feat(monitor): Expose how coarse a case battery reading is

PodDevice.batteryCase merges an AAP notification, the encrypted advertisement
payload, the public advertisement nibble and the cache into one Float, and the
provenance is gone by the time anyone reads it. The public nibble only carries
deciles, so 20% there can mean anything up to 29%.

batteryCaseReading walks the same precedence and keeps the step size of the
source that won, leaving batteryCase untouched for every existing caller.

app/src/main/java/eu/darken/capod/monitor/core/BatteryReading.kt

@@ -0,0 +1,29 @@
+package eu.darken.capod.monitor.core
+
+import eu.darken.capod.pods.core.apple.ble.BATTERY_RESOLUTION_DECILE
+import eu.darken.capod.pods.core.apple.ble.BATTERY_RESOLUTION_PERCENT
+import eu.darken.capod.pods.core.apple.ble.devices.HasCase
+
+/**
+ * A battery percentage together with the step size of the source it came from. A reading of 20%
+ * from a decile source means "somewhere in [20%, 30%)", which is the difference between a claim we
+ * can make and one we can't.
+ */
+data class BatteryReading(
+    val percent: Float,
+    val resolution: Float,
+)
+
+/**
+ * [PodDevice.batteryCase] resolved through the same precedence, but keeping the resolution of
+ * whichever source won. Cache entries store no resolution, so they report the coarsest one.
+ */
+val PodDevice.batteryCaseReading: BatteryReading?
+    get() {
+        aap?.batteryCase?.let { return BatteryReading(it, BATTERY_RESOLUTION_PERCENT) }
+        (ble as? HasCase)?.let { snapshot ->
+            snapshot.batteryCasePercent?.let { return BatteryReading(it, snapshot.batteryCaseResolution) }
+        }
+        cached?.case?.percent?.let { return BatteryReading(it, BATTERY_RESOLUTION_DECILE) }
+        return null
+    }

app/src/main/java/eu/darken/capod/pods/core/apple/ble/PodDeviceExtensions.kt

@@ -21,6 +21,12 @@ import kotlin.math.roundToInt
 
 const val BATTERY_UNKNOWN = -1f
 
+/** Step size of a percent-granularity reading: AAP battery notifications and the encrypted payload. */
+const val BATTERY_RESOLUTION_PERCENT = 0.01f
+
+/** Step size of the public advertisement's battery nibble, which only carries deciles. */
+const val BATTERY_RESOLUTION_DECILE = 0.10f
+
 fun isKnownBattery(percent: Float): Boolean = percent.isFinite() && percent >= 0f
 
 fun batteryProgress(percent: Float): Float =

app/src/main/java/eu/darken/capod/pods/core/apple/ble/devices/DualApplePods.kt

@@ -4,6 +4,8 @@ import eu.darken.capod.common.debug.logging.log
 import eu.darken.capod.common.isBitSet
 import eu.darken.capod.common.lowerNibble
 import eu.darken.capod.common.upperNibble
+import eu.darken.capod.pods.core.apple.ble.BATTERY_RESOLUTION_DECILE
+import eu.darken.capod.pods.core.apple.ble.BATTERY_RESOLUTION_PERCENT
 import eu.darken.capod.pods.core.apple.ble.DualBlePodSnapshot
 import eu.darken.capod.pods.core.apple.ble.DualBlePodSnapshot.Pod
 
@@ -137,6 +139,13 @@ interface DualApplePods : ApplePods, HasChargeDetectionDual, DualBlePodSnapshot,
             }
         }
 
+    /** The encrypted payload carries whole percents; the public nibble only carries deciles. */
+    override val batteryCaseResolution: Float
+        get() = when {
+            payload.private?.asBatteryState(3) != null -> BATTERY_RESOLUTION_PERCENT
+            else -> BATTERY_RESOLUTION_DECILE
+        }
+
     override val isCaseCharging: Boolean
         get() {
             payload.private?.asBatteryState(3)?.let { return it.isCharging }

app/src/main/java/eu/darken/capod/pods/core/apple/ble/devices/HasCase.kt

@@ -2,14 +2,23 @@ package eu.darken.capod.pods.core.apple.ble.devices
 
 import androidx.annotation.DrawableRes
 import eu.darken.capod.R
+import eu.darken.capod.pods.core.apple.ble.BATTERY_RESOLUTION_DECILE
 
 interface HasCase {
 
     val batteryCasePercent: Float?
 
+    /**
+     * Step size of [batteryCasePercent]. Deciles unless a decoder knows it read a finer source, so
+     * a caller reasoning about how much the true value may exceed the reading errs on the coarse
+     * side.
+     */
+    val batteryCaseResolution: Float
+        get() = BATTERY_RESOLUTION_DECILE
+
     val isCaseCharging: Boolean
 
     @get:DrawableRes
     val caseIcon: Int
         get() = R.drawable.device_airpods_gen1_case
-}
+}