{ config, options, lib, pkgs, utils, modules, baseModules, extraModules, modulesPath, ... }:

with lib;

let

  cfg = config.documentation;
  allOpts = options;

  /* Modules for which to show options even when not imported. */
  extraDocModules = [ ../virtualisation/qemu-vm.nix ];

  canCacheDocs = m:
    let
      f = import m;
      instance = f (mapAttrs (n: _: abort "evaluating ${n} for `meta` failed") (functionArgs f));
    in
      cfg.nixos.options.splitBuild
        && builtins.isPath m
        && isFunction f
        && instance ? options
        && instance.meta.buildDocsInSandbox or true;

  docModules =
    let
      p = partition canCacheDocs (baseModules ++ extraDocModules);
    in
      {
        lazy = p.right;
        eager = p.wrong ++ optionals cfg.nixos.includeAllModules (extraModules ++ modules);
      };

  manual = import ../../doc/manual rec {
    inherit pkgs config;
    version = config.system.nixos.release;
    revision = "release-${version}";
    extraSources = cfg.nixos.extraModuleSources;
    options =
      let
        scrubbedEval = evalModules {
          modules = [ {
            _module.check = false;
          } ] ++ docModules.eager;
          specialArgs = {
            pkgs = scrubDerivations "pkgs" pkgs;
            # allow access to arbitrary options for eager modules, eg for getting
            # option types from lazy modules
            options = allOpts;
            inherit modulesPath utils;
          };
        };
        scrubDerivations = namePrefix: pkgSet: mapAttrs
          (name: value:
            let wholeName = "${namePrefix}.${name}"; in
            if isAttrs value then
              scrubDerivations wholeName value
              // (optionalAttrs (isDerivation value) { outPath = "\${${wholeName}}"; })
            else value
          )
          pkgSet;
      in scrubbedEval.options;
    baseOptionsJSON =
      let
        filterIntoStore =
          builtins.filterSource
            (n: t:
              (t == "directory" -> baseNameOf n != "tests")
              && (t == "file" -> hasSuffix ".nix" n)
            );

        # Figure out if Nix runs in pure evaluation mode. May return true in
        # impure mode, but this is highly unlikely.
        # We need to know because of https://github.com/NixOS/nix/issues/1888
        # and https://github.com/NixOS/nix/issues/5868
        isPureEval = builtins.getEnv "PATH" == "" && builtins.getEnv "_" == "";

