require "qerub/singleton_class"

module TextFilter
  class Error < StandardError
  end

  class << self
    def process(filename, data, variables = {})
      find_filter_chain(filename).inject(data) { |data, filter| filter.process(data, variables) }
    end

    def find_filter_chain(filename)
      basename, *extensions = filename.split(".")

      require("enumerator")
      steps = extensions.reverse.enum_cons(2).to_a

      plugins = TextFilter::Plugin.subclasses

      steps.map do |from, to|
        plugin = plugins.find do |plugin|
          plugin.handles[from] == to
        end

        plugin ||= plugins.find do |plugin|
          plugin.handles[from] == "*"
        end

        if plugin
          next plugin
        else
          raise Error, "Could not find a TextFilter plugin for converting from #{from} to #{to}"
        end
      end
    end
  end

  class Plugin
    class << self
      def subclasses
        @subclasses ||= []
      end

      def inherited(klass)
        subclasses << klass
      end

      def process(*args)
        new.filter(*args)
      end

      def handles
        @handles ||= {}
      end

      def handles=(value)
        @handles = value
      end
    end
  end

  class Context
    def initialize(env)
      env.each do |key, value|
        singleton_class.instance_eval do
          define_method(key) { value }
        end
      end
    end

    def context_binding
      binding
    end
  end

  module Plugins
    class TextileFilter < Plugin
      self.handles = { "textile" => "html" }

      def filter(text, env)
        require("redcloth")
        RedCloth.new(text).to_html
      end
    end

    class HTMLStripper < Plugin
      self.handles = { "html" => "txt" }

      def filter(text, env)
        text.gsub(/<.*?>/, "")
      end
    end

    class ERBFilter < Plugin
      self.handles = { "erb" => "*" }

      def filter(text, env)
        require("erb")

        context = Context.new(env)
        context.extend(ERB::Util)

        ERB.new(text, nil, "-").result(context.context_binding)
      end
    end
  end
end