Change kMaxLowEntropySize to have 8000 values instead of 8192.

chrome/browser/metrics/metrics_service.cc

Index: chrome/browser/metrics/metrics_service.cc
===================================================================
--- chrome/browser/metrics/metrics_service.cc	(revision 207251)
+++ chrome/browser/metrics/metrics_service.cc	(working copy)
@@ -291,9 +291,9 @@
 }
 
 // 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.
@@ -1043,13 +1043,17 @@
   // 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_;
     }