# encoding: UTF-8
module Asciidoctor
# Extensions provide a way to participate in the parsing and converting
# phases of the AsciiDoc processor or extend the AsciiDoc syntax.
#
# The various extensions participate in AsciiDoc processing as follows:
#
# 1. After the source lines are normalized, {Preprocessor}s modify or replace
#    the source lines before parsing begins.  {IncludeProcessor}s are used to
#    process include directives for targets which they claim to handle.
# 2. The Parser parses the block-level content into an abstract syntax tree.
#    Custom blocks and block macros are processed by associated {BlockProcessor}s
#    and {BlockMacroProcessor}s, respectively.
# 3. {Treeprocessor}s are run on the abstract syntax tree.
# 4. Conversion of the document begins, at which point inline markup is processed
#    and converted. Custom inline macros are processed by associated {InlineMacroProcessor}s.
# 5. {Postprocessor}s modify or replace the converted document.
# 6. The output is written to the output stream.
#
# Extensions may be registered globally using the {Extensions.register} method
# or added to a custom {Registry} instance and passed as an option to a single
# Asciidoctor processor.
module Extensions

  # Public: An abstract base class for document and syntax processors.
  #
  # This class provides access to a class-level Hash for holding default
  # configuration options defined using the {Processor.option} method. This
  # style of default configuration is specific to the native Ruby environment
  # and is only consulted inside the initializer. An overriding configuration
  # Hash can be passed to the initializer. Once the processor is initialized,
  # the configuration is accessed using the {Processor#config} instance variable.
  #
  # Instances of the Processor class provide convenience methods for creating
  # AST nodes, such as Block and Inline, and for parsing child content.
  class Processor
    class << self
      # Public: Get the static configuration for this processor class.
      # 
      # Returns a configuration [Hash]
      def config
        @config ||= {}
      end

      # Public: Assigns a default value for the specified option that gets
      # applied to all instances of this processor.
      #
      # Examples
      #
      #   option :contexts, [:open, :paragraph]
      #
      # Returns nothing
      def option key, default_value
        config[key] = default_value
      end

      # Include the DSL class for this processor into this processor class or instance.
      #
      # This method automatically detects whether to use the include or extend keyword
      # based on what is appropriate.
      #
      # NOTE Inspiration for this DSL design comes from https://corcoran.io/2013/09/04/simple-pattern-ruby-dsl/
      #
      # Returns nothing
      def use_dsl
        if self.name.nil_or_empty?
          # NOTE contants(false) doesn't exist in Ruby 1.8.7
          #include const_get :DSL if constants(false).grep :DSL
          include const_get :DSL if constants.grep :DSL
        else
          # NOTE contants(false) doesn't exist in Ruby 1.8.7
          #extend const_get :DSL if constants(false).grep :DSL
          extend const_get :DSL if constants.grep :DSL
        end
      end
      alias :extend_dsl :use_dsl
      alias :include_dsl :use_dsl
    end

    # Public: Get the configuration Hash for this processor instance.
    attr_reader :config

    def initialize config = {}
      @config = self.class.config.merge config
    end

    def update_config config
      @config.update config
    end

    def process *args
      raise ::NotImplementedError
    end

    def create_block parent, context, source, attrs, opts = {}
      Block.new parent, context, { :source => source, :attributes => attrs }.merge(opts)
    end

    def create_image_block parent, attrs, opts = {}
      create_block parent, :image, nil, attrs, opts
    end

    def create_inline parent, context, text, opts = {}
      Inline.new parent, context, text, opts
    end

    # Public: Parses blocks in the content and attaches the block to the parent.
    #
    # Returns nothing
    #--
    # QUESTION is parse_content the right method name? should we wrap in open block automatically?
    def parse_content parent, content, attributes = {}
      reader = (content.is_a? Reader) ? content : (Reader.new content)
      while reader.has_more_lines?
        block = Parser.next_block reader, parent, attributes
        parent << block if block
      end
      nil
    end

    # TODO fill out remaining methods
    [
      [:create_paragraph,     :create_block,  :paragraph],
      [:create_open_block,    :create_block,  :open],
      [:create_example_block, :create_block,  :example],
      [:create_pass_block,    :create_block,  :pass],
      [:create_listing_block, :create_block,  :listing],
      [:create_literal_block, :create_block,  :literal],
      [:create_anchor,        :create_inline, :anchor]
    ].each do |method_name, delegate_method_name, context|
      define_method method_name do |*args|
        send delegate_method_name, *args.dup.insert(1, context)
      end
    end
  end

  # Internal: Overlays a builder DSL for configuring the Processor instance.
  # Includes a method to define configuration options and another to define the
  # {Processor#process} method.
  module ProcessorDsl
    def option key, value
      config[key] = value
    end

    def process *args, &block
      # need to check for both block/proc and lambda
      # TODO need test for this!
      #if block_given? || (args.size == 1 && ((block = args[0]).is_a? ::Proc))
      if block_given?
        @process_block = block
      elsif @process_block
        # NOTE Proc automatically expands a single array argument
        # ...but lambda doesn't (and we want to accept lambdas too)
        # TODO need a test for this!
        @process_block.call(*args)
      else
        raise ::NotImplementedError
      end
    end
    #alias :process_with :process

    def process_block_given?
      defined? @process_block
    end
  end

  # Public: Preprocessors are run after the source text is split into lines and
  # normalized, but before parsing begins.
  #
  # Prior to invoking the preprocessor, Asciidoctor splits the source text into
  # lines and normalizes them. The normalize process strips trailing whitespace
  # from each line and leaves behind a line-feed character (i.e., "\n").
  #
  # Asciidoctor passes a reference to the Reader and a copy of the lines Array
  # to the {Processor#process} method of an instance of each registered
  # Preprocessor. The Preprocessor modifies the Array as necessary and either
  # returns a reference to the same Reader or a reference to a new Reader.
  #
  # Preprocessor implementations must extend the Preprocessor class.
  class Preprocessor < Processor
    def process document, reader
      raise ::NotImplementedError
    end
  end
  Preprocessor::DSL = ProcessorDsl

