Count distinct devices (by MAC) in active scanner status

The ARP and SNMP scanners reported every online sighting, so a device
seen on multiple IPs or via duplicate ARP replies was counted more than
once. Fold the dedup into ActiveStatus::record_scan, which now takes the
device slice and reports the number of distinct MAC addresses.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TODO.md

@@ -20,7 +20,7 @@
 
 ## Backend
 
-- [ ] In the active scanners status, it should count the number of distinct devices it saw on the last scan (avoid duplicates)
+- [x] In the active scanners status, it should count the number of distinct devices it saw on the last scan (avoid duplicates)
 - [ ] Implement the pushover API call directly to support HTML content and review notification text to use it
 
 ## Frontend

backend/src/scanners/arp/scanner.rs

@@ -21,7 +21,7 @@ pub async fn scan() -> Result<(), Box<dyn std::error::Error>> {
 
         info!("Done with ARP probes");
         info!("Found {} online devices", devices.len());
-        status::record_scan(devices.len() as u64);
+        status::record_scan(&devices);
 
         // Process found devices
         for device in devices.iter() {

backend/src/scanners/arp/status.rs

@@ -2,6 +2,7 @@ use chrono::{DateTime, Utc};
 use once_cell::sync::OnceCell;
 use std::sync::Mutex;
 
+use crate::model::devices::Device;
 use crate::scanners::common::active_status::{ActiveSnapshot, ActiveStatus};
 
 static STATUS: OnceCell<Mutex<ActiveStatus>> = OnceCell::new();
@@ -22,9 +23,9 @@ pub fn set_waiting(next_scan_at: DateTime<Utc>) {
     }
 }
 
-pub fn record_scan(devices_seen: u64) {
+pub fn record_scan(devices: &[Device]) {
     if let Some(m) = STATUS.get() {
-        m.lock().unwrap().record_scan(devices_seen);
+        m.lock().unwrap().record_scan(devices);
     }
 }
 

backend/src/scanners/common/active_status.rs

@@ -1,4 +1,6 @@
+use crate::model::devices::Device;
 use chrono::{DateTime, Utc};
+use std::collections::HashSet;
 
 /// Status state for the active (polling) scanners — ARP and SNMP. Each scanner owns its own
 /// `OnceCell<Mutex<ActiveStatus>>` and delegates to these methods (see e.g.
@@ -41,9 +43,16 @@ impl ActiveStatus {
         self.next_scan_at = Some(next_scan_at);
     }
 
-    /// Record the result of a completed scan.
+    /// Record the result of a completed scan. A single scan can surface the same device more than
-    pub fn record_scan(&mut self, devices_seen: u64) {
+    /// once — duplicate ARP replies, or one MAC bound to several IPs in an SNMP ARP cache — so the
-        self.last_scan_devices_seen = Some(devices_seen);
+    /// reported count is the number of distinct devices (by MAC address), not raw sightings.
+    pub fn record_scan(&mut self, devices: &[Device]) {
+        let distinct = devices
+            .iter()
+            .map(|device| &device.mac_address)
+            .collect::<HashSet<_>>()
+            .len();
+        self.last_scan_devices_seen = Some(distinct as u64);
         self.last_scan_at = Some(Utc::now());
     }
 
@@ -83,15 +92,37 @@ mod tests {
         assert_eq!(snapshot.next_scan_at.unwrap(), next);
     }
 
+    fn device_with_mac(mac: &str, ip: &str) -> Device {
+        Device::new(mac.to_string(), ip.to_string(), String::new(), Utc::now())
+    }
+
     #[test]
     fn record_scan_stores_count_and_time() {
         let mut status = ActiveStatus::new();
-        status.record_scan(7);
+        let devices = vec![
+            device_with_mac("aa:bb:cc:dd:ee:01", "192.168.0.2"),
+            device_with_mac("aa:bb:cc:dd:ee:02", "192.168.0.3"),
+        ];
+        status.record_scan(&devices);
         let snapshot = status.snapshot();
-        assert_eq!(snapshot.last_scan_devices_seen, Some(7));
+        assert_eq!(snapshot.last_scan_devices_seen, Some(2));
         assert!(snapshot.last_scan_at.is_some());
     }
 
+    #[test]
+    fn record_scan_counts_distinct_devices() {
+        let mut status = ActiveStatus::new();
+        // Same MAC seen twice (e.g. duplicate ARP reply or one MAC on two IPs) plus a second
+        // distinct device — only two unique devices should be reported.
+        let devices = vec![
+            device_with_mac("aa:bb:cc:dd:ee:01", "192.168.0.2"),
+            device_with_mac("aa:bb:cc:dd:ee:01", "192.168.0.9"),
+            device_with_mac("aa:bb:cc:dd:ee:02", "192.168.0.3"),
+        ];
+        status.record_scan(&devices);
+        assert_eq!(status.snapshot().last_scan_devices_seen, Some(2));
+    }
+
     #[test]
     fn initial_state_is_empty() {
         let snapshot = ActiveStatus::new().snapshot();

backend/src/scanners/snmp/scanner.rs

@@ -32,7 +32,7 @@ pub async fn scan() -> Result<(), Box<dyn std::error::Error>> {
         match finder::find(config).await {
             Ok(devices) => {
                 info!("SNMP poll found {} devices in the ARP cache", devices.len());
-                status::record_scan(devices.len() as u64);
+                status::record_scan(&devices);
                 for device in devices.iter() {
                     pipeline::record_sighting(device.clone(), DeviceEventScanner::Snmp);
                 }

backend/src/scanners/snmp/status.rs

@@ -2,6 +2,7 @@ use chrono::{DateTime, Utc};
 use once_cell::sync::OnceCell;
 use std::sync::Mutex;
 
+use crate::model::devices::Device;
 use crate::scanners::common::active_status::{ActiveSnapshot, ActiveStatus};
 
 static STATUS: OnceCell<Mutex<ActiveStatus>> = OnceCell::new();
@@ -22,9 +23,9 @@ pub fn set_waiting(next_scan_at: DateTime<Utc>) {
     }
 }
 
-pub fn record_scan(devices_seen: u64) {
+pub fn record_scan(devices: &[Device]) {
     if let Some(m) = STATUS.get() {
-        m.lock().unwrap().record_scan(devices_seen);
+        m.lock().unwrap().record_scan(devices);
     }
 }