require "random"

class String
  @@nearby_chars = {
    'q' => ['w', 'a'],
    'w' => ['q', 'e', 's'],
    'e' => ['w', 'r', 'd'],
    'r' => ['e', 't', 'f'],
    't' => ['r', 'y', 'g'],
    'y' => ['t', 'h', 'u'],
    'u' => ['y', 'j', 'i'],
    'i' => ['u', 'k', 'o'],
    'o' => ['i', 'l', 'p'],
    'p' => ['o', 'l'],
    'a' => ['q', 's', 'z'],
    's' => ['w', 'r', 'a', 'x'],
    'd' => ['s', 'e', 'f', 'x', 'c'],
    'f' => ['d', 'r', 'x', 'c', 'v'],
    'g' => ['f', 't', 'h', 'v', 'b'],
    'h' => ['g', 'y', 'j', 'b', 'n'],
    'j' => ['h', 'u', 'k', 'n', 'm'],
    'k' => ['j', 'i', 'l', 'm'],
    'l' => ['k', 'o', 'p'],
    'z' => ['a', 's', 'x'],
    'x' => ['z', 's', 'd', 'c'],
    'c' => ['x', 'd', 'f', 'v'],
    'v' => ['c', 'f', 'g', 'b'],
    'b' => ['v', 'g', 'h', 'n'],
    'n' => ['b', 'h', 'j', 'm'],
    'm' => ['n', 'j', 'k', 'l'],
    ' ' => [' ']
  }

  def mutate
    return self if Random.rand > 0.1
    mutations = [
      ->mutate_modify,
      ->mutate_insert
    ]
    return mutations.sample(1)[0].call.mutate
  end

  def mutate_modify
    index = Random.rand(size)
    was_upcase = self[index].uppercase?
    char = self[index].downcase
    char = @@nearby_chars[char].sample(1)[0]
    char = char.upcase if was_upcase
    new_chars = chars
    new_chars[index] = char
    return new_chars.join("")
  end

  def mutate_insert
    index = Random.rand(size)
    return insert(index, ' ')
  end
end