add substitution and differentiation

lib/ruby_algebra/interpreter.rb

@@ -50,7 +50,7 @@ module RubyAlgebra
           @locals[command.lhs] = result
           return "#{command.lhs} = #{result}"
         when :subs
-          result = operand1.evaluate(operand2)
+          result = operand1.substitute(operand2)
           @locals[command.lhs] = result
           return "#{command.lhs} = #{result}"
         end

lib/ruby_algebra/parser.rb

@@ -261,10 +261,17 @@ module RubyAlgebra
                 raise ParserError, "unexpected token #{n.type}, expected ="
               end
               n = tokenizer.next_token
+              unless n.type == :num || n.type == :plus || n.type == :minus
+                raise ParserError, "unexpected token #{n.type}, expected -, + or number"
+              end
+              if n.type == :plus || n.type == :minus
+                negative = true if n.type == :minus
+                n = tokenizer.next_token
+              end
               unless n.type == :num
                 raise ParserError, "unexpected token #{n.type}, expected number"
               end
-              substitutions[variable] = n.value
+              substitutions[variable] = if negative then -n.value else n.value end
             end
             n = tokenizer.next_token
             unless n.type == :paren && n.closing == true

lib/ruby_algebra/polynomial.rb

@@ -33,16 +33,12 @@ module RubyAlgebra
       result
     end
 
-    def diff
-      raise NotImplementedError
+    def diff(variable)
+      Polynomial.new(@terms.map { |term| term.diff(variable) })
     end
 
     def substitute(values)
-      result = 0
-      @terms.each do |term|
-        # ...
-      end
-      result
+      Polynomial.new(@terms.map { |term| term.substitute(values) })
     end
 
     def +(other)
@@ -190,12 +186,28 @@ module RubyAlgebra
       end
     end
 
-    def diff(var)
-      raise NotImplementedError
+    def diff(variable)
+      return Term.new if @variables[variable].nil?
+      new_coeff = @coeff
+      new_variables = @variables.filter { |var, power| var != variable }
+      unless @variables[variable] == 1
+        new_coeff *= @variables[variable]
+        new_variables[variable] = @variables[variable] - 1
+      end
+      Term.new(new_coeff, new_variables)
     end
 
     def substitute(values)
-      raise NotImplementedError
+      new_coeff = @coeff
+      new_variables = {}
+      @variables.each do |var, pow|
+        if values[var].nil?
+          new_variables[var] = pow
+        else
+          new_coeff *= values[var] ** pow
+        end
+      end
+      Term.new(new_coeff, new_variables)
     end
 
     def +(other)