        # Return a nixpkgs subpath with minimal copying.
        #
        # The sources for the base options json derivation can come in one of
        # two forms:
        #   - single source: a store path with all of nixpkgs, postfix with
        #     subpaths to access various directories. This has the benefit of
        #     not creating copies of these subtrees in the Nix store, but
        #     can cause unnecessary rebuilds if you update the Nixpkgs `pkgs`
        #     tree often.
        #   - split sources: multiple store paths with subdirectories of
        #     nixpkgs that exclude the bulk of the pkgs directory.
        #     This requires more copying and hashing during evaluation but
        #     requires fewer files to be copied. This method produces fewer
        #     unnecessary rebuilds of the base options json.
        #
        # Flake
        #
        # Flakes always put a copy of the full nixpkgs sources in the store,
        # so we can use the "single source" method. This method is ideal
        # for using nixpkgs as a dependency, as the base options json will be
        # substitutable from cache.nixos.org.
        #
        # This requires that the `self.outPath` is wired into `pkgs` correctly,
        # which is done for you if `pkgs` comes from the `lib.nixosSystem` or
        # `legacyPackages` flake attributes.
        #
        # Other Nixpkgs invocation
        #
        # If you do not use the known-correct flake attributes, but rather
        # invoke Nixpkgs yourself, set `config.path` to the correct path value,
        # e.g. `import nixpkgs { config.path = nixpkgs; }`.
        #
        # Choosing between single or split source paths
        #
        # We make assumptions based on the type and contents of `pkgs.path`.
        # By passing a different `config.path` to Nixpkgs, you can influence
        # how your documentation cache is evaluated and rebuilt.
        #
        # Single source
        #  - If pkgs.path is a string containing a store path, the code has no
        #    choice but to create this store path, if it hasn't already been.
        #    We assume that the "single source" method is most efficient.
        #  - If pkgs.path is a path value containing that is a store path,
        #    we try to convert it to a string with context without copying.
        #    This occurs for example when nixpkgs was fetched and using its
        #    default `config.path`, which is `./.`.
        #    Nix currently does not allow this conversion when evaluating in
        #    pure mode. If the conversion is not possible, we use the
        #    "split source" method.
        #
        # Split source
        #  - If pkgs.path is a path value that is not a store path, we assume
        #    that it's unlikely for all of nixpkgs to end up in the store for
        #    other reasons and try to keep both the copying and rebuilds low.
        pull =
          if builtins.typeOf pkgs.path == "string" && isStorePath pkgs.path then
            dir: "${pkgs.path}/${dir}"
          else if !isPureEval && isStorePath pkgs.path then
            dir: "${builtins.storePath pkgs.path}/${dir}"
          else
            dir: filterIntoStore "${toString pkgs.path}/${dir}";
      in
        pkgs.runCommand "lazy-options.json" {
          libPath = pull "lib";
          pkgsLibPath = pull "pkgs/pkgs-lib";
          nixosPath = pull "nixos";
          modules = map (p: ''"${removePrefix "${modulesPath}/" (toString p)}"'') docModules.lazy;
        } ''
          export NIX_STORE_DIR=$TMPDIR/store
          export NIX_STATE_DIR=$TMPDIR/state
          ${pkgs.buildPackages.nix}/bin/nix-instantiate \
            --show-trace \
            --eval --json --strict \
            --argstr libPath "$libPath" \
            --argstr pkgsLibPath "$pkgsLibPath" \
            --argstr nixosPath "$nixosPath" \
            --arg modules "[ $modules ]" \
            --argstr stateVersion "${options.system.stateVersion.default}" \
            --argstr release "${config.system.nixos.release}" \
            $nixosPath/lib/eval-cacheable-options.nix > $out \
            || {
              echo -en "\e[1;31m"
              echo 'Cacheable portion of option doc build failed.'
              echo 'Usually this means that an option attribute that ends up in documentation (eg' \
                '`default` or `description`) depends on the restricted module arguments' \
                '`config` or `pkgs`.'
              echo
              echo 'Rebuild your configuration with `--show-trace` to find the offending' \
                'location. Remove the references to restricted arguments (eg by escaping' \
                'their antiquotations or adding a `defaultText`) or disable the sandboxed' \
                'build for the failing module by setting `meta.buildDocsInSandbox = false`.'
              echo -en "\e[0m"
              exit 1
            } >&2
        '';
    inherit (cfg.nixos.options) warningsAreErrors;
  };


  nixos-help = let
    helpScript = pkgs.writeShellScriptBin "nixos-help" ''
      # Finds first executable browser in a colon-separated list.
      # (see how xdg-open defines BROWSER)
      browser="$(
        IFS=: ; for b in $BROWSER; do
          [ -n "$(type -P "$b" || true)" ] && echo "$b" && break
        done
      )"
      if [ -z "$browser" ]; then
        browser="$(type -P xdg-open || true)"
        if [ -z "$browser" ]; then
          browser="${pkgs.w3m-nographics}/bin/w3m"
        fi
      fi
      exec "$browser" ${manual.manualHTMLIndex}
    '';

    desktopItem = pkgs.makeDesktopItem {
      name = "nixos-manual";
      desktopName = "NixOS Manual";
      genericName = "View NixOS documentation in a web browser";
      icon = "nix-snowflake";
      exec = "nixos-help";
      categories = "System";
    };

    in pkgs.symlinkJoin {
      name = "nixos-help";
      paths = [
        helpScript
        desktopItem
      ];
    };

in

{
  imports = [
    (mkRenamedOptionModule [ "programs" "info" "enable" ] [ "documentation" "info" "enable" ])
    (mkRenamedOptionModule [ "programs" "man"  "enable" ] [ "documentation" "man"  "enable" ])
    (mkRenamedOptionModule [ "services" "nixosManual" "enable" ] [ "documentation" "nixos" "enable" ])
  ];

  options = {

    documentation = {

      enable = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to install documentation of packages from
          <option>environment.systemPackages</option> into the generated system path.

