{ config, pkgs, ... }:

{
  # Systemd Journal Monitoring.
  # Alternative:
  #   journal-biref
  #     https://github.com/twaugh/journal-brief
  #     https://opensource.com/article/20/7/systemd-journals-email

  # Write to Systemd Journal:
  #   echo 'hello' | systemd-cat -p emerg
  #   echo 'hello' | systemd-cat -t someapp -p emerg

  # View Systemd Journal.
  #   Filter by app:
  #     journalctl -b -t someapp
  #   Filter by priority:
  #     journalctl -b -p 5

  # Manually execute journalwatch timer:
  #   sudo systemctl start journalwatch.service

  # Find a message and view its details
  #   journalctl -b -p5 -o json-pretty
  # Then press "/" and enter a pattern, then press "Enter".

  assertions = [{
    assertion = config.services.opensmtpd.enable;
    message = "journalwatch requires a configured sendmail MTA, see sendmail-mta.nix.";
  }];

  services.journalwatch = {
    enable = true;
    # TODO: Same as configured by sendmail MTA.
    mailFrom = "langbein@mail.de";
    mailTo = "daniel+journalwatch@systemli.org";
    #interval = "hourly";

    # Lowest priority of message to be considered. A value between 7 (“debug”), and 0 (“emerg”). Defaults to 6 (“info”). If you don’t care about anything with “info” priority, you can reduce this to e.g. 5 (“notice”) to considerably reduce the amount of messages without needing many filterBlocks.
    priority = 5;

    # Default patterns: https://github.com/The-Compiler/journalwatch/blob/363725ac4b8aa841d87654fa8a63403a59ad1275/journalwatch.py#L71
    # If the value of `match` starts and ends with a slash, it is interpreted as a regular expression, if not, it's an exact match.
    # `filters` are always regular expressions.
    # All regular expressions have to match the full string!
    filterBlocks = [
      { # yodaTux
        filters = ''
         .*
        '';
        match = "_SYSTEMD_UNIT = bluetooth.service";
      }
      { # yodaTux
        filters = ''
          .*
        '';
        match = "_SYSTEMD_UNIT = cups.service";
      }
      { # yodaTux. If the user `yoda` runs a command with `sudo`.
        filters = ''
          \s+yoda : TTY=pts/7 ; PWD=/.+ ; USER=root ; COMMAND=/.+
        '';
        match = "SYSLOG_IDENTIFIER = sudo";
      }
      { # yodaTux
        filters = ''
          The system will suspend now!
        '';
        match = "_SYSTEMD_UNIT = systemd-logind.service";
      }
      { # yodaTux
        filters = ''
          Reexecuting.
          finished switching to system configuration /nix/store/.+-nixos-system-.+-[0-9]+\.[0-9]+pre-git
        '';
        match = "_SYSTEMD_UNIT = user@0.service";
      }
      { # yodaTux
        filters = ''
          Reexecuting.
          (finished )?switching to system configuration /nix/store/.+-nixos-system-.+-[0-9]+\.[0-9]+pre-git
        '';
        match = "_SYSTEMD_UNIT = user@1000.service";
      }
      { # yodaYoga
        filters = ''
          (finished )?switching to system configuration /nix/store/.+-nixos-system-.+-[0-9]+\.[0-9]+pre-git
        '';
        match = "SYSLOG_IDENTIFIER = nixos";
      }
      { # yodaTux
        filters = ''
          Reloading rules
          Collecting garbage unconditionally...
          Loading rules from directory /.+
          Finished loading, compiling and executing [0-9]+ rules
        '';
        match = "_SYSTEMD_UNIT = polkit.service";
      }
      { # yodaTux
        filters = ''
          .+ error name="org\.bluez\.MediaEndpoint1\.Error\.NotImplemented" .+
        '';
        match = "_SYSTEMD_UNIT = dbus.service";
      }
      { # yodaTux
        filters = ''
          .*
        '';
        match = "SYSLOG_IDENTIFIER = //nix/store/.+/libexec/gdm-x-session/";
      }
      { # yodaTux
        filters = ''
          ACPI: FW issue: working around C-state latencies out of order
          # Kernel WiFi driver bug.
          iwlwifi 0000:01:00.0: Unhandled alg: 0x707
        '';
        match = "SYSLOG_IDENTIFIER = kernel";
      }
   ];
  };
}