gistfile1.rb

class StringCalculator
  def add(numbers)
    return 0 if numbers.empty?
    raise ArgumentError if is_negative?(numbers)
    return handle_more_than_one_number(extract_numbers(numbers), extract_delimiter(numbers)) if has_optional_delimiter(numbers)
    return handle_more_than_one_number(numbers, ",") if has_more_than_one_number?(numbers)
    return numbers.to_i
  end

  def is_negative?(numbers)
    numbers.index "-"
  end 

  def extract_numbers(numbers)
    numbers[4,numbers.size] 
  end 

  def extract_delimiter(numbers)
    numbers[2,1] 
  end 

  def has_more_than_one_number?(numbers)
    numbers.index ','
  end 

  def has_optional_delimiter(numbers)
    numbers.index '//'
  end 

  def handle_more_than_one_number(numbers, delimiter)
    numbers.gsub! "\n", delimiter 
    numbers.split(delimiter).inject(0) { |sum, num| sum + num.to_i }
  end 
end