module Sass
  # A namespace for nodes in the Sass parse tree.
  #
  # The Sass parse tree has three states: dynamic, static Sass, and static CSS.
  #
  # When it's first parsed, a Sass document is in the dynamic state.
  # It has nodes for mixin definitions and `@for` loops and so forth,
  # in addition to nodes for CSS rules and properties.
  # Nodes that only appear in this state are called **dynamic nodes**.
  #
  # {Tree::Visitors::Perform} creates a static Sass tree, which is different.
  # It still has nodes for CSS rules and properties
  # but it doesn't have any dynamic-generation-related nodes.
  # The nodes in this state are in the same structure as the Sass document:
  # rules and properties are nested beneath one another.
  # Nodes that can be in this state or in the dynamic state
  # are called **static nodes**.
  #
  # {Tree::Visitors::Cssize} is then used to create a static CSS tree.
  # This is like a static Sass tree,
  # but the structure exactly mirrors that of the generated CSS.
  # Rules and properties can't be nested beneath one another in this state.
  #
  # Finally, {Tree::Visitors::ToCss} can be called on a static CSS tree
  # to get the actual CSS code as a string.
  module Tree
    # The abstract superclass of all parse-tree nodes.
    class Node
      include Enumerable

      # The child nodes of this node.
      #
      # @return [Array<Tree::Node>]
      attr_accessor :children

      # Whether or not this node has child nodes.
      # This may be true even when \{#children} is empty,
      # in which case this node has an empty block (e.g. `{}`).
      #
      # @return [Boolean]
      attr_accessor :has_children

      # The line of the document on which this node appeared.
      #
      # @return [Fixnum]
      attr_accessor :line

      # The name of the document on which this node appeared.
      #
      # @return [String]
      attr_writer :filename

      # The options hash for the node.
      # See {file:SASS_REFERENCE.md#sass_options the Sass options documentation}.
      #
      # @return [{Symbol => Object}]
      attr_reader :options

      def initialize
        @children = []
      end

      # Sets the options hash for the node and all its children.
      #
      # @param options [{Symbol => Object}] The options
      # @see #options
      def options=(options)
        children.each {|c| c.options = options}
        @options = options
      end

      # @private
      def children=(children)
        self.has_children ||= !children.empty?
        @children = children
      end

      # The name of the document on which this node appeared.
      #
      # @return [String]
      def filename
        @filename || (@options && @options[:filename])
      end

      # Appends a child to the node.
      #
      # @param child [Tree::Node, Array<Tree::Node>] The child node or nodes
      # @raise [Sass::SyntaxError] if `child` is invalid
      def <<(child)
        return if child.nil?
        if child.is_a?(Array)
          child.each {|c| self << c}
        else
          self.has_children = true
          @children << child
        end
      end

      # Compares this node and another object (only other {Tree::Node}s will be equal).
      # This does a structural comparison;
      # if the contents of the nodes and all the child nodes are equivalent,
      # then the nodes are as well.
      #
      # Only static nodes need to override this.
      #
      # @param other [Object] The object to compare with
      # @return [Boolean] Whether or not this node and the other object
      #   are the same
      # @see Sass::Tree
      def ==(other)
        self.class == other.class && other.children == children
      end

      # True if \{#to\_s} will return `nil`;
      # that is, if the node shouldn't be rendered.
      # Should only be called in a static tree.
      #
      # @return [Boolean]
      def invisible?; false; end

      # The output style. See {file:SASS_REFERENCE.md#sass_options the Sass options documentation}.
      #
      # @return [Symbol]
      def style
        @options[:style]
      end

      # Computes the CSS corresponding to this static CSS tree.
      #
      # @return [String, nil] The resulting CSS
      # @see Sass::Tree
      def to_s
        Sass::Tree::Visitors::ToCss.visit(self)
      end

      # Converts a static CSS tree (e.g. the output of \{Tree::Visitors::Cssize})
      # into another static CSS tree,
      # with the given extensions applied to all relevant {RuleNode}s.
      #
      # @todo Link this to the reference documentation on `@extend`
      #   when such a thing exists.
      #
      # @param extends [Sass::Util::SubsetMap{Selector::Simple => Selector::Sequence}]
      #   The extensions to perform on this tree
      # @return [Tree::Node] The resulting tree of static CSS nodes.
      # @raise [Sass::SyntaxError] Only if there's a programmer error
      #   and this is not a static CSS tree
      def do_extend(extends)
        node = dup
        node.children = children.map {|c| c.do_extend(extends)}
        node
      rescue Sass::SyntaxError => e
        e.modify_backtrace(:filename => filename, :line => line)
        raise e
      end

      # Iterates through each node in the tree rooted at this node
      # in a pre-order walk.
      #
      # @yield node
      # @yieldparam node [Node] a node in the tree
      def each(&block)
        yield self
        children.each {|c| c.each(&block)}
      end

      # Converts a node to Sass code that will generate it.
      #
      # @param options [{Symbol => Object}] An options hash (see {Sass::CSS#initialize})
      # @return [String] The Sass code corresponding to the node
      def to_sass(options = {})
        Sass::Tree::Visitors::Convert.visit(self, options, :sass)
      end

      # Converts a node to SCSS code that will generate it.
      #
      # @param options [{Symbol => Object}] An options hash (see {Sass::CSS#initialize})
      # @return [String] The Sass code corresponding to the node
      def to_scss(options = {})
        Sass::Tree::Visitors::Convert.visit(self, options, :scss)
      end

      # Return a deep clone of this node.
      # The child nodes are cloned, but options are not.
      #
      # @return [Node]
      def deep_copy
        node = dup
        node.children = children.map {|c| c.deep_copy}
        node
      end

      protected

      # @see Sass::Shared.balance
      # @raise [Sass::SyntaxError] if the brackets aren't balanced
      def balance(*args)
        res = Sass::Shared.balance(*args)
        return res if res
        raise Sass::SyntaxError.new("Unbalanced brackets.", :line => line)
      end
    end
  end
end