          See "Multiple-output packages" chapter in the nixpkgs manual for more info.
        '';
        # which is at ../../../doc/multiple-output.chapter.md
      };

      man.enable = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to install manual pages.
          This also includes <literal>man</literal> outputs.
        '';
      };

      man.generateCaches = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to generate the manual page index caches.
          This allows searching for a page or
          keyword using utilities like
          <citerefentry>
            <refentrytitle>apropos</refentrytitle>
            <manvolnum>1</manvolnum>
          </citerefentry>
          and the <literal>-k</literal> option of
          <citerefentry>
            <refentrytitle>man</refentrytitle>
            <manvolnum>1</manvolnum>
          </citerefentry>.
        '';
      };

      info.enable = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to install info pages and the <command>info</command> command.
          This also includes "info" outputs.
        '';
      };

      doc.enable = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to install documentation distributed in packages' <literal>/share/doc</literal>.
          Usually plain text and/or HTML.
          This also includes "doc" outputs.
        '';
      };

      dev.enable = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether to install documentation targeted at developers.
          <itemizedlist>
          <listitem><para>This includes man pages targeted at developers if <option>documentation.man.enable</option> is
                    set (this also includes "devman" outputs).</para></listitem>
          <listitem><para>This includes info pages targeted at developers if <option>documentation.info.enable</option>
                    is set (this also includes "devinfo" outputs).</para></listitem>
          <listitem><para>This includes other pages targeted at developers if <option>documentation.doc.enable</option>
                    is set (this also includes "devdoc" outputs).</para></listitem>
          </itemizedlist>
        '';
      };

      nixos.enable = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to install NixOS's own documentation.
          <itemizedlist>
          <listitem><para>This includes man pages like
                    <citerefentry><refentrytitle>configuration.nix</refentrytitle>
                    <manvolnum>5</manvolnum></citerefentry> if <option>documentation.man.enable</option> is
                    set.</para></listitem>
          <listitem><para>This includes the HTML manual and the <command>nixos-help</command> command if
                    <option>documentation.doc.enable</option> is set.</para></listitem>
          </itemizedlist>
        '';
      };

      nixos.options.splitBuild = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to split the option docs build into a cacheable and an uncacheable part.
          Splitting the build can substantially decrease the amount of time needed to build
          the manual, but some user modules may be incompatible with this splitting.
        '';
      };

      nixos.options.warningsAreErrors = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Treat warning emitted during the option documentation build (eg for missing option
          descriptions) as errors.
        '';
      };

      nixos.includeAllModules = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Whether the generated NixOS's documentation should include documentation for all
          the options from all the NixOS modules included in the current
          <literal>configuration.nix</literal>. Disabling this will make the manual
          generator to ignore options defined outside of <literal>baseModules</literal>.
        '';
      };

      nixos.extraModuleSources = mkOption {
        type = types.listOf (types.either types.path types.str);
        default = [ ];
        description = ''
          Which extra NixOS module paths the generated NixOS's documentation should strip
          from options.
        '';
        example = literalExpression ''
          # e.g. with options from modules in ''${pkgs.customModules}/nix:
          [ pkgs.customModules ]
        '';
      };

    };

  };

  config = mkIf cfg.enable (mkMerge [
    {
      assertions = [
        {
          assertion = !(cfg.man.man-db.enable && cfg.man.mandoc.enable);
          message = ''
            man-db and mandoc can't be used as the default man page viewer at the same time!
          '';
        }
      ];
    }

    # The actual implementation for this lives in man-db.nix or mandoc.nix,
    # depending on which backend is active.
    (mkIf cfg.man.enable {
      environment.pathsToLink = [ "/share/man" ];
      environment.extraOutputsToInstall = [ "man" ] ++ optional cfg.dev.enable "devman";
    })

    (mkIf cfg.info.enable {
      environment.systemPackages = [ pkgs.texinfoInteractive ];
      environment.pathsToLink = [ "/share/info" ];
      environment.extraOutputsToInstall = [ "info" ] ++ optional cfg.dev.enable "devinfo";
      environment.extraSetup = ''
        if [ -w $out/share/info ]; then
          shopt -s nullglob
          for i in $out/share/info/*.info $out/share/info/*.info.gz; do
              ${pkgs.buildPackages.texinfo}/bin/install-info $i $out/share/info/dir
          done
        fi
      '';
    })

    (mkIf cfg.doc.enable {
      environment.pathsToLink = [ "/share/doc" ];
      environment.extraOutputsToInstall = [ "doc" ] ++ optional cfg.dev.enable "devdoc";
    })

    (mkIf cfg.nixos.enable {
      system.build.manual = manual;

      environment.systemPackages = []
        ++ optional cfg.man.enable manual.manpages
        ++ optionals cfg.doc.enable [ manual.manualHTML nixos-help ];

      services.getty.helpLine = mkIf cfg.doc.enable (
          "\nRun 'nixos-help' for the NixOS manual."
      );
    })

  ]);

}