rot_n_jesse.rb

def rot_n_char(char, rot_by)
  fail ArgumentError, "First argument must be a 1-letter String (got `#{char}')" unless char.length == 1

  case char
  when ('a'..'z')
    ((char.ord - 'a'.ord + rot_by) % 26 + 'a'.ord).chr
  when ('A'..'Z')
    ((char.ord - 'A'.ord + rot_by) % 26 + 'A'.ord).chr
  else
    char
  end
end

def rot_n(string, rot_by)
  string.chars.map { |char| rot_n_char(char, rot_by) }.join
end

if __FILE__ == $0
  p rot_n("The quick brown fox jumps over the lazy dog.", 14) == "Hvs eiwqy pfckb tcl xiadg cjsf hvs zonm rcu."

  %w(abcdef bcdefg cdefgh defghi).each_with_index do |output_str, i|
    input_str = "abcdef"
    p rot_n(input_str, i)        == output_str
    p rot_n(input_str.upcase, i) == output_str.upcase
    p rot_n(input_str + input_str.upcase, i) == output_str + output_str.upcase
  end
end

rot_n_katz.rb

def rot_n(string, n)
  rotn_ary = []
  n = n % 26
  n > 13 ? n2 = (26 - n)  : n2 = n   # n2 for numbers greater than 13
  string.each_char do |char|
    if char.ord < 65 || (char.ord > 90 && char.ord < 97) || char.ord > 122
      rotn_ary.push(char)
    elsif "z".ord - n >= char.downcase.ord && "z".ord - char.downcase.ord + n >= 26
      rotn_ary.push((char.ord + n).chr)
    else
      rotn_ary.push((char.ord - n2).chr)
    end
  end
  rotn_ary.join

end

if __FILE__ == $0
  p rot_n("People who eat beef are the enemy of vegetarians.", 22)
  p rot_n("The quick brown fox jumps over the lazy dog.", 14) == "Hvs eiwqy pfckb tcl xiadg cjsf hvs zonm rcu."
  end