Use a different algorithm with the low entropy source for field trials.

chrome/common/metrics/entropy_provider.h

+// Copyright (c) 2012 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 CHROME_COMMON_METRICS_ENTROPY_PROVIDER_H_
+#define CHROME_COMMON_METRICS_ENTROPY_PROVIDER_H_
+
+#include <functional>
+#include <string>
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/metrics/field_trial.h"
+#include "third_party/mt19937ar/mt19937ar.h"
+
+// Internals of entropy_provider.cc exposed for testing.
+namespace internal {

[jar (doing other things), 2012/08/17 19:06:19]

This seems like a strange choice of global namespace names, and doesn't match
any of the path elements. Style guide suggests:

"choose the name based on the project, and possibly its path"

Could you consider something with the word "metrics," or nested inside of a
"metrics" namespace?

[Alexei Svitkine (slow), 2012/08/20 15:57:40]

Done.

+
+// A functor that generates random numbers based on a seed, using the Mersenne
+// Twister algorithm. Suitable for use with std::random_shuffle().
+struct SeededRandGenerator : std::unary_function<uint32, uint32> {
+  explicit SeededRandGenerator(uint32 seed);
+  ~SeededRandGenerator();
+
+  // Returns a random number in range [0, range).
+  uint32 operator()(uint32 range);
+
+  MersenneTwister mersenne_twister_;
+};
+
+// Creates unique identifier for the trial by hashing a name string, whether
+// it's for the field trial or the group name.
+// TODO(asvitkine): Share the implementation with variations_util.cc.
+uint32 HashName(const std::string& name);
+
+}  // namespace internal
+
+// SHA1EntropyProvider is an entropy provider suitable for high entropy
+// sources. It works by taking the first 64 bits of the SHA1 hash of the
+// entropy source concatenated with the trial name and using that for the
+// final entropy value.
+class SHA1EntropyProvider : public base::FieldTrial::EntropyProvider {
+ public:
+  // Creates a SHA1EntropyProvider with the given |entropy_source|, which
+  // should contain a large amount of entropy - for example, a textual
+  // representation of a persistent randomly-generated 64-bit value.
+  explicit SHA1EntropyProvider(const std::string& entropy_source);
+  virtual ~SHA1EntropyProvider();
+
+  // base::FieldTrial::EntropyProvider implementation:
+  virtual double GetEntropyForTrial(const std::string& trial_name) const
+      OVERRIDE;
+
+ private:
+  std::string entropy_source_;
+
+  DISALLOW_COPY_AND_ASSIGN(SHA1EntropyProvider);
+};
+
+// PermutedEntropyProvider is an entropy provider suitable for low entropy
+// sources (below 16 bits). It uses the field trial name to generate a
+// permutation of a mapping array from an initial entropy value to a new value.
+// Note: This provider's performance is O(n^2), where n is the number of bits

[hfung, 2012/08/17 17:35:40]

Isn't it O(low_entropy_source_max), since it has to shuffle that many elements,
so O(2^n)?

[Alexei Svitkine (slow), 2012/08/17 17:57:40]

That's correct - I accidentally transposed the two. Fixed.

+// in the entropy source.
+class PermutedEntropyProvider : public base::FieldTrial::EntropyProvider {
+ public:
+  // Creates a PermutedEntropyProvider with the given |low_entropy_source|,
+  // which should have a value in the range of [0, low_entropy_source_max).
+  PermutedEntropyProvider(uint16 low_entropy_source,
+                          size_t low_entropy_source_max);
+  virtual ~PermutedEntropyProvider();
+
+  // base::FieldTrial::EntropyProvider implementation:
+  virtual double GetEntropyForTrial(const std::string& trial_name) const
+      OVERRIDE;
+
+ private:
+  uint16 low_entropy_source_;
+  size_t low_entropy_source_max_;
+
+  DISALLOW_COPY_AND_ASSIGN(PermutedEntropyProvider);
+};
+
+#endif  // CHROME_COMMON_METRICS_ENTROPY_PROVIDER_H_