def modulo_3?(number)
  # (number % 3).zero? 
  number.modulo(3).zero? # i guess this is more idiomatic Ruby, though I like the previous better
end

def commify(number)
  reversed = number.to_s.reverse
  number_size = number.to_s.size
  
  result = ""

  number_size.times do |i|
    result << reversed[i]
    result << "," if modulo_3?(i + 1)
  end

  result.reverse
end

# > commify(25164150)
# => "25,164,150"
# > commify(1234567890)
# => "1,234,567,890"