{ registryFromDownloadUrl, lib }:

{ indexUrl
, configSha256
, fetchurlExtraArgs ? { }
}:

let
  slashTerminatedIndexUrl = if lib.hasSuffix "/" indexUrl then indexUrl else "${indexUrl}/";
  configContents = builtins.readFile "${builtins.fetchurl {
    url = "${slashTerminatedIndexUrl}config.json";
    sha256 = configSha256;
  }}";

  config = builtins.fromJSON configContents;
  dl = config.dl or (throw ''
    registry config does not have a "dl" endpoint:
    ${configContents}
  '');
in
registryFromDownloadUrl {
  inherit dl fetchurlExtraArgs;
  registryPrefix = "sparse+";
  indexUrl = slashTerminatedIndexUrl;
}