"Give me a Diamond"

give_me_a_diamond.rb

=begin
https://www.codewars.com/kata/5503013e34137eeeaa001648/train/ruby
Give me a Diamond

Jamie is a programmer, and James' girlfriend. She likes diamonds, and wants a diamond string from James. Since James doesn't know how to make this happen, he needs your help.

Task
You need to return a string that looks like a diamond shape when printed on the screen, using asterisk (*) characters. Trailing spaces should be removed, and every line must be terminated with a newline character (\n).

Return null/nil/None/... if the input is an even number or negative, as it is not possible to print a diamond of even or negative size.

Examples
A size 3 diamond:

 *
***
 *
...which would appear as a string of " *\n***\n *\n"

A size 5 diamond:

  *
 ***
*****
 ***
  *
...that is: " *\n ***\n*****\n ***\n *\n"

A size 7 diamond:

   *
  ***
 *****
*******
 *****
  ***
   *
=end

def diamond(n)
  # Make some diamonds!
  if (n.even? || n <= 0)
    nil
  else
    counter = n
    whitespace = 1
    middle = ("*" * n) + "\n"
    results = []
    results.push middle
    until counter.eql? 1
      counter = counter - 2
      step = (" " * whitespace) + ("*" * counter ) + "\n"
      whitespace = whitespace + 1
      results.push(step)
      results.unshift(step)
    end
    results.join("")
  end
end