# `nixos-diff`: after a rebuild, report (1) what packages changed and (2) whether the
# change needs a reboot, a re-login, or nothing. NixOS keeps two markers —
# /run/booted-system (what you booted) and /run/current-system (what the last `switch`
# activated). A switch applies userland live, but:
#   - the kernel, initrd, and kernel modules are frozen until the next BOOT — detected
#     exactly, by comparing those store paths between the two systems;
#   - session-mapped components (the Wayland compositor, GL stack, portals) a running
#     graphical session keeps until you log out and back in — detected HEURISTICALLY,
#     by matching changed package names against `local.rebuildDiff.reloginPackages`.
# Generic and topology-free, so it rides in the baseline; disable with
# `local.rebuildDiff.enable = false`.
{ config, lib, pkgs, ... }:

let
  cfg = config.local.rebuildDiff;
  reloginPattern = lib.concatStringsSep "|" cfg.reloginPackages;

  nixos-diff = pkgs.writeShellApplication {
    name = "nixos-diff";
    runtimeInputs = [ pkgs.nvd pkgs.coreutils pkgs.gnugrep ];
    text = ''
      booted=/run/booted-system
      current=/run/current-system

      # Package-level diff of the running system vs the activated one. Force colour
      # (nvd auto-disables it when captured); only version parts are coloured, so the
      # package-name grep below is unaffected.
      diff=$(nvd --color always diff "$booted" "$current")
      printf '%s\n' "$diff"

      # Reboot (exact): kernel/initrd/modules only take effect on the next boot.
      reboot=0
      for c in kernel initrd kernel-modules; do
        [ "$(readlink -f "$booted/$c" 2>/dev/null)" = "$(readlink -f "$current/$c" 2>/dev/null)" ] \
          || reboot=1
      done

      # Re-login (heuristic): session-mapped components a running Wayland session
      # keeps until you log out and back in.
      relogin_hits=""
      ${lib.optionalString (reloginPattern != "") ''
        relogin_hits=$(printf '%s\n' "$diff" | grep -Ew "${reloginPattern}" || true)
      ''}

      echo
      if [ "$reboot" -eq 1 ]; then
        printf '>>> REBOOT needed (kernel/initrd/modules changed).\n'
      elif [ -n "$relogin_hits" ]; then
        printf '>>> RE-LOGIN recommended — session components changed:\n'
        printf '%s\n' "$relogin_hits"
      else
        printf '>>> No reboot/re-login needed.\n'
      fi
    '';
  };
in
{
  options.local.rebuildDiff = {
    enable = lib.mkOption {
      type = lib.types.bool;
      default = true;
      description = ''
        Install `nixos-diff`: after a `nixos-rebuild switch`, print an `nvd` closure
        diff of the running vs activated system plus a reboot / re-login / live verdict.
      '';
    };

    reloginPackages = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      default = [ "mesa" "wayland" "hyprland" "xdg-desktop-portal" ];
      description = ''
        Whole-word package-name patterns matched against the diff; a match means a
        running Wayland session should be restarted (re-login) to pick the change up —
        the compositor, GL stack, and portals. Extend per host (e.g. a specific GPU
        driver). Empty disables the re-login heuristic.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    environment.systemPackages = [ nixos-diff pkgs.nvd ];
  };
}