deps@{ formats, lib, lychee, stdenv, writeShellApplication }:
let
  inherit (lib) mapAttrsToList throwIf;
  inherit (lib.strings) hasInfix hasPrefix escapeNixString;

  toURL = v:
    let s = "${v}";
    in if hasPrefix builtins.storeDir s
    then # lychee requires that paths on the file system are prefixed with file://
      "file://${s}"
    else s;

  withCheckedName = name:
    throwIf (hasInfix " " name) ''
      lycheeLinkCheck: remap patterns must not contain spaces.
      A space marks the end of the regex in lychee.toml.

      Please change attribute name 'remap.${escapeNixString name}'
    '';

  # See https://nixos.org/manual/nixpkgs/unstable/#tester-lycheeLinkCheck
  # or doc/build-helpers/testers.chapter.md
  lycheeLinkCheck = {
    site,
    remap ? { },
    lychee ? deps.lychee,
    extraConfig ? { },
  }:
    stdenv.mkDerivation (finalAttrs: {
      name = "lychee-link-check";
      inherit site;
      nativeBuildInputs = [ finalAttrs.passthru.lychee ];
      configFile = (formats.toml {}).generate "lychee.toml" finalAttrs.passthru.config;

      # These can be overriden with overrideAttrs if needed.
      passthru = {
        inherit lychee remap;
        config = {
          include_fragments = true;
        } // lib.optionalAttrs (finalAttrs.passthru.remap != { }) {
          remap =
            mapAttrsToList
              (name: value: withCheckedName name "${name} ${toURL value}")
              finalAttrs.passthru.remap;
        } // extraConfig;
        online = writeShellApplication {
          name = "run-lychee-online";
          runtimeInputs = [ finalAttrs.passthru.lychee ];
          # Comment out to run shellcheck:
          checkPhase = "";
          text = ''
            site=${finalAttrs.site}
            configFile=${finalAttrs.configFile}
            echo Checking links on $site
            exec lychee --config $configFile $site "$@"
          '';
        };
      };
      buildCommand = ''
        echo Checking internal links on $site
        lychee --offline --config $configFile $site
        touch $out
      '';
    });

in
{
  inherit lycheeLinkCheck;
}