Optimizations to histogram protobuf encoding.

components/metrics/histogram_encoder.cc

 // Copyright 2014 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.
 
 #include "components/metrics/histogram_encoder.h"
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/metrics_hashes.h"
 
 using base::SampleCountIterator;
 
 namespace metrics {
 
 void EncodeHistogramDelta(const std::string& histogram_name,
                           const base::HistogramSamples& snapshot,
                           ChromeUserMetricsExtension* uma_proto) {
   DCHECK_NE(0, snapshot.TotalCount());
   DCHECK(uma_proto);
 
   // We will ignore the MAX_INT/infinite value in the last element of range[].
 
   HistogramEventProto* histogram_proto = uma_proto->add_histogram_event();
   histogram_proto->set_name_hash(base::HashMetricName(histogram_name));
-  histogram_proto->set_sum(snapshot.sum());
+  if (snapshot.sum() != 0)
+    histogram_proto->set_sum(snapshot.sum());
 
   for (scoped_ptr<SampleCountIterator> it = snapshot.Iterator(); !it->Done();
        it->Next()) {
     base::Histogram::Sample min;
     base::Histogram::Sample max;
     base::Histogram::Count count;
     it->Get(&min, &max, &count);
     HistogramEventProto::Bucket* bucket = histogram_proto->add_bucket();
     bucket->set_min(min);
     bucket->set_max(max);
-    bucket->set_count(count);
+    if (count != 1)

[rkaplow, 2016/02/29 22:46:44]

Might just add a comment that 1 is the default in the proto. Less worried about
sum since 0 is a pretty natural default but 1 is not so looks slightly odd.

[Alexei Svitkine (slow), 2016/02/29 22:55:53]

Done.

+      bucket->set_count(count);
   }
 
   // Omit fields to save space (see rules in histogram_event.proto comments).
   for (int i = 0; i < histogram_proto->bucket_size(); ++i) {
     HistogramEventProto::Bucket* bucket = histogram_proto->mutable_bucket(i);
     if (i + 1 < histogram_proto->bucket_size() &&
         bucket->max() == histogram_proto->bucket(i + 1).min()) {
       bucket->clear_max();
     } else if (bucket->max() == bucket->min() + 1) {
       bucket->clear_min();
     }
   }
 }
 
 }  // namespace metrics