module RubyAlgebra
  class Interpreter
    def initialize
      @locals = {}
    end

    def execute(command)
      case command.type
      when :assignment
        unless command.operand1.is_a?(Polynomial)
          operand1 = @locals[command.operand1]
          return "Undeclared polynomial: #{command.operand1}" if operand1.nil?
        else
          operand1 = command.operand1
        end

        unless [:assign, :scale, :diff, :subs].include?(command.operation)
          operand2 = @locals[command.operand2]
          return "Undeclared polynomial: #{command.operand2}" if operand2.nil?
        else
          operand2 = command.operand2
        end

        case command.operation
        when :assign
          @locals[command.lhs] = operand1
          return "#{command.lhs} = #{operand1}"
        when :add
          result = operand1 + operand2
          @locals[command.lhs] = result
          return "#{command.lhs} = #{result}"
        when :sub
          result = operand1 - operand2
          @locals[command.lhs] = result
          return "#{command.lhs} = #{result}"
        when :mult, :scale
          result = operand1 * operand2
          @locals[command.lhs] = result
          return "#{command.lhs} = #{result}"
        when :div
          result = operand1 / operand2
          @locals[command.lhs[0]] = result[0]
          @locals[command.lhs[1]] = result[1]
          return "Quotient: #{command.lhs[0]} = #{result[0]}, remainder: #{command.lhs[1]} = #{result[1]}"
        when :diff
          result = operand1
          operand2.each do |variable|
            result = result.diff(variable)
          end
          @locals[command.lhs] = result
          return "#{command.lhs} = #{result}"
        when :subs
          result = operand1.evaluate(operand2)
          @locals[command.lhs] = result
          return "#{command.lhs} = #{result}"
        end
      when :display
        unless command.item.is_a? Polynomial
          unless @locals[command.item].nil?
            return @locals[command.item].to_s
          else
            return "Undeclared polynomial: #{command.item}"
          end
        else
          return command.item.to_s
        end
      end
    end
  end
end