implement interpreter and demo + bug fixes

- implement interpreter and demo
- replace StandardError with appropriate subclasses
- implement division
- rewrite Polynomial.simplify()

lib/ruby_algebra/interpreter.rb

@@ -0,0 +1,70 @@
+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