require "nokogiri"
require "pathname"

files = ARGV[0..-1]

class LineInfo
  attr_accessor :links

  def initialize
    @links = []
  end
end

class AgdaContext
  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_node = { :from => offset-at.content.length, :to => offset }
          new_node[:href] = href if href
          new_node[:id] = id if id

          line_info.links << new_node
        end
      end
    end
    return line_infos
  end
end

class FileGroup
  def initialize(agda_context)
    @agda_context = agda_context
    # Agda HTML href -> list of (file, Hugo HTML node that links to it)
    @nodes_referencing_href = {}
    # Agda HTML href -> (its new ID in Hugo-land, file in which it's defined)
    # This supports cross-post linking within a seires.
    @global_seen_hrefs = {}
    # file name -> Agda HTML href -> its new ID in Hugo-land
    # This supports linking within a particular post.
    @local_seen_hrefs = Hash.new { {} }
    # Global counter to generate fresh IDs. There's no reason for it to
    # be global within a series (since IDs are namespaced by the file they're in),
    # but it's just more convenient this way.
    @id_counter = 0
  end

  def note_defined_href(file, href)
    file_hrefs = @local_seen_hrefs.fetch(file) do
      @local_seen_hrefs[file] = {}
    end

    uniq_id = file_hrefs.fetch(href) do
      new_id = "agda-unique-ident-#{@id_counter}"
      @id_counter += 1
      file_hrefs[href] = new_id
    end

    unless @global_seen_hrefs.include? href
      @global_seen_hrefs[href] = { :file => file, :id => uniq_id }
    end

    return uniq_id
  end

  def note_used_href(file, node, href)
    ref_list = @nodes_referencing_href.fetch(href) { @nodes_referencing_href[href] = [] }
    ref_list << { :file => file, :node => node }
    return href
  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, document)
    # Process each highlight group that's been marked as an Agda 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

      if first_line and last_line
        line_range = first_line..last_line
      else
        line_range = 1..
      end

      # Sometimes, code is deeply nested in the source file, but we don't
      # want to show the leading space. In that case, the generator sets
      # data-source-offset with how much leading space was stripped off.
      initial_offset = 0
      if source_offset_attr = t.attribute("data-source-offset")
        initial_offset = source_offset_attr.to_s.to_i
      end

      full_path = t.attribute("data-file-path").to_s
      full_path_dirs = Pathname(full_path).each_filename.to_a

      # The name of an Agda module is determined from its directory
      # structure: A/B/C.agda creates A.B.C.html. Depending on where
      # the code is included, there might be some additional folders
      # that precede A that we want to ignore.
      base_path = t.attribute("data-base-path").to_s
      base_dir_depth = 0
      if base_path.empty?
        # No submodules were used. Assume code/<X> is the root, since
        # that's the code layout of the blog right now.
        base_dir_depth = 1
        base_path = full_path_dirs[0]
      else
        # The code is in a submodule. Assume that the base path / submodule
        # root is the Agda module root, ignore all folders before that.
        base_path_dirs = Pathname(base_path).each_filename.to_a
        base_dir_depth = base_path_dirs.length
      end

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

      agda_info = @agda_context.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(line_range).each do |line, line_no|
        line_info = agda_info[line_no]
        next unless line_info

        offset = initial_offset
        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_node = 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_node['href'] = note_used_href file, new_node, href
              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
                new_node['id'] = note_defined_href file, "#{html_file}##{id}"
              end
              new_node.content = content[relative_from...relative_to]

              replace_with << new_node
              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
  end

  def cross_link_files
    # Now, we have a complete list of all the IDs visible in scope.
    # Redirect relevant links to these IDs. This achieves within-post
    # links.
    @nodes_referencing_href.each do |href, references|
      references.each do |reference|
        file = reference[:file]
        node = reference[:node]

        local_targets = @local_seen_hrefs[file]
        if local_targets.include? href
          # A code block in this fine provides this href, create a local link.
          node['href'] = "##{local_targets[href]}"
        elsif @global_seen_hrefs.include? href
          # A code block in this series, but not in this file, defines
          # this href. Create a cross-file link.
          target = @global_seen_hrefs[href]
          other_file = target[:file]
          id = target[:id]

          relpath = Pathname.new(other_file).dirname.relative_path_from(Pathname.new(file).dirname)
          node['href'] = "#{relpath}##{id}"
        else
          # No definitions in any blog page. For now, just delete the anchor.
          node.replace node.content
        end
      end
    end
  end
end

agda_context = AgdaContext.new

file_documents = {}
series_groups = files.group_by do |file|
  file_documents[file] = document = Nokogiri::HTML.parse(File.open(file))
  document.css("meta[name=blog-series]")&.attribute("content")&.to_s
end

# For the 'nil' group, process individually.
if files_with_no_series = series_groups.delete(nil)
  files_with_no_series.each do |file|
    file_group = FileGroup.new agda_context
    file_group.process_source_file file, file_documents[file]
    file_group.cross_link_files
  end
end

# For groups, process them together to allow cross-linking
series_groups.each do |_, files_in_series|
  file_group = FileGroup.new agda_context
  files_in_series.each do |file|
    file_group.process_source_file file, file_documents[file]
  end
  file_group.cross_link_files
end

# Having modified all the HTML files, save them.
file_documents.each do |file, document|
  File.write(file, document.to_html(encoding: 'UTF-8'))
end