snippet.rb

module Calculator
  def self.calculate(expression)
    raise ArgumentError, 'Must be a string expression' unless expression.is_a? String
    raise ArgumentError, 'Must be numeric arithmetic' unless expression =~ /\d\s[\+\-\*\/]\s\d/
    
    parts = []
    def parts.combine(value)
      replace([value.to_s])
    end
    expression.split(/\s/).each do |val|
      parts << val
      if parts.size == 3
        [0, 2].each {|i| parts[i] = parts[i].to_i}
        case parts[1]
        when "+"
          parts.combine(parts[0] + parts[2])
        when "-"
          parts.combine(parts[0] - parts[2])
        when "/"
          parts.combine(parts[0] / parts[2])
        when "*"
          parts.combine(parts[0] * parts[2])
        end
      end
    end

    parts[0]
  end
end