Display a Fir Tree On Terminal

my_fir.rb

def usage
  abort 'Wrong number of argument: ruby my_fir.rb FIR_SIZE'
end

def display_trunk(width)
  trunk_width = trunk_height = ((width / 2) - 2).round
  (1..trunk_height).each { puts ("|" * trunk_width).center(width) }
end

def display_level(width, line_length)
  (1..3).each do |line|
    puts ("*" * line_length).center(width)
    line_length += 2  unless line == 3
  end
  line_length - 2
end

def length_of_last_line(fir_size)
  5 + ((fir_size - 1) * 2) # I expect to get a scholarship from MIT with this formula..
end

def my_fir(fir_size)
  width = length_of_last_line(fir_size)
  first_line_of_level = 1

  (1..fir_size).each do
    first_line_of_level = display_level(width, first_line_of_level)
  end
  display_trunk(width)
end

# Sorry for this C/C++ coding style :)
def main
  usage if ARGV.length != 1 || ARGV.first.to_i < 1
  my_fir(ARGV.first.to_i)
end

main

output

$> ruby my_fir.rb 3
    *    
   ***   
  *****  
   ***   
  *****  
 ******* 
  *****  
 ******* 
*********
   ||    
   ||    
$>