refactor and cleanup [skip ci]

submodules/Postbox/Sources/TimestampBasedMessageAttributesView.swift

@@ -0,0 +1,52 @@
+import Foundation
+
+final class MutableTimestampBasedMessageAttributesView {
+    let tag: UInt16
+    var head: TimestampBasedMessageAttributesEntry?
+    
+    init(tag: UInt16, getHead: (UInt16) -> TimestampBasedMessageAttributesEntry?) {
+        self.tag = tag
+        self.head = getHead(tag)
+    }
+    
+    func replay(operations: [TimestampBasedMessageAttributesOperation], getHead: (UInt16) -> TimestampBasedMessageAttributesEntry?) -> Bool {
+        var updated = false
+        var invalidatedHead = false
+        for operation in operations {
+            switch operation {
+                case let .add(entry):
+                    if entry.tag == self.tag {
+                        if let head = self.head {
+                            if entry.index < head.index {
+                                self.head = entry
+                                updated = true
+                            }
+                        } else {
+                            self.head = entry
+                            updated = true
+                        }
+                    }
+                case let .remove(entry):
+                    if entry.tag == self.tag {
+                        if let head = self.head, head.index == entry.index {
+                            self.head = nil
+                            updated = true
+                            invalidatedHead = true
+                        }
+                    }
+            }
+        }
+        if invalidatedHead {
+            self.head = getHead(self.tag)
+        }
+        return updated
+    }
+}
+
+public final class TimestampBasedMessageAttributesView {
+    public let head: TimestampBasedMessageAttributesEntry?
+    
+    init(_ view: MutableTimestampBasedMessageAttributesView) {
+        self.head = view.head
+    }
+}