# `builtins.fetchTarball` only accepts a `sha256` argument in Nix version 1.12
# or later, so here we provide a function that can provide a compatible interface
# to Nix 1.11 or Nix 1.12
#
# TODO FIXME: remove this sometime after Nix 1.12 goes stable

{ url                             # URL of the nixpkgs tarball to download
, rev                             # The Git revision of nixpkgs to fetch
, sha256                          # The SHA256 of the downloaded data
, system ? builtins.currentSystem # This is overridable if necessary
}:

with {
  ifThenElse = { bool, thenValue, elseValue }: (
    if bool then thenValue else elseValue);
};

ifThenElse {
  bool = (0 <= builtins.compareVersions builtins.nixVersion "1.12");

  # In Nix 1.12, we can just give a `sha256` to `builtins.fetchTarball`.
  thenValue = (builtins.fetchTarball { inherit url sha256; });

  # This hack should at least work for Nix 1.11
  elseValue = (
    (rec {
      tarball = import <nix/fetchurl.nix> { inherit url sha256; };
      builtin-paths = import <nix/config.nix>;

      script = builtins.toFile "nixpkgs-unpacker" ''
        "$coreutils/mkdir" "$out"
        cd "$out"
        "$gzip" --decompress < "$tarball" | "$tar" -x --strip-components=1
      '';

      nixpkgs = builtins.derivation {
        name = "nixpkgs-${builtins.substring 0 6 rev}";

        builder = builtins.storePath builtin-paths.shell;
        args = [ script ];

        inherit tarball system;
        tar       = builtins.storePath builtin-paths.tar;
        gzip      = builtins.storePath builtin-paths.gzip;
        coreutils = builtins.storePath builtin-paths.coreutils;
      };
    }).nixpkgs);
}