[+]: Add Implementation of NFA

Author: AlexiaChen

machine/nfa.rb

@@ -33,4 +33,57 @@ def follow_rules_for(state, character)
   def rules_for(state, character)
     rules.select { |rule| rule.applies_to?(state, character) } # select * where 
   end
-end
+end
+
+=begin
+#<struct NFARuleSet rules=[#<FARule 1 --a--> 1>, #<FARule 1 --b--> 1>, #<FARule 1 --b--> 2>, 
+#<FARule 2 --a--> 3>, #<FARule 2 --b--> 3>, #<FARule 3 --a--> 4>, #<FARule 3 --b--> 4>]>
+=end
+ruleset = NFARuleSet.new([
+  FARule.new(1, 'a', 1), FARule.new(1, 'b', 1), FARule.new(1, 'b', 2),
+  FARule.new(2, 'a', 3), FARule.new(2, 'b', 3), FARule.new(3, 'a', 4),
+  FARule.new(3, 'b', 4)
+])
+
+# => #<Set: {1, 2}>
+ruleset.next_states(Set[1], 'b')
+# => #<Set: {1, 3, 4}>
+ruleset.next_states(Set[1,2,3], 'a')
+# => #<Set: {1, 2, 4}>
+ruleset.next_states(Set[1,3], 'b')
+
+class NFA < Struct.new(:current_states, :final_states, :ruleset)
+  def accepting?
+    (current_states & final_states).any?       # set intersection operation is empty?
+  end
+
+  def read_char(character)
+    self.current_states = ruleset.next_states(current_states,character)
+  end
+
+  def read_string(str)
+    str.chars.each do |character| 
+      read_char(character)
+    end
+  end
+end
+
+class NFAMaker < Struct.new(:start_state, :final_states, :ruleset)
+  def make_nfa
+    NFA.new(Set[start_state], final_states, ruleset)
+  end
+  
+  def accepts?(str)
+    make_nfa.tap { |nfa| nfa.read_string(str) }.accepting?
+  end
+end
+
+nfa = NFAMaker.new(1, [4], ruleset)
+# => true
+nfa.accepts?('bab')
+# => true
+nfa.accepts?('bababab')
+# => false
+nfa.accepts?('babababa')
+# => true
+nfa.accepts?('bababababbbbbb')