  # Public: Treeprocessors are run on the Document after the source has been
  # parsed into an abstract syntax tree (AST), as represented by the Document
  # object and its child Node objects (e.g., Section, Block, List, ListItem).
  #
  # Asciidoctor invokes the {Processor#process} method on an instance of each
  # registered Treeprocessor.
  #
  # Treeprocessor implementations must extend Treeprocessor.
  #--
  # QUESTION should the treeprocessor get invoked after parse header too?
  class Treeprocessor < Processor
    def process document
      raise ::NotImplementedError
    end
  end
  Treeprocessor::DSL = ProcessorDsl

  # Public: Postprocessors are run after the document is converted, but before
  # it is written to the output stream.
  #
  # Asciidoctor passes a reference to the converted String to the {Processor#process}
  # method of each registered Postprocessor. The Preprocessor modifies the
  # String as necessary and returns the String replacement.
  #
  # The markup format in the String is determined by the backend used to convert
  # the Document. The backend and be looked up using the backend method on the
  # Document object, as well as various backend-related document attributes.
  #
  # TIP: Postprocessors can also be used to relocate assets needed by the published
  # document.
  #
  # Postprocessor implementations must Postprocessor.
  class Postprocessor < Processor
    def process document, output
      raise ::NotImplementedError
    end
  end
  Postprocessor::DSL = ProcessorDsl

  # Public: IncludeProcessors are used to process `include::<target>[]`
  # directives in the source document.
  #
  # When Asciidoctor comes across a `include::<target>[]` directive in the
  # source document, it iterates through the IncludeProcessors and delegates
  # the work of reading the content to the first processor that identifies
  # itself as capable of handling that target.
  #
  # IncludeProcessor implementations must extend IncludeProcessor.
  #--
  # TODO add file extension or regexp to shortcut handles?
  class IncludeProcessor < Processor
    def process document, reader, target, attributes
      raise ::NotImplementedError
    end

    def handles? target
      true
    end
  end
  IncludeProcessor::DSL = ProcessorDsl

  # Public: DocinfoProcessors are used to add additional content to
  # the header and/or footer of the generated document.
  #
  # The placement of docinfo content is controlled by the converter.
  #
  # DocinfoProcessors implementations must extend DocinfoProcessor.
  # If a location is not specified, the DocinfoProcessor is assumed
  # to add content to the header.
  class DocinfoProcessor < Processor
    attr_accessor :location

    def initialize config = {}
      super config
      @config[:location] ||= :head
    end

    def process document
      raise ::NotImplementedError
    end
  end

  module DocinfoProcessorDsl
    include ProcessorDsl

    def at_location value
      option :location, value
    end
  end
  DocinfoProcessor::DSL = DocinfoProcessorDsl

  # Public: BlockProcessors are used to handle delimited blocks and paragraphs
  # that have a custom name.
  #
  # When Asciidoctor encounters a delimited block or paragraph with an
  # unrecognized name while parsing the document, it looks for a BlockProcessor
  # registered to handle this name and, if found, invokes its {Processor#process}
  # method to build a cooresponding node in the document tree.
  #
  # AsciiDoc example:
  #
  #   [shout]
  #   Get a move on.
  #
  # Recognized options:
  #
  # * :named - The name of the block (required: true)
  # * :contexts - The blocks contexts on which this style can be used (default: [:paragraph, :open]
  # * :content_model - The structure of the content supported in this block (default: :compound)
  # * :positional_attributes - A list of attribute names used to map positional attributes (default: nil)
  # * ...
  #
  # BlockProcessor implementations must extend BlockProcessor.
  class BlockProcessor < Processor
    attr_accessor :name

    def initialize name = nil, config = {}
      super config
      @name = name || @config[:name]
      # assign fallbacks
      case @config[:contexts]
      when ::NilClass
        @config[:contexts] ||= [:open, :paragraph].to_set
      when ::Symbol
        @config[:contexts] = [@config[:contexts]].to_set
      else
        @config[:contexts] = @config[:contexts].to_set
      end
      # QUESTION should the default content model be raw??
      @config[:content_model] ||= :compound
    end

    def process parent, reader, attributes
      raise ::NotImplementedError
    end
  end

  module BlockProcessorDsl
    include ProcessorDsl

    # FIXME this isn't the prettiest thing
    def named value
      if self.is_a? Processor
        @name = value
      else
        option :name, value
      end
    end
    alias :match_name :named
    alias :bind_to :named

    def contexts *value
      option :contexts, value.flatten
    end
    alias :on_contexts :contexts
    alias :on_context :contexts

