# = PluginHost
#
# $Id: plugin.rb 188 2006-10-17 11:18:01Z murphy $
#
# A simple subclass plugin system.
#
#  Example:
#    class Generators < PluginHost
#      plugin_path 'app/generators'
#    end
#    
#    class Generator
#      extend Plugin
#      PLUGIN_HOST = Generators
#    end
#    
#    class FancyGenerator < Generator
#      register_for :fancy
#    end
#
#    Generators[:fancy]  #-> FancyGenerator
#    # or
#    require_plugin 'Generators/fancy'
module PluginHost

  # Raised if Encoders::[] fails because:
  # * a file could not be found
  # * the requested Encoder is not registered
  PluginNotFound = Class.new Exception
  HostNotFound = Class.new Exception

  PLUGIN_HOSTS = []
  PLUGIN_HOSTS_BY_ID = {}  # dummy hash

  # Loads all plugins using list and load.
  def load_all
    for plugin in list
      load plugin
    end
  end

  # Returns the Plugin for +id+.
  #
  # Example:
  #  yaml_plugin = MyPluginHost[:yaml]
  def [] id, *args, &blk
    plugin = validate_id(id)
    begin
      plugin = plugin_hash.[] plugin, *args, &blk
    end while plugin.is_a? Symbol
    plugin
  end

  # Alias for +[]+.
  alias load []

  def require_helper plugin_id, helper_name
    path = path_to File.join(plugin_id, helper_name)
    require path
  end

  class << self

    # Adds the module/class to the PLUGIN_HOSTS list.
    def extended mod
      PLUGIN_HOSTS << mod
    end

    # Warns you that you should not #include this module.
    def included mod
      warn "#{name} should not be included. Use extend."
    end

    # Find the PluginHost for host_id.
    def host_by_id host_id
      unless PLUGIN_HOSTS_BY_ID.default_proc
        ph = Hash.new do |h, a_host_id|
          for host in PLUGIN_HOSTS
            h[host.host_id] = host
          end
          h.fetch a_host_id, nil
        end
        PLUGIN_HOSTS_BY_ID.replace ph
      end
      PLUGIN_HOSTS_BY_ID[host_id]
    end

  end

  # The path where the plugins can be found.
  def plugin_path *args
    unless args.empty?
      @plugin_path = File.expand_path File.join(*args)
      load_map
    end
    @plugin_path
  end

  # The host's ID.
  #
  # If PLUGIN_HOST_ID is not set, it is simply the class name.
  def host_id
    if self.const_defined? :PLUGIN_HOST_ID
      self::PLUGIN_HOST_ID
    else
      name
    end
  end

  # Map a plugin_id to another.
  #
  # Usage: Put this in a file plugin_path/_map.rb.
  #
  #  class MyColorHost < PluginHost
  #    map :navy => :dark_blue,
  #      :maroon => :brown,
  #      :luna => :moon
  #  end
  def map hash
    for from, to in hash
      from = validate_id from
      to = validate_id to
      plugin_hash[from] = to unless plugin_hash.has_key? from
    end
  end

  # Define the default plugin to use when no plugin is found
  # for a given id.
  #
  # See also map.
  #
  #  class MyColorHost < PluginHost
  #    map :navy => :dark_blue
  #    default :gray
  #  end
  def default id
    id = validate_id id
    plugin_hash[nil] = id
  end

  # Every plugin must register itself for one or more
  # +ids+ by calling register_for, which calls this method.
  #
  # See Plugin#register_for.
  def register plugin, *ids
    for id in ids
      unless id.is_a? Symbol
        raise ArgumentError,
          "id must be a Symbol, but it was a #{id.class}"
      end
      plugin_hash[validate_id(id)] = plugin
    end
  end

  # A Hash of plugion_id => Plugin pairs.
  def plugin_hash
    @plugin_hash ||= create_plugin_hash
  end

  # Returns an array of all .rb files in the plugin path.
  #
  # The extension .rb is not included.
  def list
    Dir[path_to('*')].select do |file|
      File.basename(file)[/^(?!_)\w+\.rb$/]
    end.map do |file|
      File.basename file, '.rb'
    end
  end

