fix: send wifiStrength only for real signal changes

At -70 dBm the RSSI moves a dBm or two constantly, so the >1 dBm check
every 1.5s sent wifiStrength to every client about 270 times an hour.
That is extra traffic on an already lossy link and a state write in
Home Assistant for each one.

Broadcast on a channel change, or for a change of at least 3 dBm no
more than every 10s. A newly connected client still gets the current
value straight away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ESPNetwork.cpp

@@ -182,7 +182,8 @@ bool ESPNetwork::changeAP(const uint8_t *bssid, const int32_t channel) {
 void ESPNetwork::emitSockets() {
   this->emitHeap();
   if(this->needsBroadcast ||
-    (this->connType == conn_types_t::wifi && (abs(abs(WiFi.RSSI()) - abs(this->lastRSSI)) > 1 || WiFi.channel() != this->lastChannel))) {
+    (this->connType == conn_types_t::wifi && rssiNeedsEmit(WiFi.RSSI(), this->lastRSSI, WiFi.channel(), this->lastChannel,
+      millis() - this->lastRSSIEmit, RSSI_EMIT_MIN_DBM, RSSI_EMIT_MIN_MS))) {
     this->emitSockets(255);
     sockEmit.loop();
     this->needsBroadcast = false;
@@ -212,6 +213,7 @@ void ESPNetwork::emitSockets(uint8_t num) {
         sockEmit.endEmit(num);
         this->lastRSSI = WiFi.RSSI();
         this->lastChannel = WiFi.channel();
+        this->lastRSSIEmit = millis();
       }
       else {
         JsonSockEvent *json = sockEmit.beginEmit("wifiStrength");
@@ -231,6 +233,7 @@ void ESPNetwork::emitSockets(uint8_t num) {
         sockEmit.endEmit(num);
         this->lastRSSI = -100;
         this->lastChannel = -1;
+        this->lastRSSIEmit = millis();
       }
   }
   this->emitHeap(num);

src/ESPNetwork.h

@@ -8,10 +8,15 @@
 
 #define CONNECT_TIMEOUT 20000
 #define SSID_SCAN_INTERVAL 600000
+// A weak signal moves a dBm or two all the time, so only broadcast real
+// changes and not more often than this.
+#define RSSI_EMIT_MIN_DBM 3
+#define RSSI_EMIT_MIN_MS 10000
 class ESPNetwork {
   protected:
     unsigned long lastEmit = 0;
     unsigned long lastMDNS = 0;
+    unsigned long lastRSSIEmit = 0;
     int lastRSSI = 0;
     int lastChannel = 0;
     int linkSpeed = 0;

src/Utils.cpp

@@ -130,3 +130,8 @@ int Timestamp::tzOffset() {
   time(&now);
   return Timestamp::calcTZOffset(&now);
 }
+bool rssiNeedsEmit(int rssi, int lastRssi, int channel, int lastChannel, uint32_t sinceLastMs, int minDelta, uint32_t minMs) {
+  if(channel != lastChannel) return true;
+  int delta = rssi > lastRssi ? rssi - lastRssi : lastRssi - rssi;
+  return delta >= minDelta && sinceLastMs >= minMs;
+}

src/Utils.h

@@ -56,6 +56,10 @@ struct rebootDelay_t {
   else if(str[0] == 'f' || str[0] == 'F' || str[0] == '0') return false;
   return def;
 }
+// Whether a wifiStrength broadcast is worth sending: always on a channel change,
+// otherwise only for an RSSI change of at least minDelta dBm and not more often
+// than every minMs.
+bool rssiNeedsEmit(int rssi, int lastRssi, int channel, int lastChannel, uint32_t sinceLastMs, int minDelta, uint32_t minMs);
 
 class Timestamp {
   char _timeBuffer[128];

test/test_utils/test_utils.cpp

@@ -168,6 +168,26 @@ static void test_parseUTCTime_ignores_local_zone() {
   TEST_ASSERT_EQUAL_INT64((int64_t)SAMPLE_UTC, (int64_t)Timestamp::parseUTCTime("2024-03-05T07:08:09Z"));
 }
 
+/*********************************************************************
+ * rssiNeedsEmit
+ ********************************************************************/
+static void test_rssiNeedsEmit_channel_change_always_emits() {
+  TEST_ASSERT_TRUE(rssiNeedsEmit(-70, -70, 6, 11, 0, 3, 10000));
+}
+static void test_rssiNeedsEmit_small_change_is_ignored() {
+  TEST_ASSERT_FALSE(rssiNeedsEmit(-72, -70, 6, 6, 60000, 3, 10000));
+}
+static void test_rssiNeedsEmit_real_change_after_interval_emits() {
+  TEST_ASSERT_TRUE(rssiNeedsEmit(-73, -70, 6, 6, 10000, 3, 10000));
+}
+static void test_rssiNeedsEmit_real_change_too_soon_waits() {
+  TEST_ASSERT_FALSE(rssiNeedsEmit(-73, -70, 6, 6, 5000, 3, 10000));
+}
+static void test_rssiNeedsEmit_either_direction() {
+  TEST_ASSERT_TRUE(rssiNeedsEmit(-67, -70, 6, 6, 10000, 3, 10000));
+  TEST_ASSERT_TRUE(rssiNeedsEmit(-73, -70, 6, 6, 10000, 3, 10000));
+}
+
 int main(int argc, char **argv) {
   (void)argc;
   (void)argv;
@@ -193,5 +213,10 @@ int main(int argc, char **argv) {
   RUN_TEST(test_calcTZOffset_half_hour_west);
   RUN_TEST(test_parseUTCTime_in_utc);
   RUN_TEST(test_parseUTCTime_ignores_local_zone);
+  RUN_TEST(test_rssiNeedsEmit_channel_change_always_emits);
+  RUN_TEST(test_rssiNeedsEmit_small_change_is_ignored);
+  RUN_TEST(test_rssiNeedsEmit_real_change_after_interval_emits);
+  RUN_TEST(test_rssiNeedsEmit_real_change_too_soon_waits);
+  RUN_TEST(test_rssiNeedsEmit_either_direction);
   return UNITY_END();
 }