    def content_model value
      option :content_model, value
    end
    alias :parse_content_as :content_model

    def positional_attributes *value
      option :pos_attrs, value.flatten
    end
    alias :pos_attrs :positional_attributes
    alias :name_attributes :positional_attributes
    alias :name_positional_attributes :positional_attributes

    def default_attrs value
      option :default_attrs, value
    end
    alias :seed_attributes_with :default_attrs
  end
  BlockProcessor::DSL = BlockProcessorDsl

  class MacroProcessor < Processor
    attr_accessor :name

    def initialize name = nil, config = {}
      super config
      @name = name || @config[:name]
      @config[:content_model] ||= :attributes
    end

    def process parent, target, attributes
      raise ::NotImplementedError
    end
  end

  module MacroProcessorDsl
    include ProcessorDsl
    # QUESTION perhaps include a SyntaxDsl?

    def named value
      if self.is_a? Processor
        @name = value
      else
        option :name, value
      end
    end
    alias :match_name :named
    alias :bind_to :named

    def content_model value
      option :content_model, value
    end
    alias :parse_content_as :content_model

    def positional_attributes *value
      option :pos_attrs, value.flatten
    end
    alias :pos_attrs :positional_attributes
    alias :name_attributes :positional_attributes
    alias :name_positional_attributes :positional_attributes

    def default_attrs value
      option :default_attrs, value
    end
    alias :seed_attributes_with :default_attrs
  end

  # Public: BlockMacroProcessors are used to handle block macros that have a
  # custom name.
  #
  # BlockMacroProcessor implementations must extend BlockMacroProcessor.
  class BlockMacroProcessor < MacroProcessor
  end
  BlockMacroProcessor::DSL = MacroProcessorDsl

  # Public: InlineMacroProcessors are used to handle block macros that have a
  # custom name.
  #
  # InlineMacroProcessor implementations must extend InlineMacroProcessor.
  #--
  # TODO break this out into different pattern types
  # for example, FormalInlineMacro, ShortInlineMacro (no target) and other patterns
  # FIXME for inline passthrough, we need to have some way to specify the text as a passthrough
  class InlineMacroProcessor < MacroProcessor
    # Lookup the regexp option, resolving it first if necessary.
    # Once this method is called, the regexp is considered frozen.
    def regexp
      @config[:regexp] ||= (resolve_regexp @name, @config[:format])
    end

    def resolve_regexp name, format
      # TODO memoize these regular expressions!
      if format == :short
        %r(\\?#{name}:\[((?:\\\]|[^\]])*?)\])
      else
        %r(\\?#{name}:(\S+?)\[((?:\\\]|[^\]])*?)\])
      end
    end
  end

  module InlineMacroProcessorDsl
    include MacroProcessorDsl

    def using_format value
      option :format, value
    end

    def match value
      option :regexp, value
    end
  end
  InlineMacroProcessor::DSL = InlineMacroProcessorDsl

  # Public: Extension is a proxy object for an extension implementation such as
  # a processor. It allows the preparation of the extension instance to be
  # separated from its usage to provide consistency between different
  # interfaces and avoid tight coupling with the extension type.
  #
  # The proxy encapsulates the extension kind (e.g., :block), its config Hash
  # and the extension instance. This Proxy is what gets stored in the extension
  # registry when activated.
  #--
  # QUESTION call this ExtensionInfo?
  class Extension
    attr :kind
    attr :config
    attr :instance

    def initialize kind, instance, config
      @kind = kind
      @instance = instance
      @config = config
    end
  end

  # Public: A specialization of the Extension proxy that additionally stores a
  # reference to the {Processor#process} method. By storing this reference, its
  # possible to accomodate both concrete extension implementations and Procs.
  class ProcessorExtension < Extension
    attr :process_method

    def initialize kind, instance, process_method = nil
      super kind, instance, instance.config
      @process_method = process_method || (instance.method :process)
    end
  end

  # Public: A Group is used to register one or more extensions with the Registry.
  #
  # The Group should be subclassed and registered with the Registry either by
  # invoking the {Group.register} method or passing the subclass to the
  # {Extensions.register} method. Extensions are registered with the Registry
  # inside the {Group#activate} method.
  class Group
    class << self
      def register name = nil
        Extensions.register name, self
      end
    end

    def activate registry
      raise ::NotImplementedError
    end
  end

  # Public: The primary entry point into the extension system.
  #
  # Registry holds the extensions which have been registered and activated, has
  # methods for registering or defining a processor and looks up extensions
  # stored in the registry during parsing.
  class Registry
    # Public: Returns the {Asciidoctor::Document} on which the extensions in this registry are being used.
    attr_reader :document

    # Public: Returns the Array of {Group} classes, instances and/or Procs that have been registered.
    attr_reader :groups

    def initialize groups = {}
      @groups = groups
      @preprocessor_extensions = @treeprocessor_extensions = @postprocessor_extensions = @include_processor_extensions = @docinfo_processor_extensions =nil
      @block_extensions = @block_macro_extensions = @inline_macro_extensions = nil
      @document = nil
    end

