require "nokogiri"
require "pathname"

files = ARGV[0..-1]

class LineInfo
  attr_accessor :links

  def initialize
    @links = []
  end
end

class Handler
  def initialize
    @file_infos = {}
  end

  # Traverse the preformatted Agda block in the given Agda HTML file
  # and find which textual ranges have IDs and links to other ranges.
  # Store this information in a hash, line => LineInfo.
  def process_agda_html_file(file)
    return @file_infos[file] if @file_infos.include? file

    @file_infos[file] = line_infos = {}
    unless File.exists?(file)
      return line_infos
    end

    document = Nokogiri::HTML.parse(File.open(file))
    pre_code = document.css("pre.Agda")[0]

    # The traversal postorder; we always visit children before their
    # parents, and we visit leaves in sequence.
    offset = 0
    line = 1
    pre_code.traverse do |at|
      # Text nodes are always leaves; visiting a new leaf means we've advanced
      # in the text by the length of that text. However, if there are newlines
      # -- since this is a preformatted block -- we also advanced by a line.
      # At this time, do not support links that span multiple lines, but
      # Agda doesn't produce those either.
      if at.text?
        if at.content.include? "\n"
          # This textual leaf is at least part whitespace. The logic
          # assumes that links can't span multiple pages, and that links
          # aren't nested, so ensure that the parent of the textual node
          # is the preformatted block itself.
          raise "unsupported Agda HTML output" if at.parent.name != "pre"

          # Increase the line and track the final offset. Written as a loop
          # in case we eventually want to add some handling for the pieces
          # sandwiched between newlines.
          at.content.split("\n", -1).each_with_index do |bit, idx|
            line += 1 unless idx == 0
            offset = bit.length
          end
        else
          # It's not a newline node, so it could be anywhere. All we need to
          # do is adjust the offset within the full pre block's text.
          offset += at.content.length
        end
      elsif at.name == "a"
        # We found a link. Agda emits both links and "things to link to" as
        # 'a' nodes, so check for either, and record them. Even if
        # the link is nested, the .content.length accessor will only
        # retrieve the textual content, and thus -- assuming the link
        # isn't split across lines -- will find the proper from-to range.

        line_info = line_infos.fetch(line) { line_infos[line] = LineInfo.new }
        href = at.attribute("href")
        id = at.attribute("id")
        if href or id
          new_link = { :from => offset-at.content.length, :to => offset }
          new_link[:href] = href if href
          new_link[:id] = id if id

          line_info.links << new_link
        end
      end
    end
    return line_infos
  end

  # Given a Hugo HTML file which references potentially several Agda modules
  # in code blocks, insert links into the code blocks.
  #
  # There are several things we need to take care of:
  # 1. Finding the HTML files associated with each referenced Agda module.
  #    For this, we make use of the data-base-path etc. attributes that
  #    the vanilla theme inserts.
  # 2. "zipping together" the Agda and Hugo HTML representations. Each of
  #    them encode the code, but they use different HTML elements and structures.
  #    So, given a Hugo HTML code block, traverse its textual contents
  #    and find any that are covered by links in the related Agda HTML file.
  # 3. Fixing up links: the Agda HTML links assume each module has its own HTML
  #    file. This isn't true for us: multiple modules are stitched into
  #    one Hugo HTML file. Also, we don't include the entire Agda HTML
  #    file in the Hugo HTML, so some links may be broken. So, find IDs
  #    that are visible in the Hugo file, rename them to be globally unique,
  #    and re-write cross-file links that reference these IDs to point
  #    inside the Hugo file.
  def process_source_file(file)
    nodes_referencing_href = {} # Agda HTML ID -> all (Hugo HTML) nodes that link to it
    seen_hrefs = {} # Agda HTML ID -> its new ID in Hugo-land
    id_counter = 0

    # Process each highlight group that's been marked as an Agda file.
    document = Nokogiri::HTML.parse(File.open(file))
    document.css('div[data-agda-block]').each do |t|
      first_line, last_line = nil, nil

      if first_line_attr = t.attribute("data-first-line")
        first_line = first_line_attr.to_s.to_i
      end
      if last_line_attr = t.attribute("data-last-line")
        last_line = last_line_attr.to_s.to_i
      end

