calculator

calc.rb

NUM_REGEX = /^-?[0-9\.]+/

def calc(expression)
  expression = expression.gsub(' ', '').gsub('--', '+').gsub(/([\d\)])-([\(\d])/, '\1+-\2')
  calculate(expression)
end

def extract_brackets(expression)
  brackets_cnt = 0
  expression.chars.each_with_index do |char, i|
    brackets_cnt += 1 if char == '('
    brackets_cnt -= 1 if char == ')'
    next if brackets_cnt > 0
    brackets_expression = expression.slice!(0, i + 1)
    return calculate(brackets_expression[1...-1])
  end
end

def get_num(expression)
  return extract_brackets(expression) if expression[0] == '('
  if expression[0] == '-' && expression[1] == '('
    expression.slice!(0, 1)
    return -extract_brackets(expression)
  end
  num = expression.slice!(NUM_REGEX).to_f
end

def get_op(expression)
  expression.slice!(0)
end

def calculate(expression)
  expression = expression.gsub(' ', '').gsub(/(\d)-(\d)/, '\1+-\2')
  num = get_num(expression)
  until expression.empty?
    op = get_op(expression)
    case op
    when '+'
      return num + calc(expression)
    when '*'
      num *= get_num(expression)
    when '/'
      num /= get_num(expression)
    end
  end
  num
end