# encoding: utf-8
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('(?:<.+?>)|(?:\p{Word}+)|(?:[ \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