      # The name of an Agda module is determined from its directory
      # structure: A/B/C.agda creates A.B.C.html. Assume that the
      # base path is the Agda module root, and use the remaining directories
      # to build the HTML file name.
      base_path = t.attribute("data-base-path").to_s
      base_path_dirs = Pathname(base_path).each_filename.to_a
      full_path = t.attribute("data-file-path").to_s
      full_path_dirs = Pathname(full_path).each_filename.to_a

      dirs_in_base = full_path_dirs[base_path_dirs.length..-1]
      html_file = dirs_in_base.join(".").gsub(/\.agda$/, ".html")
      html_path = File.join(["code", base_path, "html", html_file])

      agda_info = process_agda_html_file(html_path)

      # Hugo conveniently generates a bunch of spans, each encoding a line
      # of code output. We can iterate over these and match them up with
      # the line numbers we got from reading the Agda HTML output.
      lines = t.css("pre.chroma code[data-lang] .line")
      lines.zip((first_line..last_line)).each do |line, line_no|
        line_info = agda_info[line_no]
        next unless line_info

        offset = 0
        line.traverse do |lt|
          if lt.text?
            content = lt.content
            new_offset = offset + content.length

            # The span/a/etc. structure of the Agda and Hugo HTML files
            # need not line up; it's possible for there to be a single link
            # in the Agda file that's broken up across multiple HTML nodes
            # in the Hugo output. For now, just don't link those, since inserting
            # such overlapping links is relatively complicated. Instead,
            # require links to fit fully within a current text node (and thus,
            # not overlap the boundaries of any HTML).
            matching_links = line_info.links.filter do |link|
              link[:from] >= offset and link[:to] <= new_offset
            end

            # A given text node can be broken into any number of sub-nodes,
            # where some sub-nodes are still text, and others have been turned
            # into links. Store the new pieces in replace_with. E.g.,
            #
            # Original:
            #   abc
            #
            # New:
            #   a<a href="..">b</a>c
            #
            # replace_with:
            #   ["a", <Nokogiri::XML::Node...>, "c"]
            #
            # match_offset represents how much of the original text we've
            # already converted. The below iteration assumes that matching
            # links are in order, and don't overlap.
            replace_with = []
            replace_offset = 0
            matching_links.each do |match|
              # The link's range is an offset from the beginning of the line,
              # but the text piece we're splitting up might be partway into
              # the line. Convert the link coordinates to piece-relative ones.
              relative_from = match[:from] - offset
              relative_to = match[:to] - offset

              # If the previous link ended some time before the new link
              # began (or if the current link is the first one, and is not
              # at the beginning), ensure that the plain text "in between"
              # is kept.
              replace_with << content[replace_offset...relative_from]

              tag = (match.include? :href) ? 'a' : 'span'
              new_link = Nokogiri::XML::Node.new(tag, document)
              if match.include? :href
                # For nodes with links, note what they're referring to, so
                # we can adjust their hrefs when we assign global IDs.
                href = match[:href].to_s
                new_link['href'] = href
                ref_list = nodes_referencing_href.fetch(href) { nodes_referencing_href[href] = [] }
                ref_list << new_link
              end
              if match.include? :id
                # For nodes with IDs visible in the current Hugo file, we'll
                # want to redirect links that previously go to other Agda
                # module HTML files. So, note the ID that we want to redirect,
                # and pick a new unique ID to replace it with.
                id = match[:id].to_s
                uniq_id = "agda-unique-ident-#{id_counter}"
                id_counter += 1
                new_link['id'] = uniq_id
                seen_hrefs["#{html_file}##{id}"] = uniq_id
              end
              new_link.content = content[relative_from...relative_to]

              replace_with << new_link
              replace_offset = relative_to
            end
            replace_with << content[replace_offset..-1]

            # Finally, replace the node under consideration with the new
            # pieces.
            replace_with.each do |replacement|
              lt.add_previous_sibling replacement
            end
            lt.remove

            offset = new_offset
          end
        end
      end
    end

    # Now, we have a complete list of all the IDs visible in scope.
    # Redirect relevant links to these IDs. This achieves within-post
    # links.
    seen_hrefs.each do |agda_id, hugo_id|
      nodes_to_rewrite = nodes_referencing_href[agda_id]
      next unless nodes_to_rewrite

      nodes_to_rewrite.each do |node|
        node['href'] = "##{hugo_id}"
      end

      nodes_referencing_href.delete agda_id
    end

    # The remaining nodes reference symbols not in the current Hugo document,
    # so just replace them with their contents for now (we'd need some
    # external hosting to make this work otherwise).
    nodes_referencing_href.each do |href, nodes|
      nodes.each do |node|
        node.replace node.content
      end
    end

    File.write(file, document.to_html(encoding: 'UTF-8'))
  end
end

handler = Handler.new
files.each do |file|
  handler.process_source_file file
end