gistfile1.txt

# Create a simple calculator that given a string of operators (+ - * and /) 
# and numbers separated by spaces returns the value of that expression

# Example:

# Calculator.new.evaluate("2 / 2 + 3 * 4 - 6") # => 7
# Remember about the order of operations! Multiplications and divisions have 
# a higher priority and should be performed left-to-right. Additions and 
# subtractions have a lower priority and should also be performed left-to-right.

class Calculator
  def initialize
    @priority_operators = ['/', '*']
  end

  def evaluate(equation)
    operands = equation.split(' ').select.with_index {|v, i| i.even?}.map(&:to_i)
    operators = equation.split(' ').select.with_index {|v, i| i.odd?}
    operation(operands, operators)
  end

  def operation(operands, operators)
    return operands.pop if operands.size == 1

    operators.each_with_index do |val, index|
      if @priority_operators.include?(val)
        operands.insert(index, calculation(operands.slice!(index, 2), operators.slice!(index)))
        return operation(operands, operators)
      end
    end

    operands.unshift(calculation(operands.shift(2), operators.shift))
    operation(operands, operators)
  end

  def calculation(operands, operator)
    operands[0].send(operator, operands[1])
  end
end

puts Calculator.new.evaluate("2 / 2 + 3 * 4 - 6")