  # Makes a map of all loaded plugins.
  def inspect
    map = plugin_hash.dup
    map.each do |id, plugin|
      map[id] = plugin.to_s[/(?>[\w_]+)$/]
    end
    "#{name}[#{host_id}]#{map.inspect}"
  end

protected
  # Created a new plugin list and stores it to @plugin_hash.
  def create_plugin_hash
    @plugin_hash =
      Hash.new do |h, plugin_id|
        id = validate_id(plugin_id)
        path = path_to id
        begin
          require path
        rescue LoadError => boom
          if h.has_key? nil  # default plugin
            h[id] = h[nil]
          else
            raise PluginNotFound, 'Could not load plugin %p: %s' % [id, boom]
          end
        else
          # Plugin should have registered by now
          unless h.has_key? id
            raise PluginNotFound,
              "No #{self.name} plugin for #{id.inspect} found in #{path}."
          end
        end
        h[id]
      end
  end

  # Loads the map file (see map).
  #
  # This is done automatically when plugin_path is called.
  def load_map
    mapfile = path_to '_map'
    if File.exist? mapfile
      require mapfile
    elsif $DEBUG
      warn 'no _map.rb found for %s' % name
    end
  end

  # Returns the Plugin for +id+.
  # Use it like Hash#fetch.
  #
  # Example:
  #  yaml_plugin = MyPluginHost[:yaml, :default]
  def fetch id, *args, &blk
    plugin_hash.fetch validate_id(id), *args, &blk
  end

  # Returns the expected path to the plugin file for the given id.
  def path_to plugin_id
    File.join plugin_path, "#{plugin_id}.rb"
  end

  # Converts +id+ to a Symbol if it is a String,
  # or returns +id+ if it already is a Symbol.
  #
  # Raises +ArgumentError+ for all other objects, or if the
  # given String includes non-alphanumeric characters (\W).
  def validate_id id
    if id.is_a? Symbol or id.nil?
      id
    elsif id.is_a? String
      if id[/\w+/] == id
        id.to_sym
      else
        raise ArgumentError, "Invalid id: '#{id}' given."
      end
    else
      raise ArgumentError,
        "String or Symbol expected, but #{id.class} given."
    end
  end

end


# = Plugin
#
#  Plugins have to include this module.
#
#  IMPORTANT: use extend for this module.
#
#  Example: see PluginHost.
module Plugin

  def included mod
    warn "#{name} should not be included. Use extend."
  end

  # Register this class for the given langs.
  # Example:
  #   class MyPlugin < PluginHost::BaseClass
  #     register_for :my_id
  #     ...
  #   end
  #
  # See PluginHost.register.
  def register_for *ids
    plugin_host.register self, *ids
  end

  # The host for this Plugin class.
  def plugin_host host = nil
    if host and not host.is_a? PluginHost
      raise ArgumentError,
        "PluginHost expected, but #{host.class} given."
    end
    self.const_set :PLUGIN_HOST, host if host
    self::PLUGIN_HOST
  end

  # Require some helper files.
  #
  # Example:
  #
  #  class MyPlugin < PluginHost::BaseClass
  #     register_for :my_id
  #     helper :my_helper
  #
  # The above example loads the file myplugin/my_helper.rb relative to the
  # file in which MyPlugin was defined.
  def helper *helpers
    for helper in helpers
      self::PLUGIN_HOST.require_helper plugin_id, helper.to_s
    end
  end

  # Returns the pulgin id used by the engine.
  def plugin_id
    name[/[\w_]+$/].downcase
  end

end


# Convenience method for plugin loading.
# The syntax used is:
#
#  require_plugin '<Host ID>/<Plugin ID>'
#
# Returns the loaded plugin.
def require_plugin path
  host_id, plugin_id = path.split '/', 2
  host = PluginHost.host_by_id(host_id)
  raise PluginHost::HostNotFound,
    "No host for #{host_id.inspect} found." unless host
  host.load plugin_id
end