    # Public: Activates all the global extension {Group}s and the extension {Group}s
    # associated with this registry.
    #
    # document - the {Asciidoctor::Document} on which the extensions are to be used.
    #
    # Returns the instance of this [Registry].
    def activate document
      @document = document
      (Extensions.groups.values + @groups.values).each do |group|
        case group
        when ::Proc
          case group.arity
          when 0, -1
            instance_exec(&group)
          when 1
            group.call self
          end
        when ::Class
          group.new.activate self
        else
          group.activate self
        end
      end
      self
    end

    # Public: Registers a {Preprocessor} with the extension registry to process
    # the AsciiDoc source before parsing begins.
    #
    # The Preprocessor may be one of four types:
    #
    # * A Preprocessor subclass
    # * An instance of a Preprocessor subclass
    # * The String name of a Preprocessor subclass
    # * A method block (i.e., Proc) that conforms to the Preprocessor contract
    #
    # Unless the Preprocessor is passed as the method block, it must be the
    # first argument to this method.
    #
    # Examples
    #
    #   # as a Preprocessor subclass
    #   preprocessor FrontMatterPreprocessor
    #
    #   # as an instance of a Preprocessor subclass
    #   preprocessor FrontMatterPreprocessor.new
    #
    #   # as a name of a Preprocessor subclass
    #   preprocessor 'FrontMatterPreprocessor'
    #
    #   # as a method block
    #   preprocessor do
    #     process |reader, lines|
    #       ...
    #     end
    #   end
    #
    # Returns the [Extension] stored in the registry that proxies the
    # instance of this Preprocessor.
    def preprocessor *args, &block
      add_document_processor :preprocessor, args, &block
    end

    # Public: Checks whether any {Preprocessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any Preprocessor extensions are registered.
    def preprocessors?
      !!@preprocessor_extensions
    end

    # Public: Retrieves the {Extension} proxy objects for all
    # Preprocessor instances in this registry.
    #
    # Returns an [Array] of Extension proxy objects.
    def preprocessors
      @preprocessor_extensions
    end

    # Public: Registers a {Treeprocessor} with the extension registry to process
    # the AsciiDoc source after parsing is complete.
    #
    # The Treeprocessor may be one of four types:
    #
    # * A Treeprocessor subclass
    # * An instance of a Treeprocessor subclass
    # * The String name of a Treeprocessor subclass
    # * A method block (i.e., Proc) that conforms to the Treeprocessor contract
    #
    # Unless the Treeprocessor is passed as the method block, it must be the
    # first argument to this method.
    #
    # Examples
    #
    #   # as a Treeprocessor subclass
    #   treeprocessor ShellTreeprocessor
    #
    #   # as an instance of a Treeprocessor subclass
    #   treeprocessor ShellTreeprocessor.new
    #
    #   # as a name of a Treeprocessor subclass
    #   treeprocessor 'ShellTreeprocessor'
    #
    #   # as a method block
    #   treeprocessor do
    #     process |document|
    #       ...
    #     end
    #   end
    #
    # Returns the [Extension] stored in the registry that proxies the
    # instance of this Treeprocessor.
    def treeprocessor *args, &block
      add_document_processor :treeprocessor, args, &block
    end

    # Public: Checks whether any {Treeprocessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any Treeprocessor extensions are registered.
    def treeprocessors?
      !!@treeprocessor_extensions
    end

    # Public: Retrieves the {Extension} proxy objects for all
    # Treeprocessor instances in this registry.
    #
    # Returns an [Array] of Extension proxy objects.
    def treeprocessors
      @treeprocessor_extensions
    end

    # Public: Registers a {Postprocessor} with the extension registry to process
    # the output after conversion is complete.
    #
    # The Postprocessor may be one of four types:
    #
    # * A Postprocessor subclass
    # * An instance of a Postprocessor subclass
    # * The String name of a Postprocessor subclass
    # * A method block (i.e., Proc) that conforms to the Postprocessor contract
    #
    # Unless the Postprocessor is passed as the method block, it must be the
    # first argument to this method.
    #
    # Examples
    #
    #   # as a Postprocessor subclass
    #   postprocessor AnalyticsPostprocessor
    #
    #   # as an instance of a Postprocessor subclass
    #   postprocessor AnalyticsPostprocessor.new
    #
    #   # as a name of a Postprocessor subclass
    #   postprocessor 'AnalyticsPostprocessor'
    #
    #   # as a method block
    #   postprocessor do
    #     process |document, output|
    #       ...
    #     end
    #   end
    #
    # Returns the [Extension] stored in the registry that proxies the
    # instance of this Postprocessor.
    def postprocessor *args, &block
      add_document_processor :postprocessor, args, &block
    end

    # Public: Checks whether any {Postprocessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any Postprocessor extensions are registered.
    def postprocessors?
      !!@postprocessor_extensions
    end

    # Public: Retrieves the {Extension} proxy objects for all
    # Postprocessor instances in this registry.
    #
    # Returns an [Array] of Extension proxy objects.
    def postprocessors
      @postprocessor_extensions
    end

