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 to [0, 7999]. Map
+    // 8192 to 0, so that the 0th bucket remains uniform, while re-generating
+    // the low entropy source for old values in the [8000, 8191] range.
+    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;
 }