decrypt' do
    it 'a simple string' do
      code = rotx.decrypt 'mn'
      code2 = rotx_10.decrypt 'rovvy'

      code.should eq('hi')
      code2.should eq('hello')
    end

    it 'a string with puncuation' do
      code = rotx.decrypt 'mn!'
      code2 = rotx_10.decrypt 'rovvy,'

      code.should eq('hi!')
      code2.should eq('hello,')
    end

    it 'a string with whitespace' do
      code  = rotx.decrypt('mn izij')

      code.should eq('hi dude')
    end

    it 'a string with capitalization' do
      code  = rotx.decrypt('Mn Izij!')
      code2 = rotx_10.decrypt('Rovvy, Gybvn')

      code.should eq('Hi Dude!')
      code2.should eq('Hello, World')
    end
  end

  describe 'class method for instance calling' do
    it 'mimics the above encrypt method' do
      code = RotationalCipher.rotx(10, 'Hello, World')
      code.should eq('Rovvy, Gybvn')
    end

    it 'calls a decrypt method on the instance' do
      code = RotationalCipher.rotx(10, 'Rovvy, Gybvn', false)
      code.should eq('Hello, World')
    end
  end

end