Create a histogram serialization mechanism in components/metrics

base/metrics/sparsehistogram_sample.cc

Index: base/metrics/sparsehistogram_sample.cc
diff --git a/base/metrics/sparsehistogram_sample.cc b/base/metrics/sparsehistogram_sample.cc
new file mode 100644
index 0000000000000000000000000000000000000000..47294e2f1fc0f85a0c78f1b139556c5468e49104
--- /dev/null
+++ b/base/metrics/sparsehistogram_sample.cc
@@ -0,0 +1,45 @@
+// Copyright (c) 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 <stdio.h>
+
+#include <string>
+
+#include "base/logging.h"

[gauravsh, 2014/04/11 23:50:51]

Is this being used?

+#include "base/metrics/chromeos_metrics.h"
+#include "base/metrics/metric_sample.h"
+#include "base/metrics/sparsehistogram_sample.h"

[gauravsh, 2014/04/11 23:50:51]

this needs to be after this #include block.

+#include "base/strings/stringprintf.h"
+
+using std::string;
+
+namespace base {
+
+SparseHistogramSample::SparseHistogramSample(const std::string& histname,
+                                             int sample)
+    : MetricSample(MetricSample::SPARSE_HISTOGRAM) {
+  name_ = histname;

[gauravsh, 2014/04/11 23:50:51]

Use constructor initializer lists wherever possible.

+  sample_ = sample;
+}
+
+int SparseHistogramSample::sample() { return sample_; }
+
+SparseHistogramSample* SparseHistogramSample::ReadSparseHistogram(
+    const std::string& hist_repr) {
+  int sample;
+  char name[128];
+  int result = sscanf(hist_repr.c_str(), "%127s %d", name, &sample);

[gauravsh, 2014/04/11 23:50:51]

There's probably a better way of doing this than using sscanf. Have you look at
base/strings/ and seen anything there that might be let you do this without
having to worry about overflow and other issues?

+  if (result != 2) {
+    return NULL;
+  }
+  return new SparseHistogramSample(name, sample);
+}
+
+SparseHistogramSample::~SparseHistogramSample() {}
+
+string SparseHistogramSample::toString() const {
+  return StringPrintf(
+      "sparsehistogram%c%s %d%c", '\0', name_.c_str(), sample_, '\0');

[gauravsh, 2014/04/11 23:50:51]

This is returning a string with embedded NULL bytes, when would a caller use
this?

[bsimonnet, 2014/04/14 23:03:05]

This is used for the serialization. The message format to store the metrics in
the /var/log/metrics/uma-events is: message type, \0, message content, \0.
ToString is meant to return the serialized version on the metric sample.

+}
+}  // namespace base