# Nick's web site: css_darkmode filter.  Cut out dark-mode media queries
# from a stylesheet and either re-insert them at the end of the same
# stylesheet, or produce a separate stylesheet for use as an alternate.
#
# Copyright © 2022 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 CssDarkModeFilter < Nanoc::Filter
    identifier :css_darkmode

    require 'crass'

    def filter_whitespace(nodes)
        return nodes.reject {|x| x[:node] == :whitespace }
    end

    def is_media_dark_block(x)
        return false unless x[:node] == :simple_block

        list = filter_whitespace(x[:value])
        list.length == 3 and
            list[0][:node] == :ident and
            list[0][:value] == "prefers-color-scheme" and
            list[1][:node] == :colon and
            list[2][:node] == :ident and
            list[2][:value] == "dark"
    end

    def is_media_dark(x)
        return false unless x[:node] == :at_rule and x[:name] == "media"

        x[:prelude].index {|y| is_media_dark_block(y)}
    end

    # Remove (prefers-color-scheme: dark) conditions from a media query.
    # If the resulting query is empty, returns the query's block alone.
    # Otherwise, returns the modified query.
    def prune_media_dark(x)
        start_index = 0
        end_index = 0
        state = :init

        x[:prelude].each do |y|
            case state
            when :init
                end_index += 1
                if is_media_dark_block(y)
                    state = :post
                elsif y[:node] == :ident and y[:value] == "and"
                    state = :pre
                else
                    start_index = end_index
                    state = :init
                end
            when :pre
                end_index += 1
                if is_media_dark_block(y)
                    state = :post
                elsif y[:node] == :whitespace
                    state = :pre
                else
                    start_index = end_index
                    state = :init
                end
            when :post
                if y[:node] == :whitespace
                    end_index += 1
                    state = :post
                elsif y[:node] == :ident and y[:value] == "and"
                    end_index += 1
                    state = :post
                else
                    break
                end
            end
        end

        x[:prelude].slice!(start_index..end_index-1)
        return x[:block] if filter_whitespace(x[:prelude]).empty?
        x
    end

    def equiv_query(a, b)
        return false unless a.class == b.class

        case a
        when Array
            a = filter_whitespace(a)
            b = filter_whitespace(b)

            return false unless a.length == b.length
            return a.map.with_index{|x,i| equiv_query(x, b[i])}.all?
        when Hash
            return false unless a[:node] == b[:node]
            case a[:node]
            when :at_rule
                return equiv_query(a[:prelude], b[:prelude])
            when :simple_block
                return equiv_query(a[:value], b[:value])
            else
                return (a[:value] == b[:value] and a[:unit] == b[:unit])
            end
        end
    end

    def run(content, params = {})
        params = {
            preserve_comments: true,
            preserve_hacks: true
            }.merge(params)

        tree = Crass.parse(content, params)

        darknodes = []
        tree.delete_if { |x| darknodes << x if is_media_dark(x) }

        # Combine consecutive equivalent media queries into a single query
        darknodes = darknodes.slice_when{|a,b| !equiv_query(a, b)}.each.map \
        do |x|
            combined = x[0].merge({block: x.map{|x| x[:block]}.flatten})
        end

        # In alternate mode, remove prefers-color-scheme queries.
        if params[:alternate]
            darknodes.map!{|x| prune_media_dark(x)}
        end

        darkcss = ""
        darknodes.each do |x|
            darkcss += "#{Crass::Parser.stringify(x).rstrip}\n"
        end
        darkcss.sub!(/^\n*/, "")

        if params[:alternate]
            prologue = tree.take_while do |x|
                x[:node] == :comment or x[:node] == :whitespace
            end

            "#{Crass::Parser.stringify(prologue).rstrip}\n\n#{darkcss}"
        else
            output = "#{Crass::Parser.stringify(tree).rstrip}\n"
            output += "\n#{darkcss}" unless darkcss.empty?
            output
        end
    end
end