    # Public: Registers an {IncludeProcessor} with the extension registry to have
    # a shot at handling the include directive.
    #
    # The IncludeProcessor may be one of four types:
    #
    # * A IncludeProcessor subclass
    # * An instance of a IncludeProcessor subclass
    # * The String name of a IncludeProcessor subclass
    # * A method block (i.e., Proc) that conforms to the IncludeProcessor contract
    #
    # Unless the IncludeProcessor is passed as the method block, it must be the
    # first argument to this method.
    #
    # Examples
    #
    #   # as an IncludeProcessor subclass
    #   include_processor GitIncludeProcessor
    #
    #   # as an instance of a Postprocessor subclass
    #   include_processor GitIncludeProcessor.new
    #
    #   # as a name of a Postprocessor subclass
    #   include_processor 'GitIncludeProcessor'
    #
    #   # as a method block
    #   include_processor do
    #     process |document, output|
    #       ...
    #     end
    #   end
    #
    # Returns the [Extension] stored in the registry that proxies the
    # instance of this IncludeProcessor.
    def include_processor *args, &block
      add_document_processor :include_processor, args, &block
    end

    # Public: Checks whether any {IncludeProcessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any IncludeProcessor extensions are registered.
    def include_processors?
      !!@include_processor_extensions
    end

    # Public: Retrieves the {Extension} proxy objects for all the
    # IncludeProcessor instances stored in this registry.
    #
    # Returns an [Array] of Extension proxy objects.
    def include_processors
      @include_processor_extensions
    end

    # Public: Registers an {DocinfoProcessor} with the extension registry to
    # add additionnal docinfo to the document.
    #
    # The DocinfoProcessor may be one of four types:
    #
    # * A DocinfoProcessor subclass
    # * An instance of a DocinfoProcessor subclass
    # * The String name of a DocinfoProcessor subclass
    # * A method block (i.e., Proc) that conforms to the DocinfoProcessor contract
    #
    # Unless the DocinfoProcessor is passed as the method block, it must be the
    # first argument to this method.
    #
    # Examples
    #
    #   # as an DocinfoProcessor subclass
    #   docinfo_processor MetaRobotsDocinfoProcessor
    #
    #   # as an instance of a DocinfoProcessor subclass with an explicit location
    #   docinfo_processor JQueryDocinfoProcessor.new, :location => :footer
    #
    #   # as a name of a DocinfoProcessor subclass
    #   docinfo_processor 'MetaRobotsDocinfoProcessor'
    #
    #   # as a method block
    #   docinfo_processor do
    #     process |doc|
    #       at_location :footer
    #       'footer content'
    #     end
    #   end
    #
    # Returns the [Extension] stored in the registry that proxies the
    # instance of this DocinfoProcessor.
    def docinfo_processor *args, &block
      add_document_processor :docinfo_processor, args, &block
    end

    # Public: Checks whether any {DocinfoProcessor} extensions have been registered.
    #
    # location - A Symbol for selecting docinfo extensions at a given location (:head or :footer) (default: nil)
    #
    # Returns a [Boolean] indicating whether any DocinfoProcessor extensions are registered.
    def docinfo_processors? location = nil
      if @docinfo_processor_extensions
        if location
          @docinfo_processor_extensions.find {|ext| ext.config[:location] == location }
        else
          true
        end
      else
        false
      end
    end

    # Public: Retrieves the {Extension} proxy objects for all the
    # DocinfoProcessor instances stored in this registry.
    #
    # location - A Symbol for selecting docinfo extensions at a given location (:head or :footer) (default: nil)
    #
    # Returns an [Array] of Extension proxy objects.
    def docinfo_processors location = nil
      if @docinfo_processor_extensions
        if location
          @docinfo_processor_extensions.select {|ext| ext.config[:location] == location }
        else
          @docinfo_processor_extensions
        end
      else
        nil
      end
    end

    # Public: Registers a {BlockProcessor} with the extension registry to
    # process the block content (i.e., delimited block or paragraph) in the
    # AsciiDoc source annotated with the specified block name (i.e., style).
    #
    # The BlockProcessor may be one of four types:
    #
    # * A BlockProcessor subclass
    # * An instance of a BlockProcessor subclass
    # * The String name of a BlockProcessor subclass
    # * A method block (i.e., Proc) that conforms to the BlockProcessor contract
    #
    # Unless the BlockProcessor is passed as the method block, it must be the
    # first argument to this method. The second argument is the name (coersed
    # to a Symbol) of the AsciiDoc block content (i.e., delimited block or
    # paragraph) that this processor is registered to handle. If a block name
    # is not passed as an argument, it gets read from the name property of the
    # BlockProcessor instance. If a name still cannot be determined, an error
    # is raised.
    # 
    # Examples
    #
    #   # as a BlockProcessor subclass
    #   block ShoutBlock
    #
    #   # as a BlockProcessor subclass with an explicit block name
    #   block ShoutBlock, :shout
    #
    #   # as an instance of a BlockProcessor subclass
    #   block ShoutBlock.new
    #
    #   # as an instance of a BlockProcessor subclass with an explicit block name
    #   block ShoutBlock.new, :shout
    #
    #   # as a name of a BlockProcessor subclass
    #   block 'ShoutBlock'
    #
    #   # as a name of a BlockProcessor subclass with an explicit block name
    #   block 'ShoutBlock', :shout
    #
    #   # as a method block
    #   block do
    #     named :shout 
    #     process |parent, reader, attrs|
    #       ...
    #     end
    #   end
    #
    #   # as a method block with an explicit block name
    #   block :shout do
    #     process |parent, reader, attrs|
    #       ...
    #     end
    #   end
    #
    # Returns an instance of the [Extension] proxy object that is stored in the
    # registry and manages the instance of this BlockProcessor.
    def block *args, &block
      add_syntax_processor :block, args, &block
    end

