module PatternMatching
  # Domain Specific Language style methods for inside block
  module DSL_INSIDE
    #A pattern matches description inside block
    #=== Usage
    # seems as {some pattern...} do ... end
    def as(&block)
      block
    end
    
    #A pattern restriction, must boolean
    #=== Usage
    # seems as {PATTERN}, with {CONDITION} do ... end
    def with(&block)
      block
    end
    
    #A pattern matches anything
    #=== Usage
    # seems something do ... end 
    def something
      proc {_}
    end    
  end
  
  # Domain Specific Language style methods for outside
  module DSL_OUTSIDE
    include DSL_INSIDE
    #Build structured data
    #=== Usage
    # build {[foo(bar, 100), foo(buzz, "abc")]}
    def build(&block)
      NodeBuilder.new.instance_eval(&block)
    end
    
    #Build structured data
    #=== Usage
    # PatternMatching.build {[foo(bar, 100), foo(buzz, "abc")]}
    def self.build(&block)
      NodeBuilder.new.instance_eval(&block)
    end
    
    #Do pattern matching 
    #===Usage
    # make TARGET do 
    #   seems as {PATTERN_1} do ACTION_1 end
    #   seems as {PATTERN_2} do ACTION_2 end
    #   seems something do ACTION_DEFAULT end
    # end
    def make(target, &block)
      patterns = []
      PatternFragments.new(patterns).instance_eval(&block)
      MatchExec.exec_as(target, patterns, self)
    end
    
    #Define method as partial style
    #=== Usage
    # func(NAME).seems as {PATTERN_1} do ACTION_1 end
    # func(NAME).seems as {PATTERN_2} do ACTION_2 end
    # func(NAME).seems something do ACTION_DEFAULT end
    #
    #or
    #
    # func NAME do 
    #   seems as {PATTERN_1} do ACTION_1 end
    #   seems as {PATTERN_2} do ACTION_2 end
    #   seems something do ACTION_DEFAULT end
    # end
    #
    def func(name, &block)
      pattern_name = ("@_pattern_" + name.to_s ).to_sym
      unless method_defined?(name)
        patterns = []
        instance_variable_set(pattern_name, patterns)
        define_method(name) do |target|
          MatchExec.exec_as(target, patterns, self)
        end
      end
      patterns = instance_variable_get(pattern_name)
      fragments = PatternFragments.new(patterns)
      if block
        fragments.instance_eval(&block)
      end
      fragments
    end
  end
  include DSL_OUTSIDE
  
  #Class for structured data/patterns
  class Node
    def initialize(name, children)
      @name = name
      @children = children
    end
    attr :name
    def [](index)
      @children[index]
    end
    def size
      @children.size
    end
  end
  
  private
  #Private Exception for stop matching
  class NotMatched < Exception
  end
  
  #Private class for build structured data
  class NodeBuilder
    private
    def method_missing(name, *args)
      Node.new(name, args)
    end
  end
  
  #Private module for pattern matching and to collect var/val pairs
  module Collector
    def self.walk(source, target, list)
      case source
      when Symbol
        list[source] = target
      when Node
        walk_node(source, target, list)
      when Hash
        walk_hash(source, target, list)
      when Enumerable
        walk_enumerable(source, target, list)
      else
        raise NotMatched.new unless source === target
      end
    end
    
    def self.walk_node(source, target, list)
      return if source.name == :_ and source.size == 0
      begin
        if source.size != target.size
          raise NotMatched.new 
        end
        if source.name != :_ and source.name != target.name
          raise NotMatched.new
        end
      rescue
        raise NotMatched.new
      end
      source.size.times do |i|
        walk(source[i], target[i], list)
      end
    end
    
    def self.walk_enumerable(source, target, list)
      if source.size == 0 and not (target == nil or target.size == 0)
        raise NotMatched.new 
      end
      source.size.times do |i|
        svalue = source[i]
        if i + 1 == source.size 
          if  svalue.class == Node and svalue.name == :_! and 
              svalue.size == 1
            tvalue = target[i .. -1]
            tvalue = if tvalue == nil then [] else tvalue end
            walk(svalue[0], tvalue, list)
          else
            raise NotMatched.new if source.size != target.size
            walk(svalue, target[i], list)
          end
        else
          raise NotMatched.new if target.size <= i
          walk(svalue, target[i], list)
        end
      end
    end
    
    def self.walk_hash(source, target, list)
      begin
        case target
        when Node
          raise NotMatched.new
        when Hash
          source.each do |key, svalue|
            walk(svalue, target[key], list)
          end
        else
          source.each do |key, svalue|
            walk(svalue, target.send(key), list)
          end
        end
      rescue
        raise NotMatched.new
      end
    end
  end
  
  #Private class to run pattern matching
  class MatchExec
    def self.exec_as(target, patterns, receiver)
      patterns.each do |tuple|
        pattern = tuple[0]
        action = tuple[1]
        condition = tuple[2]
        source = NodeBuilder.new.instance_eval(&pattern)
        args = {}
        begin
          Collector.walk(source, target, args)
        rescue NotMatched
          next
        end
        executer = ExecuteAs.new(args, receiver)
        next if condition != nil and not executer.instance_eval(&condition)
        return executer.instance_eval(&action)
      end
      nil
    end
    
    # Private class to access instance valiables of the receiver
    class InstanceVariableAccessor
      def initialize(receiver)
        @receiver = receiver
      end
      private
      def method_missing(name, *args)
        begin
          @receiver.send(name, *args)
        rescue NameError
          if name.to_s[-1,1] == "="
            field = "@" + name.to_s[0...-1]
            @receiver.instance_variable_set(field, args[0])
          else
            field = "@" + name.to_s
            @receiver.instance_variable_get(field, args[0])
          end
        end
      end
    end
    
    #Private class enabling to use the name of symbols in patterns 
    #like a local variables in action blocks
    class ExecuteAs
      def initialize(args, receiver)
        @receiver = receiver
        @wrapper = InstanceVariableAccessor.new receiver
        @args = args
      end
      def this
        @receiver
      end
      def the
        @wrapper
      end
      private
      def method_missing(name, *args)
        return @args[name] if @args.key?(name)
        @receiver.send(name, *args)
      end
    end
  end
  
  #Private class for collecting pattern/action fragments
  class PatternFragments
    include DSL_INSIDE
    
    def initialize(patterns)
      @patterns = patterns
    end
    def seems(pattern, condition = nil, &action)
      @patterns << [pattern, action, condition]
      self
    end
  end
end