Use stable hash to determine if configuration number needs bumping

The old implementation would increase the configuration number on each
server restart. This caused issues with e.g. Homepod that would reset
room configurations. We need a stable hash that is the same accross reboots
and maybe even hardware. We don't need a good distribution of this hash, but
importantly a hash that is unique for changes to device configuration.

Author: Bouke

Sources/HAP/Server/Configuration.swift

@@ -53,6 +53,11 @@ extension Device {
         // must be unique for the lifetime of the server/ client pairing.
         internal var aidForAccessorySerialNumber = [String: InstanceID]()
 
+        // Hash of the attached accessories, services and characteristics. Used
+        // to assert if the current configuration number should be updated after
+        // initializing / modifying list of accessories.
+        internal var stableHash: Int = 0
+
         private var aidGenerator = AIDGenerator()
 
         // The next aid - should be checked against existing devices to ensure it is unique

Sources/HAP/Server/Device.swift

@@ -238,7 +238,11 @@ public class Device {
     /// - write the configuration to storage
     /// - notify interested parties of the change
     func updatedConfiguration() {
-        configuration.number = configuration.number &+ 1
+        var newStableHash = generateStableHash()
+        if newStableHash != configuration.stableHash {
+            configuration.number = configuration.number &+ 1
+            configuration.stableHash = newStableHash
+        }
         if configuration.number < 1 {
             configuration.number = 1
         }
@@ -247,6 +251,22 @@ public class Device {
         notifyConfigurationChange()
     }
 
+    /// Generate uniqueness hash for device configuration, used to determine
+    /// if the configuration number should be updated.
+    func generateStableHash() -> Int {
+        var hash = 0
+        for accessory in accessories {
+            hash ^= 17 &* accessory.aid
+            for service in accessory.services {
+                hash ^= 19 &* service.iid
+                for characteristic in service.characteristics {
+                    hash ^= 23 &* characteristic.iid
+                }
+            }
+        }
+        return hash
+    }
+
     /// Notify the server that the config record has changed
     func notifyConfigurationChange() {
         server?.updateDiscoveryRecord()