{ pkgs, ... }:

{
  # A systemd path unit. Path units can be used to start
  # other services when something happens on the file
  # system, like a file being created.
  systemd.paths.ocrmypdf = {
    enable = true;
    # Enable this unit automatically when the server starts.
    wantedBy = [ "multi-user.target" ];
    description = "Start ocr-ing when there's new work.";
    pathConfig = {
      # Activate when files appear in the /data/scans-to-ocr
      # directory. This is where our scanner should upload
      # scanned files!
      DirectoryNotEmpty = "/data/scans-to-ocr";
      # If /data/scans-to-ocr does not exist, create it.
      MakeDirectory = true;
    };
  };

  # The service that does the actual work of running OCR.
  systemd.services.ocrmypdf = {
    enable = true;
    description = "Run ocrmypdf in /data/scans-to-ocr.";
    serviceConfig = {
      # Explain to systemd that this service is a script,
      # not some long-running process it needs to keep
      # alive. If the script exits after it's done that's
      # fine, systemd will call it again if there's new
      # work!
      Type = "oneshot";
      # Now we define what to run when this service gets
      # activated.
      #
      # We can pass `ExecStart` a single command to execute,
      # but the work we want to do does not fit in a single
      # command. Instead we let Nix create a shell script,
      # then tell systemd to run that script.
      ExecStart = let
        script = pkgs.writeShellScriptBin "go-ocr" ''
          #!/usr/bin/env bash

          # Run our OCR logic in turn for each scanned file.
          for file in /data/scans-to-ocr/*; do
            # generates a standard file name containing the 
            # current date and some random characters.
            output="$(mktemp -u "/tmp/$(date +%Y%m%d)_XXX.pdf")"

            # Run ocrmypdf on the scanned file.
            # --output-type   don't generate PDF/A's. This
            #                 might fail, requiring manual
            #                 intervention.
            # --rotate-pages  puts pages right-side-up
            # --skip-text     makes it so ocrmypdf skips
            #                 pages in the PDF that already
            #                 have text content, instead of
            #                 failing.
            # --language      which languages OCR should
            #                 detect. A lot (all?) languages
            #                 seem to be available by
            #                 default. Run
            #                 `tesseract --list-langs`
            #                 to find out which.
            ${pkgs.ocrmypdf}/bin/ocrmypdf \
              --output-type pdf \
              --rotate-pages \
              --skip-text \
              --language nld+eng \
              "$file" \
              "$output" \
              && rm "$file" \
              && mv "$output" /docs

            # Let healthchecks.io know whether ocrmypdf
            # succeeded or failed.
            ${pkgs.curl}/bin/curl --retry 3 \
              https://hc-ping.com/YOUR_UUID/$?
          done
        '';
      in "${script}/bin/go-ocr";
    };
  };

  # For healthchecks.io to mark a check as healthy it needs
  # to receive a periodic update, but we might not scan any
  # documents for days on end. The CRON job below will ping
  # healthchecks.io once an hour, but only if the
  # /data/scans-to-ocr directory is empty, indicating
  # ocrmypdf is doing work.
  services.cron.enable = true;
  services.cron.systemCronJobs = [
    ("0 * * * *      root    "
      + "ls -1qA /data/scans-to-ocr/ | grep -q . "
      + "|| curl -fsS -m 10 --retry 5 "
      + "-o /dev/null https://hc-ping.com/YOUR_UUID")
  ];

}