Implement tombstone() method

Based on [1] at 488e932. Haven't updated tests due to being in a bit of
a rush; deferred to #52.

[1] ably/specification#350

Author: lawrence-forooghian

Sources/AblyLiveObjects/Internal/InternalDefaultLiveCounter.swift

@@ -317,5 +317,11 @@ internal final class InternalDefaultLiveCounter: Sendable {
             data += amount
             return .update(DefaultLiveCounterUpdate(amount: amount))
         }
+
+        /// Needed for ``InternalLiveObject`` conformance.
+        mutating func resetDataToZeroValued() {
+            // RTLC4
+            data = 0
+        }
     }
 }

Sources/AblyLiveObjects/Internal/InternalDefaultLiveMap.swift

@@ -655,6 +655,12 @@ internal final class InternalDefaultLiveMap: Sendable {
             let mapUpdate = DefaultLiveMapUpdate(update: previousData.mapValues { _ in .removed })
             liveObjectMutableState.emit(.update(mapUpdate), on: userCallbackQueue)
         }
+
+        /// Needed for ``InternalLiveObject`` conformance.
+        mutating func resetDataToZeroValued() {
+            // RTLM4
+            data = [:]
+        }
     }
 
     // MARK: - Helper Methods

Sources/AblyLiveObjects/Internal/InternalLiveObject.swift

@@ -1,8 +1,50 @@
+internal import AblyPlugin
+
 /// Provides RTLO spec point functionality common to all LiveObjects.
 ///
 /// This exists in addition to ``LiveObjectMutableState`` to enable polymorphism.
 internal protocol InternalLiveObject<Update> {
     associatedtype Update: Sendable
 
     var liveObjectMutableState: LiveObjectMutableState<Update> { get set }
+
+    /// Resets the LiveObject's internal data to that of a zero-value, per RTLO4e4.
+    mutating func resetDataToZeroValued()
+}
+
+internal extension InternalLiveObject {
+    /// Convenience method for tombstoning a `LiveObject`, as specified in RTLO4e.
+    mutating func tombstone(
+        objectMessageSerialTimestamp: Date?,
+        logger: Logger,
+        clock: SimpleClock,
+    ) {
+        // RTLO4e2, RTLO4e3
+        if let objectMessageSerialTimestamp {
+            // RTLO4e3a
+            liveObjectMutableState.tombstonedAt = objectMessageSerialTimestamp
+        } else {
+            // RTLO4e3b1
+            logger.log("serialTimestamp not found in ObjectMessage, using local clock for tombstone timestamp", level: .debug)
+            // RTLO4e3b
+            liveObjectMutableState.tombstonedAt = clock.now
+        }
+
+        // RTLO4e4
+        resetDataToZeroValued()
+    }
+
+    /// Applies an `OBJECT_DELETE` operation, per RTLO5.
+    mutating func applyObjectDeleteOperation(
+        objectMessageSerialTimestamp: Date?,
+        logger: Logger,
+        clock: SimpleClock,
+    ) {
+        // RTLO5b
+        tombstone(
+            objectMessageSerialTimestamp: objectMessageSerialTimestamp,
+            logger: logger,
+            clock: clock,
+        )
+    }
 }

Sources/AblyLiveObjects/Internal/LiveObjectMutableState.swift

@@ -10,16 +10,26 @@ internal struct LiveObjectMutableState<Update: Sendable> {
     internal var siteTimeserials: [String: String] = [:]
     // RTLO3c
     internal var createOperationIsMerged = false
+    // RTLO3d
+    internal var isTombstone: Bool {
+        // TODO: Confirm that we don't need to store this (https://github.com/ably/specification/pull/350/files#r2213895661)
+        tombstonedAt != nil
+    }
+
+    // RTLO3e
+    internal var tombstonedAt: Date?
 
     /// Internal bookkeeping for subscriptions.
     private var subscriptionsByID: [Subscription.ID: Subscription] = [:]
 
     internal init(
         objectID: String,
         testsOnly_siteTimeserials siteTimeserials: [String: String]? = nil,
+        testsOnly_tombstonedAt tombstonedAt: Date? = nil,
     ) {
         self.objectID = objectID
         self.siteTimeserials = siteTimeserials ?? [:]
+        self.tombstonedAt = tombstonedAt
     }
 
     /// Represents parameters of an operation that `canApplyOperation` has decided can be applied to a `LiveObject`.