Merge commit '7621e2f8dec938cf48181c8b10afc9b01f444e68' into beta

submodules/Postbox/Sources/MessageHistoryAnchorIndex.swift

@@ -0,0 +1,51 @@
+import Foundation
+
+public enum MessageHistoryAnchorIndex: Comparable {
+    case message(MessageIndex)
+    case lowerBound
+    case upperBound
+    
+    public static func <(lhs: MessageHistoryAnchorIndex, rhs: MessageHistoryAnchorIndex) -> Bool {
+        switch lhs {
+            case let .message(lhsIndex):
+                switch rhs {
+                    case let .message(rhsIndex):
+                        return lhsIndex < rhsIndex
+                    case .lowerBound:
+                        return false
+                    case .upperBound:
+                        return true
+                }
+            case .lowerBound:
+                if case .lowerBound = rhs {
+                    return false
+                } else {
+                    return true
+                }
+            case .upperBound:
+                return false
+        }
+    }
+    
+    public func isLess(than: MessageIndex) -> Bool {
+        switch self {
+            case .lowerBound:
+                return true
+            case .upperBound:
+                return false
+            case let .message(index):
+                return index < than
+        }
+    }
+    
+    public func isLessOrEqual(to: MessageIndex) -> Bool {
+        switch self {
+            case .lowerBound:
+                return true
+            case .upperBound:
+                return false
+            case let .message(index):
+                return index <= to
+        }
+    }
+}