{ lib, pkgs, config, ... }:
{
  services.grafana.provision.alerting.rules.settings.groups = [
    {
      # orgId = 1; # Defaults to 1 for provisioned rules
      name = "DiskUsageAlerts";      # Name of the rule group
      folder = "System Alerts";       # The folder these rules belong to in Grafana UI
      interval = "1m";              # How often to evaluate rules in this group

      rules = [
        {
          uid = "high-disk-usage-alert-uid"; # Optional: provide a stable UID for the rule itself
          title = "HighDiskUsage";          # Name of the alert rule (was 'alert' in vmalert)

          # Condition for the alert to fire. 'C' refers to the refId of the threshold expression.
          condition = "D"; # Condition is now D
          # Removed rule-level relativeTimeRange

          # Data queries and expressions
          data = [
            # Query A: Calculate disk usage percentage
            {
              refId = "A";
              datasourceUid = "vm-datasource-uid"; # UID of the VictoriaMetrics datasource
              queryType = "prometheus"; # Explicitly set, though often inferred
              relativeTimeRange = { from = 60; to = 0; }; # Query-level, integer seconds
              model = {
                expr = ''
                  (
                    node_filesystem_size_bytes{fstype!~"tmpfs|rootfs",mountpoint!=""} - node_filesystem_avail_bytes{fstype!~"tmpfs|rootfs",mountpoint!=""}
                  ) / (node_filesystem_size_bytes{fstype!~"tmpfs|rootfs",mountpoint!=""} > 0) * 100
                  and node_filesystem_size_bytes{fstype!~"tmpfs|rootfs",mountpoint!=""}
                  and node_filesystem_avail_bytes{fstype!~"tmpfs|rootfs",mountpoint!=""}
                '';
                legendFormat = "{{mountpoint}} on {{instance}}"; # Example legend
                instant = false; # For range queries, default is false
              };
            }
            # Expression C: Reduce Query A to its last value, preserving labels
            {
              refId = "C";
              datasourceUid = "__expr__";
              model = {
                type = "reduce";
                expression = "A"; # Input is Query A
                reducer = "last"; # Get the last value of each series in A
              };
            }
            # Expression D: Apply math condition to the reduced values from C
            {
              refId = "D";
              datasourceUid = "__expr__";
              model = {
                type = "math";
                expression = "$C > 85"; # Check if the last value from each series in C is > 85
              };
            }
          ];
          
          for = "15m"; # Duration the condition must be met (same as vmalert)
          
          # How to handle states where data is missing or query errors
          noDataState = "NoData";   # Options: NoData, Alerting, OK
          execErrState = "Error"; # Options: Error, Alerting, OK

          annotations = {
            summary = "High disk usage on {{ $labels.instance }} at {{ $labels.mountpoint }}";
            description = ''
              Disk usage on {{ $labels.instance }} for mount point {{ $labels.mountpoint }}
              (fstype: {{ $labels.fstype }}) has been above 85% for more than 15 minutes.
              Current value: {{ if $values.C }}{{ $values.C | humanizePercentage }}{{ else }}N/A{{ end }}%.
            ''; # Using $values.C as it's the input to the math condition D
          };
          labels = {
            severity = "warning";
            category = "capacity";
            # Grafana automatically adds labels from the query result (instance, mountpoint, etc.)
            # and labels from the rule group/folder.
          };
          # isPaused = false; # Default is not paused
        }
      ];
    }
  ];
}