add cluster balance indicator

include/dsn/utility/math.h

@@ -0,0 +1,16 @@
+// Copyright (c) 2017, Xiaomi, Inc.  All rights reserved.
+// This source code is licensed under the Apache License Version 2.0, which
+// can be found in the LICENSE file in the root directory of this source tree.
+
+#pragma once
+
+#include <vector>
+#include <cstdint>
+
+namespace dsn {
+namespace utils {
+
+double mean_stddev(const std::vector<uint32_t> &result_set, bool partial_sample);
+
+} // namespace utils
+} // namespace dsn

src/core/core/math.cpp

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+// Copyright (c) 2017, Xiaomi, Inc.  All rights reserved.
+// This source code is licensed under the Apache License Version 2.0, which
+// can be found in the LICENSE file in the root directory of this source tree.
+
+#include <numeric>
+#include <algorithm>
+#include <math.h>
+#include <dsn/c/api_utilities.h>
+#include <dsn/utility/math.h>
+
+namespace dsn {
+namespace utils {
+
+double mean_stddev(const std::vector<uint32_t> &result_set, bool partial_sample)
+{
+    dassert(result_set.size() > 1, "invalid sample data input for stddev");
+
+    double sum = std::accumulate(result_set.begin(), result_set.end(), 0.0);
+    double mean = sum / result_set.size();
+
+    double accum = 0.0;
+    std::for_each(result_set.begin(), result_set.end(), [&](const double d) {
+        accum += (d - mean) * (d - mean);
+    });
+
+    double stddev;
+    if (partial_sample)
+        stddev = sqrt(accum / (result_set.size() - 1));
+    else
+        stddev = sqrt(accum / (result_set.size()));
+
+    stddev = ((double)((int)((stddev + 0.005) * 100))) / 100;
+    return stddev;
+}
+
+} // namespace utils
+} // namespace dsn