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

chrome/browser/metrics/metrics_service.cc

Index: chrome/browser/metrics/metrics_service.cc
===================================================================
--- chrome/browser/metrics/metrics_service.cc	(revision 152812)
+++ chrome/browser/metrics/metrics_service.cc	(working copy)
@@ -185,6 +185,7 @@
 #include "chrome/common/chrome_notification_types.h"
 #include "chrome/common/chrome_result_codes.h"
 #include "chrome/common/chrome_switches.h"
+#include "chrome/common/metrics/entropy_provider.h"
 #include "chrome/common/metrics/metrics_log_manager.h"
 #include "chrome/common/net/test_server_locations.h"
 #include "chrome/common/pref_names.h"
@@ -274,12 +275,12 @@
 // The argument used to generate a non-identifying entropy source. We want no
 // more than 13 bits of entropy, so use this max to return a number between 1
 // and 2^13 = 8192 as the entropy source.
-const uint32 kMaxEntropySize = (1 << 13);
+const uint32 kMaxLowEntropySize = (1 << 13);
 
 // Generates a new non-identifying entropy source used to seed persistent
 // activities.
 int GenerateLowEntropySource() {
-  return base::RandInt(1, kMaxEntropySize);
+  return base::RandInt(0, kMaxLowEntropySize - 1);
 }
 
 // Converts an exit code into something that can be inserted into our
@@ -344,7 +345,7 @@
   return codes;
 }
 
-}
+}  // namespace
 
 // static
 MetricsService::ShutdownCleanliness MetricsService::clean_shutdown_status_ =
@@ -539,7 +540,8 @@
   return client_id_;
 }
 
-std::string MetricsService::GetEntropySource(bool reporting_will_be_enabled) {
+scoped_ptr<const base::FieldTrial::EntropyProvider>
+    MetricsService::CreateEntropyProvider(bool reporting_will_be_enabled) {
   // For metrics reporting-enabled users, we combine the client ID and low
   // entropy source to get the final entropy source. Otherwise, only use the low
   // entropy source.
@@ -547,17 +549,22 @@
   //  1) It makes the entropy source less identifiable for parties that do not
   //     know the low entropy source.
   //  2) It makes the final entropy source resettable.
-  std::string low_entropy_source = base::IntToString(GetLowEntropySource());
   if (reporting_will_be_enabled) {
     if (entropy_source_returned_ == LAST_ENTROPY_NONE)
       entropy_source_returned_ = LAST_ENTROPY_HIGH;
     DCHECK_EQ(LAST_ENTROPY_HIGH, entropy_source_returned_);
-    return client_id_ + low_entropy_source;
+    const std::string high_entropy_source =
+        client_id_ + base::IntToString(GetLowEntropySource());
+    return scoped_ptr<const base::FieldTrial::EntropyProvider>(
+        new metrics::SHA1EntropyProvider(high_entropy_source));
   }
+
   if (entropy_source_returned_ == LAST_ENTROPY_NONE)
     entropy_source_returned_ = LAST_ENTROPY_LOW;
   DCHECK_EQ(LAST_ENTROPY_LOW, entropy_source_returned_);
-  return low_entropy_source;
+  return scoped_ptr<const base::FieldTrial::EntropyProvider>(
+      new metrics::PermutedEntropyProvider(GetLowEntropySource(),
+                                           kMaxLowEntropySize));
 }
 
 void MetricsService::ForceClientIdCreation() {