Change kMaxLowEntropySize to have 8000 values instead of 8192.

chrome/browser/metrics/metrics_service.cc

 // 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.
 
 //------------------------------------------------------------------------------
 // Description of the life cycle of a instance of MetricsService.
 //
 //  OVERVIEW
 //
 // A MetricsService instance is typically created at application startup.  It is
@@ skipping 273 matching lines @@
     case 400:
       return BAD_REQUEST;
     case net::URLFetcher::RESPONSE_CODE_INVALID:
       return NO_RESPONSE;
     default:
       return UNKNOWN_FAILURE;
   }
 }
 
 // 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 kMaxLowEntropySize = (1 << 13);
+// more than 13 bits of entropy, so use this max to return a number in the range
+// [0, 7999] as the entropy source (12.97 bits of entropy).
+const int kMaxLowEntropySize = 8000;
 
 // Default prefs value for prefs::kMetricsLowEntropySource to indicate that the
 // value has not yet been set.
 const int kLowEntropySourceNotSet = -1;
 
 // Generates a new non-identifying entropy source used to seed persistent
 // activities.
 int GenerateLowEntropySource() {
   return base::RandInt(0, kMaxLowEntropySize - 1);
 }
@@ skipping 729 matching lines @@
   // value are both kLowEntropySourceNotSet, which is used to identify if the
   // value has been set or not.
   if (low_entropy_source_ != kLowEntropySourceNotSet)
     return low_entropy_source_;
 
   PrefService* local_state = g_browser_process->local_state();
   const CommandLine* command_line(CommandLine::ForCurrentProcess());
   // Only try to load the value from prefs if the user did not request a reset.
   // Otherwise, skip to generating a new value.
   if (!command_line->HasSwitch(switches::kResetVariationState)) {
-    const int value = local_state->GetInteger(prefs::kMetricsLowEntropySource);
-    if (value != kLowEntropySourceNotSet) {
-      // Ensure the prefs value is in the range [0, kMaxLowEntropySize). Old
-      // versions of the code would generate values in the range of [1, 8192],
-      // so the below line ensures 8192 gets mapped to 0 and also guards against
-      // the case of corrupted values.
-      low_entropy_source_ = value % kMaxLowEntropySize;
+    int value = local_state->GetInteger(prefs::kMetricsLowEntropySource);
+    // Old versions of the code would generate values in the range of [1, 8192],
+    // before the range was switched to [0, 8191] and then [0, 7999]. Map 8192

[Ilya Sherman, 2013/06/21 01:12:53]

nit: "and then [0, 7999]" -> "and then to [0, 7999]"

[Alexei Svitkine (slow), 2013/06/21 14:29:09]

Done.

+    // to 0, so that the 0th bucket remains uniform, while re-generating the
+    // low entropy source for old values in the [8000, 8191] range.

[Ilya Sherman, 2013/06/21 01:12:53]

I wonder whether it's really worth keeping this extra bit of logic around just
to support users who might have not been migrated yet.  That's probably not very
many users, and they'd simply be re-shuffled -- is that so bad / worth keeping
this tweak around for?

[Alexei Svitkine (slow), 2013/06/21 04:41:30]

It is. We never migrated those users to have the new pref value, we just used %
to map them, but never re-saved the pref.

It's important to do this so that the 0th bucket remains uniform with respect to
the other buckets, else it would have fewer users.

+    if (value == 8192)
+      value = 0;
+    // If the value is outside the [0, kMaxLowEntropySize) range, re-generate
+    // it below.
+    if (value >= 0 && value < kMaxLowEntropySize) {
+      low_entropy_source_ = value;
       UMA_HISTOGRAM_BOOLEAN("UMA.GeneratedLowEntropySource", false);
       return low_entropy_source_;
     }
   }
 
   UMA_HISTOGRAM_BOOLEAN("UMA.GeneratedLowEntropySource", true);
   low_entropy_source_ = GenerateLowEntropySource();
   local_state->SetInteger(prefs::kMetricsLowEntropySource, low_entropy_source_);
   metrics::CachingPermutedEntropyProvider::ClearCache(local_state);
 
@@ skipping 751 matching lines @@
   if (local_state) {
     const PrefService::Preference* uma_pref =
         local_state->FindPreference(prefs::kMetricsReportingEnabled);
     if (uma_pref) {
       bool success = uma_pref->GetValue()->GetAsBoolean(&result);
       DCHECK(success);
     }
   }
   return result;
 }