module Tilt
  TOPOBJECT = defined?(BasicObject) ? BasicObject : Object

  # Base class for template implementations. Subclasses must implement
  # the #prepare method and one of the #evaluate or #precompiled_template
  # methods.
  class Template
    # Template source; loaded from a file or given directly.
    attr_reader :data

    # The name of the file where the template data was loaded from.
    attr_reader :file

    # The line number in #file where template data was loaded from.
    attr_reader :line

    # A Hash of template engine specific options. This is passed directly
    # to the underlying engine and is not used by the generic template
    # interface.
    attr_reader :options

    # Used to determine if this class's initialize_engine method has
    # been called yet.
    @engine_initialized = false
    class << self
      attr_accessor :engine_initialized
      alias engine_initialized? engine_initialized

      attr_accessor :default_mime_type
    end

    # Create a new template with the file, line, and options specified. By
    # default, template data is read from the file. When a block is given,
    # it should read template data and return as a String. When file is nil,
    # a block is required.
    #
    # All arguments are optional.
    def initialize(file=nil, line=1, options={}, &block)
      @file, @line, @options = nil, 1, {}

      [options, line, file].compact.each do |arg|
        case
        when arg.respond_to?(:to_str)  ; @file = arg.to_str
        when arg.respond_to?(:to_int)  ; @line = arg.to_int
        when arg.respond_to?(:to_hash) ; @options = arg.to_hash.dup
        when arg.respond_to?(:path)    ; @file = arg.path
        else raise TypeError
        end
      end

      raise ArgumentError, "file or block required" if (@file || block).nil?

      # call the initialize_engine method if this is the very first time
      # an instance of this class has been created.
      if !self.class.engine_initialized?
        initialize_engine
        self.class.engine_initialized = true
      end

      # used to hold compiled template methods
      @compiled_method = {}

      # used on 1.9 to set the encoding if it is not set elsewhere (like a magic comment)
      # currently only used if template compiles to ruby
      @default_encoding = @options.delete :default_encoding

      # load template data and prepare (uses binread to avoid encoding issues)
      @reader = block || lambda { |t| File.respond_to?(:binread) ? File.binread(@file) : File.read(@file) }
      @data = @reader.call(self)
      prepare
    end

    # Render the template in the given scope with the locals specified. If a
    # block is given, it is typically available within the template via
    # +yield+.
    def render(scope=Object.new, locals={}, &block)
      evaluate scope, locals || {}, &block
    end

    # The basename of the template file.
    def basename(suffix='')
      File.basename(file, suffix) if file
    end

    # The template file's basename with all extensions chomped off.
    def name
      basename.split('.', 2).first if basename
    end

    # The filename used in backtraces to describe the template.
    def eval_file
      file || '(__TEMPLATE__)'
    end

    # Whether or not this template engine allows executing Ruby script
    # within the template. If this is false, +scope+ and +locals+ will
    # generally not be used, nor will the provided block be avaiable 
    # via +yield+.
    # This should be overridden by template subclasses.
    def allows_script?
      true
    end

  protected
    # Called once and only once for each template subclass the first time
    # the template class is initialized. This should be used to require the
    # underlying template library and perform any initial setup.
    def initialize_engine
    end

    # Like Kernel#require but issues a warning urging a manual require when
    # running under a threaded environment.
    def require_template_library(name)
      if Thread.list.size > 1
        warn "WARN: tilt autoloading '#{name}' in a non thread-safe way; " +
             "explicit require '#{name}' suggested."
      end
      require name
    end

    # Do whatever preparation is necessary to setup the underlying template
    # engine. Called immediately after template data is loaded. Instance
    # variables set in this method are available when #evaluate is called.
    #
    # Subclasses must provide an implementation of this method.
    def prepare
      if respond_to?(:compile!)
        # backward compat with tilt < 0.6; just in case
        warn 'Tilt::Template#compile! is deprecated; implement #prepare instead.'
        compile!
      else
        raise NotImplementedError
      end
    end

    # Execute the compiled template and return the result string. Template
    # evaluation is guaranteed to be performed in the scope object with the
    # locals specified and with support for yielding to the block.
    #
    # This method is only used by source generating templates. Subclasses that
    # override render() may not support all features.
    def evaluate(scope, locals, &block)
      method = compiled_method(locals.keys)
      method.bind(scope).call(locals, &block)
    end

    # Generates all template source by combining the preamble, template, and
    # postamble and returns a two-tuple of the form: [source, offset], where
    # source is the string containing (Ruby) source code for the template and
    # offset is the integer line offset where line reporting should begin.
    #
    # Template subclasses may override this method when they need complete
    # control over source generation or want to adjust the default line
    # offset. In most cases, overriding the #precompiled_template method is
    # easier and more appropriate.
    def precompiled(locals)
      preamble = precompiled_preamble(locals)
      template = precompiled_template(locals)
      magic_comment = extract_magic_comment(template)
      if magic_comment
        # Magic comment e.g. "# coding: utf-8" has to be in the first line.
        # So we copy the magic comment to the first line.
        preamble = magic_comment + "\n" + preamble
      end
      parts = [
        preamble,
        template,
        precompiled_postamble(locals)
      ]
      [parts.join("\n"), preamble.count("\n") + 1]
    end

    # A string containing the (Ruby) source code for the template. The
    # default Template#evaluate implementation requires either this method
    # or the #precompiled method be overridden. When defined, the base
    # Template guarantees correct file/line handling, locals support, custom
    # scopes, and support for template compilation when the scope object
    # allows it.
    def precompiled_template(locals)
      raise NotImplementedError
    end

    # Generates preamble code for initializing template state, and performing
    # locals assignment. The default implementation performs locals
    # assignment only. Lines included in the preamble are subtracted from the
    # source line offset, so adding code to the preamble does not effect line
    # reporting in Kernel::caller and backtraces.
    def precompiled_preamble(locals)
      locals.map do |k,v|
        if k.to_s =~ /\A[a-z_][a-zA-Z_0-9]*\z/
          "#{k} = locals[#{k.inspect}]"
        else
          raise "invalid locals key: #{k.inspect} (keys must be variable names)"
        end
      end.join("\n")
    end

    # Generates postamble code for the precompiled template source. The
    # string returned from this method is appended to the precompiled
    # template source.
    def precompiled_postamble(locals)
      ''
    end

    # The compiled method for the locals keys provided.
    def compiled_method(locals_keys)
      @compiled_method[locals_keys] ||=
        compile_template_method(locals_keys)
    end

  private
    def compile_template_method(locals)
      source, offset = precompiled(locals)
      method_name = "__tilt_#{Thread.current.object_id.abs}"
      method_source = <<-RUBY
        #{extract_magic_comment source}
        TOPOBJECT.class_eval do
          def #{method_name}(locals)
            Thread.current[:tilt_vars] = [self, locals]
            class << self
              this, locals = Thread.current[:tilt_vars]
              this.instance_eval do
      RUBY
      offset += method_source.count("\n")
      method_source << source
      method_source << "\nend;end;end;end"
      Object.class_eval method_source, eval_file, line - offset
      unbind_compiled_method(method_name)
    end

    def unbind_compiled_method(method_name)
      method = TOPOBJECT.instance_method(method_name)
      TOPOBJECT.class_eval { remove_method(method_name) }
      method
    end

    def extract_magic_comment(script)
      comment = script.slice(/\A[ \t]*\#.*coding\s*[=:]\s*([[:alnum:]\-_]+).*$/)
      if comment && !%w[ascii-8bit binary].include?($1.downcase)
        comment
      elsif @default_encoding
        "# coding: #{@default_encoding}"
      end
    end
  end
end