    # Public: Checks whether any {BlockProcessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any BlockProcessor extensions are registered.
    def blocks?
      !!@block_extensions
    end

    # Public: Checks whether any {BlockProcessor} extensions are registered to
    # handle the specified block name appearing on the specified context.
    #
    # Returns the [Extension] proxy object for the BlockProcessor that matches
    # the block name and context or false if no match is found.
    def registered_for_block? name, context
      if (ext = @block_extensions[name.to_sym])
        (ext.config[:contexts].include? context) ? ext : false
      else
        false
      end
    end

    # Public: Retrieves the {Extension} proxy object for the BlockProcessor registered
    # to handle block content with the name.
    #
    # name - the String or Symbol (coersed to a Symbol) macro name
    #
    # Returns the [Extension] object stored in the registry that proxies the
    # corresponding BlockProcessor or nil if a match is not found.
    def find_block_extension name
      @block_extensions[name.to_sym]
    end

    # Public: Registers a {BlockMacroProcessor} with the extension registry to
    # process a block macro with the specified name.
    #
    # The BlockMacroProcessor may be one of four types:
    #
    # * A BlockMacroProcessor subclass
    # * An instance of a BlockMacroProcessor subclass
    # * The String name of a BlockMacroProcessor subclass
    # * A method block (i.e., Proc) that conforms to the BlockMacroProcessor contract
    #
    # Unless the BlockMacroProcessor is passed as the method block, it must be
    # the first argument to this method. The second argument is the name
    # (coersed to a Symbol) of the AsciiDoc block macro that this processor is
    # registered to handle. If a block macro name is not passed as an argument,
    # it gets read from the name property of the BlockMacroProcessor instance.
    # If a name still cannot be determined, an error is raised.
    # 
    # Examples
    #
    #   # as a BlockMacroProcessor subclass
    #   block_macro GistBlockMacro
    #
    #   # as a BlockMacroProcessor subclass with an explicit macro name
    #   block_macro GistBlockMacro, :gist
    #
    #   # as an instance of a BlockMacroProcessor subclass
    #   block_macro GistBlockMacro.new
    #
    #   # as an instance of a BlockMacroProcessor subclass with an explicit macro name
    #   block_macro GistBlockMacro.new, :gist
    #
    #   # as a name of a BlockMacroProcessor subclass
    #   block_macro 'GistBlockMacro'
    #
    #   # as a name of a BlockMacroProcessor subclass with an explicit macro name
    #   block_macro 'GistBlockMacro', :gist
    #
    #   # as a method block
    #   block_macro do
    #     named :gist
    #     process |parent, target, attrs|
    #       ...
    #     end
    #   end
    #
    #   # as a method block with an explicit macro name
    #   block_macro :gist do
    #     process |parent, target, attrs|
    #       ...
    #     end
    #   end
    #
    # Returns an instance of the [Extension] proxy object that is stored in the
    # registry and manages the instance of this BlockMacroProcessor.
    def block_macro *args, &block
      add_syntax_processor :block_macro, args, &block
    end

    # Public: Checks whether any {BlockMacroProcessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any BlockMacroProcessor extensions are registered.
    def block_macros?
      !!@block_macro_extensions
    end

    # Public: Checks whether any {BlockMacroProcessor} extensions are registered to
    # handle the block macro with the specified name.
    #
    # name - the String or Symbol (coersed to a Symbol) macro name
    #
    # Returns the [Extension] proxy object for the BlockMacroProcessor that matches
    # the macro name or false if no match is found.
    #--
    # TODO only allow blank target if format is :short
    def registered_for_block_macro? name
      (ext = @block_macro_extensions[name.to_sym]) ? ext : false
    end

    # Public: Retrieves the {Extension} proxy object for the BlockMacroProcessor registered
    # to handle a block macro with the specified name.
    #
    # name - the String or Symbol (coersed to a Symbol) macro name
    #
    # Returns the [Extension] object stored in the registry that proxies the
    # cooresponding BlockMacroProcessor or nil if a match is not found.
    def find_block_macro_extension name
      @block_macro_extensions[name.to_sym]
    end

    # Public: Registers a {InlineMacroProcessor} with the extension registry to
    # process an inline macro with the specified name.
    #
    # The InlineMacroProcessor may be one of four types:
    #
    # * An InlineMacroProcessor subclass
    # * An instance of an InlineMacroProcessor subclass
    # * The String name of an InlineMacroProcessor subclass
    # * A method block (i.e., Proc) that conforms to the InlineMacroProcessor contract
    #
    # Unless the InlineMacroProcessor is passed as the method block, it must be
    # the first argument to this method. The second argument is the name
    # (coersed to a Symbol) of the AsciiDoc block macro that this processor is
    # registered to handle. If a block macro name is not passed as an argument,
    # it gets read from the name property of the InlineMacroProcessor instance.
    # If a name still cannot be determined, an error is raised.
    # 
    # Examples
    #
    #   # as an InlineMacroProcessor subclass
    #   inline_macro ChromeInlineMacro
    #
    #   # as an InlineMacroProcessor subclass with an explicit macro name
    #   inline_macro ChromeInineMacro, :chrome
    #
    #   # as an instance of an InlineMacroProcessor subclass
    #   inline_macro ChromeInlineMacro.new
    #
    #   # as an instance of an InlineMacroProcessor subclass with an explicit macro name
    #   inline_macro ChromeInlineMacro.new, :chrome
    #
    #   # as a name of an InlineMacroProcessor subclass
    #   inline_macro 'ChromeInlineMacro'
    #
    #   # as a name of an InlineMacroProcessor subclass with an explicit macro name
    #   inline_macro 'ChromeInineMacro', :chrome
    #
    #   # as a method block
    #   inline_macro do
    #     named :chrome
    #     process |parent, target, attrs|
    #       ...
    #     end
    #   end
    #
    #   # as a method block with an explicit macro name
    #   inline_macro :chrome do
    #     process |parent, target, attrs|
    #       ...
    #     end
    #   end
    #
    # Returns an instance of the [Extension] proxy object that is stored in the
    # registry and manages the instance of this InlineMacroProcessor.
    def inline_macro *args, &block
      add_syntax_processor :inline_macro, args, &block
    end

