require 'sass/scss/rx'
require 'sass/scss/script_parser'

require 'strscan'
require 'set'

module Sass
  module SCSS
    # @todo Add a CSS-only parser that doesn't parse the SassScript extensions,
    #   so css2sass will work properly.
    class Parser
      def initialize(str)
        @scanner = StringScanner.new(str)
        @line = 1
      end

      def parse
        root = stylesheet
        expected("selector or at-rule") unless @scanner.eos?
        root
      end

      private

      include Sass::SCSS::RX

      def stylesheet
        block_contents(node(Sass::Tree::RootNode.new(@scanner.string))) {s}
      end

      def s
        nil while tok(S) || tok(CDC) || tok(CDO) || tok(COMMENT)
        true
      end

      def ss
        nil while tok(S) || tok(COMMENT)
        true
      end

      DIRECTIVES = Set[:mixin, :include, :debug, :for, :while, :if, :import]

      def directive
        return unless tok(/@/)
        name = tok!(IDENT)
        ss

        sym = name.gsub('-', '_').to_sym
        return send(sym) if DIRECTIVES.include?(sym)

        val = str do
          # Most at-rules take expressions (e.g. @media, @import),
          # but some (e.g. @page) take selector-like arguments
          expr || selector
        end
        node = node(Sass::Tree::DirectiveNode.new("@#{name} #{val}".strip))

        if tok(/\{/)
          block_contents(node)
          tok!(/\}/)
        end

        node
      end

      def mixin
        name = tok! IDENT
        args = sass_script_parser.parse_mixin_definition_arglist
        ss
        block(node(Sass::Tree::MixinDefNode.new(name, args)))
      end

      def include
        name = tok! IDENT
        args = sass_script_parser.parse_mixin_include_arglist
        ss
        node(Sass::Tree::MixinNode.new(name, args))
      end

      def debug
        node(Sass::Tree::DebugNode.new(sass_script_parser.parse))
      end

      def for
        tok!(/!/)
        var = tok! IDENT
        ss

        tok!(/from/)
        from = sass_script_parser.parse_until Set["to", "through"]
        ss

        @expected = '"to" or "through"'
        exclusive = (tok(/to/) || tok!(/through/)) == 'to'
        to = sass_script_parser.parse
        ss

        block(node(Sass::Tree::ForNode.new(var, from, to, exclusive)))
      end

      def while
        expr = sass_script_parser.parse
        ss
        block(node(Sass::Tree::WhileNode.new(expr)))
      end

      def if
        expr = sass_script_parser.parse
        ss
        block(node(Sass::Tree::IfNode.new(expr)))
      end

      def import
        if path = tok(STRING) || tok(URI)
          ss

          media = str do
            if tok IDENT
              ss
              while tok(/,/)
                ss; tok(IDENT); ss
              end
            end
          end

          return node(Sass::Tree::DirectiveNode.new("@import #{path} #{media}".strip))
        end

        # TODO: Do we want a better way of specifying imports?
        # We could use the CSS syntax,
        # but then how do users actually compile to the CSS syntax?
        node(Sass::Tree::ImportNode.new(tok(PATH).strip))
      end

      def variable
        return unless tok(/!/)
        name = tok!(IDENT)
        ss

        if tok(/\|/)
          tok!(/\|/)
          guarded = true
        end

        tok!(/=/)
        ss
        expr = sass_script_parser.parse

        node(Sass::Tree::VariableNode.new(name, expr, guarded))
      end

      def operator
        # Many of these operators (all except / and ,)
        # are disallowed by the CSS spec,
        # but they're included here for compatibility
        # with some proprietary MS properties
        ss if tok(/[\/,:.=]/)
        true
      end

      def unary_operator
        tok(/[+-]/)
      end

      def property
        return unless name = tok(IDENT)
        ss
        name
      end

      def ruleset
        rules = str do
          return unless selector

          while tok(/,/)
            ss; expr!(:selector)
          end
        end

        block(node(Sass::Tree::RuleNode.new(rules.strip)))
      end

      def block(node)
        tok!(/\{/)
        block_contents(node)
        tok!(/\}/)
        ss
        node
      end

      # A block may contain declarations and/or rulesets
      def block_contents(node)
        block_given? ? yield : ss
        node << (child = block_child)
        while tok(/;/) || (child && !child.children.empty?)
          block_given? ? yield : ss
          node << (child = block_child)
        end
        node
      end

      def block_child
        variable || directive || declaration_or_ruleset
      end

      # This is a nasty hack, and the only place in the parser
      # that requires backtracking.
      # The reason is that we can't figure out if certain strings
      # are declarations or rulesets with fixed finite lookahead.
      # For example, "foo:bar baz baz baz..." could be either a property
      # or a selector.
      #
      # To handle this, we simply check if it works as a property
      # (which is the most common case)
      # and, if it doesn't, try it as a ruleset.
      #
      # We could eke some more efficiency out of this
      # by handling some easy cases (first token isn't an identifier,
      # no colon after the identifier, whitespace after the colon),
      # but I'm not sure the gains would be worth the added complexity.
      #
      # TODO: We should handle these special cases if only so that
      # errors get reported for the proper interpretation most of the time,
      # rather than just for rulesets.
      def declaration_or_ruleset
        pos = @scanner.pos
        line = @line
        begin
          decl = declaration
        rescue Sass::SyntaxError
        end

        return decl if decl && tok?(/[;}]/)
        @line = line
        @scanner.pos = pos
        return ruleset
      end

      def selector
        # The combinator here allows the "> E" hack
        return unless combinator || simple_selector_sequence
        simple_selector_sequence while combinator
        true
      end

      def combinator
        tok(PLUS) || tok(GREATER) || tok(TILDE) || tok(S)
      end

      def simple_selector_sequence
        unless element_name || tok(HASH) || class_expr ||
            attrib || negation || pseudo
          # This allows for stuff like http://www.w3.org/TR/css3-animations/#keyframes-
          return expr
        end

