{ pkgs, options, config, version, revision, extraSources ? [] }:

with pkgs;

let
  lib = pkgs.lib;

  # Remove invisible and internal options.
  optionsListVisible = lib.filter (opt: opt.visible && !opt.internal) (lib.optionAttrSetToDocList options);

  # Replace functions by the string <function>
  substFunction = x:
    if builtins.isAttrs x then lib.mapAttrs (name: substFunction) x
    else if builtins.isList x then map substFunction x
    else if lib.isFunction x then "<function>"
    else x;

  # Generate DocBook documentation for a list of packages. This is
  # what `relatedPackages` option of `mkOption` from
  # ../../../lib/options.nix influences.
  #
  # Each element of `relatedPackages` can be either
  # - a string:  that will be interpreted as an attribute name from `pkgs`,
  # - a list:    that will be interpreted as an attribute path from `pkgs`,
  # - an attrset: that can specify `name`, `path`, `package`, `comment`
  #   (either of `name`, `path` is required, the rest are optional).
  genRelatedPackages = packages:
    let
      unpack = p: if lib.isString p then { name = p; }
                  else if lib.isList p then { path = p; }
                  else p;
      describe = args:
        let
          name = args.name or (lib.concatStringsSep "." args.path);
          path = args.path or [ args.name ];
          package = args.package or (lib.attrByPath path (throw "Invalid package attribute path `${toString path}'") pkgs);
        in "<listitem>"
        + "<para><literal>pkgs.${name} (${package.meta.name})</literal>"
        + lib.optionalString (!package.meta.available) " <emphasis>[UNAVAILABLE]</emphasis>"
        + ": ${package.meta.description or "???"}.</para>"
        + lib.optionalString (args ? comment) "\n<para>${args.comment}</para>"
        # Lots of `longDescription's break DocBook, so we just wrap them into <programlisting>
        + lib.optionalString (package.meta ? longDescription) "\n<programlisting>${package.meta.longDescription}</programlisting>"
        + "</listitem>";
    in "<itemizedlist>${lib.concatStringsSep "\n" (map (p: describe (unpack p)) packages)}</itemizedlist>";

  optionsListDesc = lib.flip map optionsListVisible (opt: opt // {
    # Clean up declaration sites to not refer to the NixOS source tree.
    declarations = map stripAnyPrefixes opt.declarations;
  }
  // lib.optionalAttrs (opt ? example) { example = substFunction opt.example; }
  // lib.optionalAttrs (opt ? default) { default = substFunction opt.default; }
  // lib.optionalAttrs (opt ? type) { type = substFunction opt.type; }
  // lib.optionalAttrs (opt ? relatedPackages) { relatedPackages = genRelatedPackages opt.relatedPackages; });

  # We need to strip references to /nix/store/* from options,
  # including any `extraSources` if some modules came from elsewhere,
  # or else the build will fail.
  #
  # E.g. if some `options` came from modules in ${pkgs.customModules}/nix,
  # you'd need to include `extraSources = [ pkgs.customModules ]`
  prefixesToStrip = map (p: "${toString p}/") ([ ./.. ] ++ extraSources);
  stripAnyPrefixes = lib.flip (lib.fold lib.removePrefix) prefixesToStrip;

  # Custom "less" that pushes up all the things ending in ".enable*"
  # and ".package*"
  optionLess = a: b:
    let
      ise = lib.hasPrefix "enable";
      isp = lib.hasPrefix "package";
      cmp = lib.splitByAndCompare ise lib.compare
                                 (lib.splitByAndCompare isp lib.compare lib.compare);
    in lib.compareLists cmp a.loc b.loc < 0;