    # Public: Checks whether any {InlineMacroProcessor} extensions have been registered.
    #
    # Returns a [Boolean] indicating whether any IncludeMacroProcessor extensions are registered.
    def inline_macros?
      !!@inline_macro_extensions
    end

    # Public: Checks whether any {InlineMacroProcessor} extensions are registered to
    # handle the inline macro with the specified name.
    #
    # name - the String or Symbol (coersed to a Symbol) macro name
    #
    # Returns the [Extension] proxy object for the InlineMacroProcessor that matches
    # the macro name or false if no match is found.
    def registered_for_inline_macro? name
      (ext = @inline_macro_extensions[name.to_sym]) ? ext : false
    end

    # Public: Retrieves the {Extension} proxy object for the InlineMacroProcessor registered
    # to handle an inline macro with the specified name.
    #
    # name - the String or Symbol (coersed to a Symbol) macro name
    #
    # Returns the [Extension] object stored in the registry that proxies the
    # cooresponding InlineMacroProcessor or nil if a match is not found.
    def find_inline_macro_extension name
      @inline_macro_extensions[name.to_sym]
    end

    # Public: Retrieves the {Extension} proxy objects for all
    # InlineMacroProcessor instances in this registry.
    #
    # Returns an [Array] of Extension proxy objects.
    def inline_macros
      @inline_macro_extensions.values
    end

    private

    def add_document_processor kind, args, &block
      kind_name = kind.to_s.tr '_', ' '
      kind_class_symbol = kind_name.split(' ').map {|word| %(#{word.chr.upcase}#{word[1..-1]}) }.join.to_sym
      kind_class = Extensions.const_get kind_class_symbol
      kind_java_class = (defined? ::AsciidoctorJ) ? (::AsciidoctorJ::Extensions.const_get kind_class_symbol) : nil
      kind_store = instance_variable_get(%(@#{kind}_extensions).to_sym) || instance_variable_set(%(@#{kind}_extensions).to_sym, [])
      # style 1: specified as block
      extension = if block_given?
        config = resolve_args args, 1
        # TODO if block arity is 0, assume block is process method
        processor = kind_class.new config
        # NOTE class << processor idiom doesn't work in Opal
        #class << processor
        #  include_dsl
        #end
        # NOTE kind_class.contants(false) doesn't exist in Ruby 1.8.7
        processor.extend kind_class.const_get :DSL if kind_class.constants.grep :DSL
        processor.instance_exec(&block)
        processor.freeze
        unless processor.process_block_given?
          raise ::ArgumentError.new %(No block specified to process #{kind_name} extension at #{block.source_location})
        end
        ProcessorExtension.new kind, processor
      else
        processor, config = resolve_args args, 2
        # style 2: specified as class or class name
        if (processor.is_a? ::Class) || ((processor.is_a? ::String) && (processor = Extensions.class_for_name processor))
          unless processor < kind_class || (kind_java_class && processor < kind_java_class)
            raise ::ArgumentError.new %(Invalid type for #{kind_name} extension: #{processor})
          end
          processor_instance = processor.new config
          processor_instance.freeze
          ProcessorExtension.new kind, processor_instance
        # style 3: specified as instance
        elsif (processor.is_a? kind_class) || (kind_java_class && (processor.is_a? kind_java_class))
          processor.update_config config
          processor.freeze
          ProcessorExtension.new kind, processor
        else
          raise ::ArgumentError.new %(Invalid arguments specified for registering #{kind_name} extension: #{args})
        end
      end

      if extension.config[:position] == :>>
        kind_store.unshift extension
      else
        kind_store << extension
      end
    end

