Helium: Adding vector clock

blimp/net/helium/vector_clock.h

Index: blimp/net/helium/vector_clock.h
diff --git a/blimp/net/helium/vector_clock.h b/blimp/net/helium/vector_clock.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BLIMP_NET_HELIUM_VECTOR_CLOCK_H_
+#define BLIMP_NET_HELIUM_VECTOR_CLOCK_H_
+
+#include <stdint.h>
+
+namespace blimp {
+
+// From wikipedia:
+// A vector clock is an algorithm for generating a partial ordering of events
+// in a distributed system and detecting causality violations. This is used
+// in Blimp to allow client and server modify a local copy of an object and
+// later be able to detect ordering or conflicts if any.
+//
+// For more info see:
+// https://en.wikipedia.org/wiki/Vector_clock
+
+typedef uint32_t Revision;
+
+class VectorClock {
+ public:
+  enum class Comparison { LessThan, EqualTo, GreaterThan, Conflict };
+
+  VectorClock(Revision local_revision, Revision remote_revision);
+  VectorClock();
+
+  // Compares two vector clocks. There are 4 possibilities for the result:
+  // * LessThan: One revision is equal and for the other is smaller.
+  // (1,0).CompareTo((2, 0));
+  // * EqualTo: Both revisions are the same.
+  // * GreaterThan: One revision is equal and for the other is greater.
+  // (2,0).CompareTo((1, 0));
+  // * Conflict: Both revisions are different. (1,0).CompareTo(0,1)
+  Comparison CompareTo(const VectorClock& other) const;
+
+  // Merges two vector clocks. This function should be used at synchronization
+  // points. i.e. when client receives data from the server or vice versa.
+  VectorClock MergeWith(const VectorClock& other) const;
+
+  // Increments local_revision_ by one. This is used when something changes
+  // in the local state like setting a property or applying a change set.
+  void IncrementLocal();
+
+  Revision local_revision() const { return local_revision_; }
+
+  void set_local_revision(Revision local_revision) {
+    local_revision_ = local_revision;
+  }
+
+  Revision remote_revision() const { return remote_revision_; }
+
+  void set_remote_revision(Revision remote_revision) {
+    remote_revision_ = remote_revision;
+  }
+
+ private:
+  Revision local_revision_ = 0;
+  Revision remote_revision_ = 0;
+};
+
+}  // namespace blimp
+
+#endif  // BLIMP_NET_HELIUM_VECTOR_CLOCK_H_