  # Customly sort option list for the man page.
  optionsList = lib.sort optionLess optionsListDesc;

  # Convert the list of options into an XML file.
  optionsXML = builtins.toFile "options.xml" (builtins.toXML optionsList);

  optionsDocBook = runCommand "options-db.xml"
    {
      nativeBuildInputs = [ buildPackages.libxslt.bin ];
    }
    ''
      optionsXML=${optionsXML}
      xsltproc \
        --stringparam program 'home-manager' \
        --stringparam revision '${revision}' \
        -o $out ${./options-to-docbook.xsl} $optionsXML
    '';

  sources = lib.sourceFilesBySuffices ./. [".xml"];

  modulesDoc = builtins.toFile "modules.xml" ''
    <section xmlns:xi="http://www.w3.org/2001/XInclude" id="modules">
    ${(lib.concatMapStrings (path: ''
      <xi:include href="${path}" />
    '') (lib.catAttrs "value" config.meta.doc))}
    </section>
  '';

  generatedSources = runCommand "generated-docbook" {} ''
    mkdir $out
    ln -s ${modulesDoc} $out/modules.xml
    ln -s ${optionsDocBook} $out/options-db.xml
    printf "%s" "${version}" > $out/version
  '';

  copySources =
    ''
      cp -prd $sources/* . # */
      ln -s ${generatedSources} ./generated
      chmod -R u+w .
    '';

  toc = builtins.toFile "toc.xml"
    ''
      <toc role="chunk-toc">
        <d:tocentry xmlns:d="http://docbook.org/ns/docbook" linkend="book-home-manager-manual"><?dbhtml filename="index.html"?>
          <d:tocentry linkend="ch-options"><?dbhtml filename="options.html"?></d:tocentry>
        </d:tocentry>
      </toc>
    '';

  manualXsltprocOptions = toString [
    "--param section.autolabel 1"
    "--param section.label.includes.component.label 1"
    "--stringparam html.stylesheet 'style.css overrides.css highlightjs/mono-blue.css'"
    "--stringparam html.script './highlightjs/highlight.pack.js ./highlightjs/loader.js'"
    "--param xref.with.number.and.title 1"
    "--param toc.section.depth 3"
    "--stringparam admon.style ''"
    "--stringparam callout.graphics.extension .svg"
    "--stringparam current.docid manual"
    "--param chunk.section.depth 0"
    "--param chunk.first.sections 1"
    "--param use.id.as.filename 1"
    "--stringparam generate.toc 'book toc appendix toc'"
    "--stringparam chunk.toc ${toc}"
  ];

  manual-combined = runCommand "home-manager-manual-combined"
    { inherit sources;
      nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin ];
      meta.description = "The Home Manager manual as plain docbook XML";
    }
    ''
      ${copySources}

      xmllint --xinclude --output ./manual-combined.xml ./manual.xml
      xmllint --xinclude --noxincludenode \
         --output ./man-pages-combined.xml ./man-pages.xml

      # outputs the context of an xmllint error output
      # LEN lines around the failing line are printed
      function context {
        # length of context
        local LEN=6
        # lines to print before error line
        local BEFORE=4

        # xmllint output lines are:
        # file.xml:1234: there was an error on line 1234
        while IFS=':' read -r file line rest; do
          echo
          if [[ -n "$rest" ]]; then
            echo "$file:$line:$rest"
            local FROM=$(($line>$BEFORE ? $line - $BEFORE : 1))
            # number lines & filter context
            nl --body-numbering=a "$file" | sed -n "$FROM,+$LEN p"
          else
            if [[ -n "$line" ]]; then
              echo "$file:$line"
            else
              echo "$file"
            fi
          fi
        done
      }

      function lintrng {
        xmllint --debug --noout --nonet \
          --relaxng ${docbook5}/xml/rng/docbook/docbook.rng \
          "$1" \
          2>&1 | context 1>&2
          # ^ redirect assumes xmllint doesn’t print to stdout
      }

      lintrng manual-combined.xml
      lintrng man-pages-combined.xml

      mkdir $out
      cp manual-combined.xml $out/
      cp man-pages-combined.xml $out/
    '';

  olinkDB = runCommand "manual-olinkdb"
    { inherit sources;
      nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin ];
    }
    ''
      xsltproc \
        ${manualXsltprocOptions} \
        --stringparam collect.xref.targets only \
        --stringparam targets.filename "$out/manual.db" \
        --nonet \
        ${docbook5_xsl}/xml/xsl/docbook/xhtml/chunktoc.xsl \
        ${manual-combined}/manual-combined.xml

      cat > "$out/olinkdb.xml" <<EOF
      <?xml version="1.0" encoding="utf-8"?>
      <!DOCTYPE targetset SYSTEM
        "file://${docbook5_xsl}/xml/xsl/docbook/common/targetdatabase.dtd" [
        <!ENTITY manualtargets SYSTEM "file://$out/manual.db">
      ]>
      <targetset>
        <targetsetinfo>
            Allows for cross-referencing olinks between the manpages
            and manual.
        </targetsetinfo>