    def add_syntax_processor kind, args, &block
      kind_name = kind.to_s.tr '_', ' '
      kind_class_basename = kind_name.split(' ').map {|word| %(#{word.chr.upcase}#{word[1..-1]}) }.join
      kind_class_symbol = %(#{kind_class_basename}Processor).to_sym
      kind_class = Extensions.const_get kind_class_symbol
      kind_java_class = (defined? ::AsciidoctorJ) ? (::AsciidoctorJ::Extensions.const_get kind_class_symbol) : nil
      kind_store = instance_variable_get(%(@#{kind}_extensions).to_sym) || instance_variable_set(%(@#{kind}_extensions).to_sym, {})
      # style 1: specified as block
      if block_given?
        name, config = resolve_args args, 2
        processor = kind_class.new as_symbol(name), config
        # NOTE class << processor idiom doesn't work in Opal
        #class << processor
        #  include_dsl
        #end
        # NOTE kind_class.contants(false) doesn't exist in Ruby 1.8.7
        processor.extend kind_class.const_get :DSL if kind_class.constants.grep :DSL
        if block.arity == 1
          yield processor
        else
          processor.instance_exec(&block)
        end
        unless (name = as_symbol processor.name)
          raise ::ArgumentError.new %(No name specified for #{kind_name} extension at #{block.source_location})
        end
        unless processor.process_block_given?
          raise ::NoMethodError.new %(No block specified to process #{kind_name} extension at #{block.source_location})
        end
        processor.freeze
        kind_store[name] = ProcessorExtension.new kind, processor
      else
        processor, name, config = resolve_args args, 3
        # style 2: specified as class or class name
        if (processor.is_a? ::Class) || ((processor.is_a? ::String) && (processor = Extensions.class_for_name processor))
          unless processor < kind_class || (kind_java_class && processor < kind_java_class)
            raise ::ArgumentError.new %(Class specified for #{kind_name} extension does not inherit from #{kind_class}: #{processor})
          end
          processor_instance = processor.new as_symbol(name), config
          unless (name = as_symbol processor_instance.name)
            raise ::ArgumentError.new %(No name specified for #{kind_name} extension: #{processor})
          end
          processor.freeze
          kind_store[name] = ProcessorExtension.new kind, processor_instance
        # style 3: specified as instance
        elsif (processor.is_a? kind_class) || (kind_java_class && (processor.is_a? kind_java_class))
          processor.update_config config
          # TODO need a test for this override!
          unless (name = name ? (processor.name = as_symbol name) : (as_symbol processor.name))
            raise ::ArgumentError.new %(No name specified for #{kind_name} extension: #{processor})
          end
          processor.freeze
          kind_store[name] = ProcessorExtension.new kind, processor
        else
          raise ::ArgumentError.new %(Invalid arguments specified for registering #{kind_name} extension: #{args})
        end
      end
    end

    def resolve_args args, expect
      opts = (args[-1].is_a? ::Hash) ? args.pop : {}
      return opts if expect == 1
      num_args = args.size
      if (missing = expect - 1 - num_args) > 0
        args.fill nil, num_args, missing
      elsif missing < 0
        args.pop(-missing)
      end
      args << opts
      args
    end

    def as_symbol name
      name ? ((name.is_a? ::Symbol) ? name : name.to_sym) : nil
    end
  end

  class << self
    def generate_name
      %(extgrp#{next_auto_id})
    end

    def next_auto_id
      @auto_id ||= -1
      @auto_id += 1
    end

    def groups
      @groups ||= {}
    end

    def build_registry name = nil, &block
      if block_given?
        name ||= generate_name
        Registry.new({ name => block })
      else
        Registry.new
      end
    end

    # Public: Registers an extension Group that subsequently registers a
    # collection of extensions.
    #
    # Registers the extension Group specified under the given name. If a name is
    # not given, one is calculated by appending the next value in a 0-based
    # index to the string "extgrp". For instance, the first unnamed extension
    # group to be registered is assigned the name "extgrp0" if a name is not
    # specified.
    #
    # The names are not yet used, but are intended for selectively activating
    # extensions in the future.
    #
    # If the extension group argument is a String or a Symbol, it gets resolved
    # to a Class before being registered.
    #
    # name    - The name under which this extension group is registered (optional, default: nil)
    # group   - A block (Proc), a Class, a String or Symbol name of a Class or
    #           an Object instance of a Class.
    #
    # Examples
    #
    #   Asciidoctor::Extensions.register UmlExtensions
    #
    #   Asciidoctor::Extensions.register :uml, UmlExtensions
    #
    #   Asciidoctor::Extensions.register do
    #     block_processor :plantuml, PlantUmlBlock
    #   end
    #
    #   Asciidoctor::Extensions.register :uml do
    #     block_processor :plantuml, PlantUmlBlock
    #   end
    #
    # Returns the [Proc, Class or Object] instance, matching the type passed to this method.
    def register *args, &block
      argc = args.length
      resolved_group = if block_given?
        block
      elsif !(group = args.pop)
        raise ::ArgumentError.new %(Extension group to register not specified)
      else
        # QUESTION should we instantiate the group class here or defer until
        # activation??
        case group
        when ::Class
          group
        when ::String
          class_for_name group
        when ::Symbol
          class_for_name group.to_s
        else
          group
        end
      end
      name = args.pop || generate_name
      unless args.empty?
        raise ::ArgumentError.new %(Wrong number of arguments (#{argc} for 1..2))
      end
      groups[name] = resolved_group
    end

    def unregister_all
      @groups = {}
    end

    # unused atm, but tested
    def resolve_class object
      (object.is_a? ::Class) ? object : (class_for_name object.to_s)
    end

    # Public: Resolves the Class object for the qualified name.
    #
    # Returns Class
    def class_for_name qualified_name
      resolved_class = ::Object
      qualified_name.split('::').each do |name|
        if name.empty?
          # do nothing
        elsif resolved_class.const_defined? name
          resolved_class = resolved_class.const_get name
        else
          raise %(Could not resolve class for name: #{qualified_name})
        end
      end
      resolved_class
    end
  end

end
end
