with builtins;

# This duplicates the overlay code in nixpkgs/pkgs/top-level/impure.nix (may require periodic resyncing).
#
# It has to be done this way because passing overlays to nixpkgs (required to insert our packages) disables all the
# default overlay mechanisms (required to pickup any user or system overlays).

let

  # overlaysDefault - List of overlay functions nixpkgs would normally use if not provided.
  #
  # This is the order overlays are searched for in accordance to what is done in nixpkgs.  If an overlay is found
  # at a given level (the leading number) the search is stopped and they are used.  If multiple overlays are found
  # at a given level then an error is generated.
  #
  # 1. <nixpkgs-overlays> (i.e., nixpkgs-overlays.nix or nixpkgs-overlays/default.nix in NIX_PATH)
  # 2. ~/.config/nixpkgs/overlays.nix
  # 2. ~/.config/nixpkgs/overlays
  #
  # This code is lifted directly from nixpkgs/pkgs/top-level/impure.nix to ensure consistency.
  #
  overlaysDefault =
    let

      # try expression default - Replace expression with default on exception.
      try = x: def: let res = tryEval x; in if res.success then res.value else def;

      # isDir path - Is path a directory (requires access).
      isDir = path: pathExists (path + "/.");

      # overlaysRetrieve path - Retrieve a list of the overlay functions from path.
      #   path is file - import the file itself (should give a list of overlay functions)
      #   path is directory - list of imports of all the *.nix files in the directory (each should give an overlay function)
      #
      overlaysRetrieve = path:
        if isDir path then 
          let content = readDir path; in
            map (n: import (path + ("/" + n)))
              (builtins.filter (n: builtins.match ".*\\.nix" n != null || pathExists (path + ("/" + n + "/default.nix")))
                (attrNames content))
        else 
          import path;

      # pathOverlays - NIX_PATH nixpkgs-overlays file or "" if not found
      pathOverlays = try <nixpkgs-overlays> "";

      # homeDir          - ~/
      # homeOverlaysFile - ~/.config/nixpkgs/overlays.nix
      # homeOverlaysDir  - ~/.config/nixpkgs/overlays
      #
      homeDir = builtins.getEnv "HOME";
      homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
      homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";

    in 

      if pathOverlays != "" && pathExists pathOverlays then overlaysRetrieve pathOverlays
      else if pathExists homeOverlaysFile && pathExists homeOverlaysDir then 
        throw ''
          Nixpkgs overlays can be specified with ${homeOverlaysFile} or ${homeOverlaysDir}, but not both.
          Please remove one of them and try again.
        ''
      else if pathExists homeOverlaysFile then 
        if isDir homeOverlaysFile then 
          throw (homeOverlaysFile + " should be a file")
        else overlaysRetrieve homeOverlaysFile
      else if pathExists homeOverlaysDir then
        if !(isDir homeOverlaysDir) then 
          throw (homeOverlaysDir + " should be a directory")
        else overlaysRetrieve homeOverlaysDir
      else [];


  # overlaysAlways - List of overlay functions provide by this package.
  #
  # The naming in this function is misleading but consistent with nixpkgs.
  #
  # self - final package set (stack fully closed) (-> pkgs in all-packages.nix)
  # super - prior package set (stack closed up to us)
  # res - next package set (stack closed up to and over us) (-> self in all-packages.nix)
  #
  overlaysAlways =
    let
      # knot - Feed final and up to and over us overlay results into overlay
      #
      # This matches what is done in nixpkgs (see pkgs/top-level/stage.nix).
      #
      knot = path: self: super:
        let res = import path res super self; in res;
    in
      map knot [
        ./temporary/all-packages.nix
        ./pkgs/all-packages.nix
      ];

  # nixpkgs - The underlying nixpkg to use.
  #
  # Get a usable nixpkgs, that is, one with a version that matches ours, or die.
  # 
  nixpkgs =
    let

      # first check list - Return first element of list that passes check list otherwise aborts.
      #
      first = check: list:
        if   list == []
        then builtins.throw ''
               Unable to locate a suitable nixpkgs directory.
               Most likely you want to git clone one into the top of the repo as 'upstream'.
             ''
        else if   check ( builtins.head list )
             then builtins.head list
             else first check ( builtins.tail list );

      # okay path - Check if path exist and match our version number and otherwise print a warning.
      #
      # Version numbers are taken from ./version files.
      #
      okay =
        let
          version = builtins.readFile ./.version;
        in
          path:
            (    builtins.pathExists (path + /.version)
              && (    builtins.readFile (path + /.version) == version
                   || builtins.trace ''
                        Skipping ${toString path} as not version ${version}.
                      '' false ) );

      # paths - Paths to search for nixpkgs.
      #
      paths = [
        ./upstream          # 1. build channel or git cloned one directly associated with us takes priority
        <nixpkgs/upstream>  # 2. if the channel is a version of ourselves the real one will be in a subdirectory
        <nixpkgs>           # 3. otherwise maybe the channel is a nixpkgs proper
      ];

    in

      first okay paths;

in

  { overlays ? overlaysDefault
  , upstream ? nixpkgs
  , ... } @ args: 

    import upstream (    builtins.removeAttrs args [ "upstream" ]
                      // { overlays = overlaysAlways ++ overlays; } )