Pull cacheinvalidations code directory into chromium repo.

third_party/cacheinvalidation/src/google/cacheinvalidation/impl/smearer.h

+// Copyright 2012 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// An abstraction to "smear" values by a given percent. Useful for randomizing
+// delays a little bit so that (say) processes do not get synchronized on time
+// inadvertently, e.g., a heartbeat task that sends a message every few minutes
+// is smeared so that all clients do not end up sending a message at the same
+// time. In particular, given a |delay|, returns a value that is randomly
+// distributed between
+// [delay - smearPercent * delay, delay + smearPercent * delay]
+
+#ifndef GOOGLE_CACHEINVALIDATION_IMPL_SMEARER_H_
+#define GOOGLE_CACHEINVALIDATION_IMPL_SMEARER_H_
+
+#include "google/cacheinvalidation/deps/logging.h"
+#include "google/cacheinvalidation/deps/random.h"
+#include "google/cacheinvalidation/deps/scoped_ptr.h"
+
+namespace invalidation {
+
+class Smearer {
+ public:
+  /* Creates a smearer with the given random number generator.
+   * REQUIRES: 0 <= smear_percent <= 100
+   * Caller continues to own space for random.
+   */
+  Smearer(Random* random, int smear_percent) : random_(random),
+          smear_fraction_(smear_percent / 100.0) {
+    CHECK((smear_percent >= 0) && (smear_percent <= 100));
+  }
+
+  /* Given a delay, returns a value that is randomly distributed between
+   * (delay - smear_percent * delay, delay + smear_percent * delay)
+   */
+  TimeDelta GetSmearedDelay(TimeDelta delay) {
+    // Get a random number between -1 and 1 and then multiply that by the
+    // smear fraction.
+    double smear_factor = (2 * random_->RandDouble() - 1.0) * smear_fraction_;
+    return TimeDelta::FromMilliseconds(
+        delay.InMilliseconds() * (1.0 + smear_factor));
+  }
+
+ private:
+  Random* random_;
+
+  /* The percentage (0, 1.0] for smearing the delay. */
+  double smear_fraction_;
+};
+}  // namespace invalidation
+
+#endif  // GOOGLE_CACHEINVALIDATION_IMPL_SMEARER_H_