        # The tok(/\*/) allows the "E*" hack
        nil while tok(HASH) || class_expr || attrib ||
          negation || pseudo || tok(/\*/)
        true
      end

      def class_expr
        return unless tok(/\./)
        tok! IDENT
      end

      def element_name
        res = tok(IDENT) || tok(/\*/)
        if tok(/\|/)
          @expected = "element name or *"
          res = tok(IDENT) || tok!(/\*/)
        end
        res
      end

      def attrib
        return unless tok(/\[/)
        ss
        attrib_name!; ss
        if tok(/=/) ||
            tok(INCLUDES) ||
            tok(DASHMATCH) ||
            tok(PREFIXMATCH) ||
            tok(SUFFIXMATCH) ||
            tok(SUBSTRINGMATCH)
          ss
          @expected = "identifier or string"
          tok(IDENT) || tok!(STRING); ss
        end
        tok!(/\]/)
      end

      def attrib_name!
        if tok(IDENT)
          # E, E|E, or E|
          # The last is allowed so that E|="foo" will work
          tok(IDENT) if tok(/\|/)
        elsif tok(/\*/)
          # *|E
          tok!(/\|/)
          tok! IDENT
        else
          # |E or E
          tok(/\|/)
          tok! IDENT
        end
      end

      def pseudo
        return unless tok(/::?/)

        @expected = "pseudoclass or pseudoelement"
        functional_pseudo || tok!(IDENT)
      end

      def functional_pseudo
        return unless tok FUNCTION
        ss
        expr! :pseudo_expr
        tok!(/\)/)
      end

      def pseudo_expr
        return unless tok(PLUS) || tok(/-/) || tok(NUMBER) ||
          tok(STRING) || tok(IDENT)
        ss
        ss while tok(PLUS) || tok(/-/) || tok(NUMBER) ||
          tok(STRING) || tok(IDENT)
        true
      end

      def negation
        return unless tok(NOT)
        ss
        @expected = "selector"
        element_name || tok(HASH) || class_expr || attrib || expr!(:pseudo)
        tok!(/\)/)
      end

      def declaration
        # The tok(/\*/) allows the "*prop: val" hack
        if tok(/\*/)
          name = '*' + expr!(:property)
        else
          return unless name = property
        end

        value =
          if tok(/=/)
            sass_script_parser.parse
          else
            @expected = '":" or "="'
            tok!(/:/); ss

            str do
              expr! :expr
              prio
            end.strip
          end

        node(Sass::Tree::PropNode.new(name, value, :new))
      end

      def prio
        return unless tok IMPORTANT
        ss
      end

      def expr
        return unless term
        nil while operator && term
        true
      end

      def term
        unless tok(NUMBER) ||
            tok(URI) ||
            function ||
            tok(STRING) ||
            tok(UNICODERANGE) ||
            tok(IDENT) ||
            hexcolor
          return unless unary_operator
          @expected = "number or function"
          tok(NUMBER) || expr!(:function)
        end
        ss
      end

      def function
        return unless tok FUNCTION
        ss
        expr
        tok!(/\)/); ss
      end

      #  There is a constraint on the color that it must
      #  have either 3 or 6 hex-digits (i.e., [0-9a-fA-F])
      #  after the "#"; e.g., "#000" is OK, but "#abcd" is not.
      def hexcolor
        return unless tok HASH
        ss
      end

      def str
        @str = ""
        yield
        @str
      ensure
        @str = nil
      end

      def node(node)
        node.line = @line
        node
      end

      def sass_script_parser
        ScriptParser.new(@scanner, @line,
          @scanner.pos - (@scanner.string.rindex("\n") || 0))
      end

      EXPR_NAMES = {
        :medium => "medium (e.g. print, screen)",
        :pseudo_expr => "expression (e.g. fr, 2n+1)",
        :expr => "expression (e.g. 1px, bold)",
      }

      TOK_NAMES = Haml::Util.to_hash(
        Sass::SCSS::RX.constants.map {|c| [Sass::SCSS::RX.const_get(c), c.downcase]}).
        merge(IDENT => "identifier")

      def tok?(rx)
        @scanner.match?(rx)
      end

      def expr!(name)
        (e = send(name)) && (return e)
        expected(EXPR_NAMES[name] || name.to_s)
      end

      def tok!(rx)
        (t = tok(rx)) && (return t)
        expected(TOK_NAMES[rx])
      end

      def expected(name)
        pos = @scanner.pos

        after = @scanner.string[0...pos]
        # Get rid of whitespace between pos and the last token,
        # but only if there's a newline in there
        after.gsub!(/\s*\n\s*$/, '')
        # Also get rid of stuff before the last newline
        after.gsub!(/.*\n/, '')
        after = "..." + after[-15..-1] if after.size > 18

        expected = @expected || name

        was = @scanner.rest.dup
        # Get rid of whitespace between pos and the next token,
        # but only if there's a newline in there
        was.gsub!(/^\s*\n\s*/, '')
        # Also get rid of stuff after the next newline
        was.gsub!(/\n.*/, '')
        was = was[0...15] + "..." if was.size > 18

        raise Sass::SyntaxError.new(
          "Invalid CSS after \"#{after}\": expected #{expected}, was \"#{was}\"",
          :line => @line)
      end

      def tok(rx)
        res = @scanner.scan(rx)
        if res
          @line += res.count("\n")
          @expected = nil
          @str << res if @str && rx != COMMENT
        end

        res
      end
    end
  end
end