{ ccpkgs                       # Checked out ccpkgs git repo
    ? { outPath = ./.;
        revCount = 1234 + 149;
        shortRev = "gabcdef";
      }
, nixpkgs                      # Checked out nixpkgs git repe
    ? { outPath = ./upstream;
        revCount = 5678 + 242760;
        shortRev = "gfedcba";
      }
, nixpkgsArgs ? { config = { allowUnfree = true; inHydra = true; }; }
, stableBranch ? false
, supportedSystems ? [ "x86_64-linx" ] }:


# Free up the nixpkgs and ccpkgs name for binding to final product.
#
let
  # *Git - git checkout
  #
  nixpkgsGit = nixpkgs;
  ccpkgsGit = ccpkgs;

# Tarball hydra release product for the ccpkgs channel.
#
# 1. Generate an unpacked channel for the associated nixpkgs in the store.
# 2. Symlink this into the top-level directory as upstream (nixpkgs is used).
# 3. Stick in the standard version files.
# 4. Let releaseTools.sourceTarball handle the details of exposing it as a hydra product.
#
in let

  # pkgs - Packages sucked in from the given ccpkgs git version.
  #
  pkgs = import ccpkgsGit ( nixpkgsArgs // { upstream = nixpkgsGit; } );

  # version        - Version number (e.g., "17.09")
  # version*Suffix - Version suffix (e.g., "5678.gfedcba")
  #
  # Code lifted from nixos/release.nix.
  #
  version =
    let
      versionNixpkgs = pkgs.lib.fileContents (nixpkgsGit + /.version);
      versionCcpkgs  = pkgs.lib.fileContents ./.version;
    in
      assert (versionNixpkgs == versionCcpkgs); versionCcpkgs;

  versionSeparator = if stableBranch then "." else "pre";

  mkVersion = base: git: rec {
    count = toString (git.revCount - base);
    commit = git.shortRev;
    suffix = "${versionSeparator}${count}.${commit}";
  };

  nixpkgsVersion = mkVersion 242760 nixpkgsGit;
  ccpkgsVersion  = mkVersion 149 ccpkgsGit;

  versionSuffix = "${versionSeparator}${ccpkgsVersion.count}.${nixpkgsVersion.count}.${ccpkgsVersion.commit}.${nixpkgsVersion.commit}";

  # nixpkgs - The store path containing the unpacked nixpkgs channel.
  #
  # 1. Generate a channel tarball from the git repo via the nixos/release.nix expression for hydra.
  # 2. Unpack this into the store using the nix/unpack-channel.nix expression used by nix-channel.
  #
  nixpkgs =
    let
      # channel - Store path containing the channel release tarballs.
      #
      # The nixos.channel attribute from nixos/release.nix builds the nixpkgs channel tarballs.
      #
      channel =
        ( import (nixpkgsGit + /nixos/release.nix) {
            inherit stableBranch supportedSystems;
            nixpkgs = nixpkgsGit;
          } ).channel;

      # nixpkgs - The store path containing the unpacked nixpkgs channel.
      #
      # The nix provided nix/unpack-channel.nix function extracts the nixpkgs from the channel,
      #
      nixpkgs =
        ( pkgs.callPackage ./unpack-channel.nix { } rec {
           name = "nixos";
           channelName = "${name}-${version}";
           src = channel + /tarballs + "/${name}-${version}${nixpkgsVersion.suffix}.tar.xz";
          } );
    in
      nixpkgs;

  # jobs - The jobs hydra is to run.
  #
  jobs =
    let
      # Extract the valid derivations from an overlay
      #
      # nameSet  - the attribute set the uncomposed overlay (just the names are required)
      # valueSet - the final attribute from the composed overlay (the values are required)
      #
      extractDerivations = valueSet: nameSet:
        builtins.listToAttrs
          ( builtins.map
              ( name: { inherit name; value = valueSet.${name}; } )
              ( pkgs.lib.filter
                  ( name: ( builtins.tryEval (pkgs.lib.isDerivation valueSet.${name}) ).value )
                  ( pkgs.lib.attrNames nameSet ) ) );

      # Provided overlays
      pkgsNamesTopLevel = let self = import ./pkgs/all-packages.nix    self { } self; in self;
      pkgsNamesPython   = let self = import ./pkgs/python-packages.nix { } self { } ; in self;
      pkgsNamesR        = let self = import ./pkgs/r-modules.nix       { } self { } ; in self;

      temporaryNamesTopLevel = let self = import ./temporary/all-packages.nix    self { } self; in self;
      temporaryNamesPython   = let self = import ./temporary/python-packages.nix { } self { } ; in self;
      temporaryNamesR        = let self = import ./temporary/r-modules.nix       { } self { } ; in self;

    in {
      # Tweak the nixos make-channel code to include the upstream channel.
      #
      # 1. Replace the git repo nixpkgs with a copy of the unpacked nixpkgs channel.
      #
      channel =
        ( import (nixpkgsGit + /nixos/lib/make-channel.nix) {
            inherit pkgs version versionSuffix;
            nixpkgs = ccpkgsGit;
        } ).overrideAttrs ( original: {
            name = "ccpkgs-channel";
            distPhase =
              ''
                rm -fr upstream
                cp -rd ${nixpkgs}/nixos-${version} upstream
              '' + original.distPhase;
        } );

      # tested - Aggregate package set required to built for automatic channel release
      #
      tested = pkgs.lib.hydraJob (pkgs.releaseTools.aggregate {
        name = "ccpkgs-${version}";

        constituents = builtins.map ( pkgs.lib.collect pkgs.lib.isDerivation ) [ jobs.pkgs jobs.temporary ];

        meta = {
          description = "Release-critical builds for the ccpkgs channel";
          maintainers = with pkgs.lib.maintainers; [ ];
        };
      });

      # pkgs - Attribute set of overlayed pkgs.
      #
      pkgs = extractDerivations pkgs pkgsNamesTopLevel // {
        python2Packages = extractDerivations pkgs.python2Packages pkgsNamesPython;
        python3Packages = extractDerivations pkgs.python3Packages pkgsNamesPython;
        rPackages       = extractDerivations pkgs.rPackages       pkgsNamesR;
      };


      # temporary - Attribute set of overlayed pkgs.
      #
      temporary = extractDerivations pkgs temporaryNamesTopLevel // {
        python2Packages = extractDerivations pkgs.python2Packages temporaryNamesPython;
        python3Packages = extractDerivations pkgs.python3Packages temporaryNamesPython;
        rPackages       = extractDerivations pkgs.rPackages       temporaryNamesR;
      };
    };

in jobs