Drop Stat1, refactor statistics to be user-providable, add median. (#428)

* Drop Stat1, refactor statistics to be user-providable, add median.

My main goal was to add median statistic. Since Stat1
calculated the stats incrementally, and did not store
the values themselves, it is was not possible. Thus,
i have replaced Stat1 with simple std::vector<double>,
containing all the values.

Then, i have refactored current mean/stdev to be a
function that is provided with values vector, and
returns the statistic. While there, it seemed to make
sense to deduplicate the code by storing all the
statistics functions in a map, and then simply iterate
over it. And the interface to add new statistics is
intentionally exposed, so they may be added easily.

The notable change is that Iterations are no longer
displayed as 0 for stdev. Is could be changed, but
i'm not sure how to nicely fit that into the API.

Similarly, this dance about sometimes (for some fields,
for some statistics) dividing by run.iterations, and
then multiplying the calculated stastic back is also
dropped, and if you do the math, i fail to see why
it was needed there in the first place.

Since that was the only use of stat.h, it is removed.

* complexity.h: attempt to fix MSVC build

* Update README.md

* Store statistics to compute in a vector, ensures ordering.

* Add a bit more tests for repetitions.

* Partially address review notes.

* Fix gcc build: drop extra ';'

clang, why didn't you warn me?

* Address review comments.

* double() -> 0.0
* early return

1 files changed, 37 insertions, 0 deletions

diff --git a/src/statistics.h b/src/statistics.h
new file mode 100644
index 0000000..7eccc85
--- /dev/null
+++ b/src/statistics.h
@@ -0,0 +1,37 @@
+// Copyright 2016 Ismael Jimenez Martinez. All rights reserved.
+// Copyright 2017 Roman Lebedev. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef STATISTICS_H_
+#define STATISTICS_H_
+
+#include <vector>
+
+#include "benchmark/benchmark.h"
+
+namespace benchmark {
+
+// Return a vector containing the mean, median and standard devation information
+// (and any user-specified info) for the specified list of reports. If 'reports'
+// contains less than two non-errored runs an empty vector is returned
+std::vector<BenchmarkReporter::Run> ComputeStats(
+    const std::vector<BenchmarkReporter::Run>& reports);
+
+double StatisticsMean(const std::vector<double>& v);
+double StatisticsMedian(const std::vector<double>& v);
+double StatisticsStdDev(const std::vector<double>& v);
+
+}  // end namespace benchmark
+
+#endif  // STATISTICS_H_