        <document targetdoc="manual">&manualtargets;</document>
      </targetset>
      EOF
    '';

in rec {
  inherit generatedSources;

  # The Home Manager options in JSON format.
  optionsJSON = runCommand "options-json"
    { meta.description = "List of Home Manager options in JSON format";
    }
    ''
      # Export list of options in different format.
      dst=$out/share/doc/home-manager
      mkdir -p $dst

      cp ${builtins.toFile "options.json" (builtins.unsafeDiscardStringContext (builtins.toJSON
        (builtins.listToAttrs (map (o: { name = o.name; value = removeAttrs o ["name" "visible" "internal"]; }) optionsList))))
      } $dst/options.json

      mkdir -p $out/nix-support
      echo "file json $dst/options.json" >> $out/nix-support/hydra-build-products
    ''; # */

  # Generate the Home Manager manual.
  manual = runCommand "home-manager-manual"
    { inherit sources;
      nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin ];
      meta.description = "The Home Manager manual in HTML format";
      allowedReferences = ["out"];
    }
    ''
      # Generate the HTML manual.
      dst=$out/share/doc/home-manager
      mkdir -p $dst
      xsltproc \
        ${manualXsltprocOptions} \
        --stringparam target.database.document "${olinkDB}/olinkdb.xml" \
        --nonet --output $dst/ \
        ${docbook5_xsl}/xml/xsl/docbook/xhtml/chunktoc.xsl \
        ${manual-combined}/manual-combined.xml

      mkdir -p $dst/images/callouts
      cp ${docbook5_xsl}/xml/xsl/docbook/images/callouts/*.svg $dst/images/callouts/

      cp ${./style.css} $dst/style.css
      cp ${./overrides.css} $dst/overrides.css
      cp -r ${pkgs.documentation-highlighter} $dst/highlightjs

      mkdir -p $out/nix-support
      echo "nix-build out $out" >> $out/nix-support/hydra-build-products
      echo "doc manual $dst" >> $out/nix-support/hydra-build-products
    ''; # */


  manualEpub = runCommand "home-manager-manual-epub"
    { inherit sources;
      buildInputs = [ libxml2.bin libxslt.bin zip ];
    }
    ''
      # Generate the epub manual.
      dst=$out/share/doc/home-manager

      xsltproc \
        ${manualXsltprocOptions} \
        --stringparam target.database.document "${olinkDB}/olinkdb.xml" \
        --nonet --xinclude --output $dst/epub/ \
        ${docbook5_xsl}/xml/xsl/docbook/epub/docbook.xsl \
        ${manual-combined}/manual-combined.xml

      mkdir -p $dst/epub/OEBPS/images/callouts
      cp -r ${docbook5_xsl}/xml/xsl/docbook/images/callouts/*.svg $dst/epub/OEBPS/images/callouts # */
      echo "application/epub+zip" > mimetype
      manual="$dst/home-manager-manual.epub"
      zip -0Xq "$manual" mimetype
      cd $dst/epub && zip -Xr9D "$manual" *

      rm -rf $dst/epub

      mkdir -p $out/nix-support
      echo "doc-epub manual $manual" >> $out/nix-support/hydra-build-products
    '';


  # Generate the Home Manager manpages.
  manpages = runCommand "home-manager-manpages"
    { inherit sources;
      nativeBuildInputs = [ buildPackages.libxml2.bin buildPackages.libxslt.bin ];
      allowedReferences = ["out"];
    }
    ''
      # Generate manpages.
      mkdir -p $out/share/man
      xsltproc --nonet \
        --param man.output.in.separate.dir 1 \
        --param man.output.base.dir "'$out/share/man/'" \
        --param man.endnotes.are.numbered 0 \
        --param man.break.after.slash 1 \
        --stringparam target.database.document "${olinkDB}/olinkdb.xml" \
        ${docbook5_xsl}/xml/xsl/docbook/manpages/docbook.xsl \
        ${manual-combined}/man-pages-combined.xml
    '';

}