# Nick's web site: xhtml_compat filter.  Perform fixups to improve
# XHTML compatibility with various user agents.
#
# Copyright © 2019-2021 Nick Bowler
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

class XhtmlCompatFilter < Nanoc::Filter
    identifier :xhtml_compat
    requires 'nokogiri'

    Xmlns = {
        math: 'http://www.w3.org/1998/Math/MathML',
        svg: 'http://www.w3.org/2000/svg',
    }.freeze

    XHTMLPublic = '-//W3C//DTD XHTML 1.1//EN'
    MathPublic  = '-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN'
    MathSystem  = 'http://www.w3.org/Math/DTD/mathml2/xhtml-math11-f.dtd'
    SVGPublic   = '-//W3C//DTD XHTML 1.1 plus MathML 2.0 plus SVG 1.1//EN'
    SVGSystem   = 'http://www.w3.org/2002/04/xhtml-math-svg/xhtml-math-svg.dtd'

    # XSLT 1.0 as implemented in Nokogiri canot construct doctypes based
    # on content.  When using MathML or SVG elements in XHTML a different
    # doctype is needed: select one based on which elements are present.
    def fix_doctype(content, params = {})
        return "#{content}" if not params[:fix_doctype]

        doc = Nokogiri::XML(content)
        doctype = doc.internal_subset

        return "#{content}" if doctype.external_id != XHTMLPublic

        if not doc.xpath("//svg:svg", Xmlns).empty?
            doctype.remove
            doc.create_internal_subset("html", SVGPublic, SVGSystem)
        elsif not doc.xpath("//math:math", Xmlns).empty?
            doctype.remove
            doc.create_internal_subset("html", MathPublic, MathSystem)
        end

        return doc.to_xml
    end

    def run(content, params = {})
        text = fix_doctype(content, params)

        # Old versions of Netscape get confused by <hr/> but have no problem
        # with <hr />, so avoid that by adding spaces to such elements.
        text.gsub!(/([^[:space:]])\/>/m, '\1 />');

        # Even older versions of Netscape interpret any script as Javascript,
        # which causes major problems with the CDATA hack; solve that by making
        # the whole thing look like a Javascript comment.
        text.gsub!("<![CDATA[]]x><!--]]>", '/*\&')
        text.gsub!("<![CDATA[-->]]>", '\&*/')

        # Delete any zero-width word joiners added for XSLT processing.
        text.delete! "\u2060"

        return text
    end
end