diamond.rb

class Array
  def palindromize
    _, *tail = reverse
    self + tail
  end
end

class String
  def make_diamond
    return "A\n" if eql? 'A'
    letters = 'A'..self
    width = (letters.count * 2) - 1
    blank_row = ' ' * width

    build_line = proc do |result, (letter, n)|
      result << blank_row.palindrome_insert(letter, n) + "\n"
    end

    letters.each_with_index.inject([], &build_line).palindromize.join
  end

  def midpoint
    raise ArgumentError if size.even?
    (size - 1) / 2
  end

  def palindrome_insert(letter, offset)
    result = dup
    result[midpoint + offset] = letter
    result[midpoint - offset] = letter
    result
  end
end

class Diamond
  def self.make_diamond(letter)
    letter.make_diamond
  end
end

Diamond.make_diamond('C')