module Danbooru
  TAG_DEL = '<del>'
  TAG_INS = '<ins>'
  TAG_DEL_CLOSE = '</del>'
  TAG_INS_CLOSE = '</ins>'
  TAG_NEWLINE = "↲\n"
  TAG_BREAK = "<br>\n"

  # Produce a formatted page that shows the difference between two versions of a page.
  def diff(old, new)
    pattern = Regexp.new('(?:<.+?>)|(?:[0-9_A-Za-z\x80-\xff]+[\x09\x20]?)|(?:[ \t]+)|(?:\r?\n)|(?:.+?)')

    thisarr = old.scan(pattern)
    otharr = new.scan(pattern)

    cbo = Diff::LCS::ContextDiffCallbacks.new
    diffs = thisarr.diff(otharr, cbo)

    escape_html = lambda {|str| str.gsub(/&/,'&amp;').gsub(/</,'&lt;').gsub(/>/,'&gt;')}

    output = thisarr;
    output.each { |q| q.replace(escape_html[q]) }

    diffs.reverse_each do |hunk|
      newchange = hunk.max{|a,b| a.old_position <=> b.old_position}
      newstart = newchange.old_position
      oldstart = hunk.min{|a,b| a.old_position <=> b.old_position}.old_position

      if newchange.action == '+'
        output.insert(newstart, TAG_INS_CLOSE)
      end

      hunk.reverse_each do |chg|
        case chg.action
        when '-'
          oldstart = chg.old_position
          output[chg.old_position] = TAG_NEWLINE if chg.old_element.match(/^\r?\n$/)
        when '+'
          if chg.new_element.match(/^\r?\n$/)
            output.insert(chg.old_position, TAG_NEWLINE)
          else
            output.insert(chg.old_position, "#{escape_html[chg.new_element]}")
          end
        end
      end

      if newchange.action == '+'
        output.insert(newstart, TAG_INS)
      end

      if hunk[0].action == '-'
        output.insert((newstart == oldstart || newchange.action != '+') ? newstart+1 : newstart, TAG_DEL_CLOSE)
        output.insert(oldstart, TAG_DEL)
      end
    end

    output.join.gsub(/\r?\n/, TAG_BREAK)
  end

  module_function :diff
end