Reduce battery-reading noise with a trimmed-mean ADC sample

Sometimes a single reading came in noticeably off from the real trend
(a regulator/RF transient during sampling), and the next normal reading
would then look like a big jump relative to that bad one -- server-side,
enough to misfire the recharge-cycle heuristic (see the paired server
commit). Went from 8 raw-averaged samples to 16, sorted, with the 3
extreme samples on each end dropped before averaging the remaining 10 --
a handful of outliers can no longer skew the result the way a plain
average let them.

firmware/main/battery.c

@@ -1,3 +1,5 @@
+#include <stdlib.h>
+
 #include "driver/gpio.h"
 #include "esp_adc/adc_cali_scheme.h"
 #include "esp_adc/adc_oneshot.h"
@@ -11,7 +13,13 @@ static const char *TAG = "battery";
 #if CONFIG_FRAME_BATTERY_ADC_GPIO >= 0
 
 #define BATTERY_ADC_GPIO CONFIG_FRAME_BATTERY_ADC_GPIO
-#define BATTERY_SAMPLES 8
+#define BATTERY_SAMPLES 16
+/* Trimmed mean: the extreme BATTERY_TRIM samples on each end (regulator/
+ * RF transients, not the true resting voltage) are dropped before
+ * averaging the rest -- a plain average lets even one or two of those
+ * skew the result enough to read as a real percent change downstream
+ * (see the recharge-jump handling in routers/device.py). */
+#define BATTERY_TRIM 3
 /* The external divider halves the battery voltage (2x200k, per the
  * Seeed-documented XIAO wiring) so a full 4.2V cell reads ~2.1V at the
  * pin, inside the 12dB-attenuation ADC range. */
@@ -34,6 +42,11 @@ static const struct {
     { 3300, 5 },   { 3000, 0 },
 };
 
+static int int_cmp(const void *a, const void *b)
+{
+    return *(const int *)a - *(const int *)b;
+}
+
 static int mv_to_percent(int mv)
 {
     int n = sizeof(LIPO_CURVE) / sizeof(LIPO_CURVE[0]);
@@ -139,19 +152,17 @@ int battery_read_percent(void)
         ESP_LOGW(TAG, "ADC calibration unavailable, using nominal scaling");
     }
 
-    int mv_sum = 0;
+    int mv_samples[BATTERY_SAMPLES];
     int samples = 0;
     for (int i = 0; i < BATTERY_SAMPLES; i++) {
         int value;
         if (calibrated) {
             if (adc_oneshot_get_calibrated_result(adc, cali, channel, &value) == ESP_OK) {
-                mv_sum += value;
+                mv_samples[samples++] = value;
-                samples++;
             }
         } else {
             if (adc_oneshot_read(adc, channel, &value) == ESP_OK) {
-                mv_sum += value * 3300 / 4095; /* nominal 12-bit full scale at 12dB */
+                mv_samples[samples++] = value * 3300 / 4095; /* nominal 12-bit full scale at 12dB */
-                samples++;
             }
         }
     }
@@ -167,7 +178,19 @@ int battery_read_percent(void)
         return -1;
     }
 
-    int battery_mv = (mv_sum / samples) * BATTERY_DIVIDER_RATIO;
+    /* Only trim if there's enough left afterward to still be a
+     * meaningful average -- falls back to a plain average of whatever
+     * came in on a wake where most reads failed. */
+    qsort(mv_samples, samples, sizeof(int), int_cmp);
+    int trim = (samples > 2 * BATTERY_TRIM) ? BATTERY_TRIM : 0;
+    int mv_sum = 0;
+    int kept = 0;
+    for (int i = trim; i < samples - trim; i++) {
+        mv_sum += mv_samples[i];
+        kept++;
+    }
+
+    int battery_mv = (mv_sum / kept) * BATTERY_DIVIDER_RATIO;
     if (battery_mv < BATTERY_MV_MIN || battery_mv > BATTERY_MV_MAX) {
         ESP_LOGI(TAG, "Reading %dmV outside plausible battery range, ignoring", battery_mv);
         return -1;