Add a parameterized ctor and m2 getter

Summary: Allows us to serialized and unserialize StreamingStats.

Reviewed By: yfeldblum

Differential Revision: D30108179

fbshipit-source-id: 7a920acfa10e6c16613e9059049b8550478e81f0

folly/stats/StreamingStats.h

@@ -37,11 +37,26 @@ class StreamingStats final {
   static_assert(std::is_floating_point_v<StatsType>);
 
  public:
+  struct StreamingState {
+    size_t count = 0;
+    StatsType mean = 0;
+    StatsType m2 = 0;
+    SampleDataType min = std::numeric_limits<SampleDataType>::max();
+    SampleDataType max = std::numeric_limits<SampleDataType>::lowest();
+  };
+
   template <class Iterator>
   StreamingStats(Iterator first, Iterator last) noexcept {
     add(first, last);
   }
 
+  explicit StreamingStats(StreamingState state)
+      : count_(state.count),
+        mean_(state.mean),
+        m2_(state.m2),
+        min_(state.min),
+        max_(state.max) {}
+
   StreamingStats() = default;
 
   ~StreamingStats() = default;
@@ -104,6 +119,11 @@ class StreamingStats final {
     return mean_;
   }
 
+  StatsType m2() const {
+    checkMinimumDataSize(1);
+    return m2_;
+  }
+
   StatsType populationVariance() const {
     checkMinimumDataSize(2);
     return var_(0);
@@ -124,6 +144,16 @@ class StreamingStats final {
     return std_(1);
   }
 
+  StreamingState state() const {
+    StreamingState state;
+    state.count = count_;
+    state.m2 = m2_;
+    state.max = max_;
+    state.mean = mean_;
+    state.min = min_;
+    return state;
+  }
+
  private:
   void checkMinimumDataSize(size_t const minElements) const {
     if (count_ < minElements) {

folly/stats/test/StreamingStatsTest.cpp

@@ -43,6 +43,20 @@ TEST(StreamingStatsTest, Constructor) {
   EXPECT_DOUBLE_EQ(stats.maximum(), 10.0);
 }
 
+TEST(StreamingStatsTest, ConstructFromValues) {
+  StreamingStats<double> stats1;
+  for (double i = 0.0; i < 100; i++) {
+    stats1.add(i);
+  }
+
+  StreamingStats<double> stats2(stats1.state());
+  EXPECT_EQ(stats1.count(), stats2.count());
+  EXPECT_EQ(stats1.mean(), stats2.mean());
+  EXPECT_EQ(stats1.m2(), stats2.m2());
+  EXPECT_EQ(stats1.minimum(), stats2.minimum());
+  EXPECT_EQ(stats1.maximum(), stats2.maximum());
+}
+
 template <typename SampleDataType>
 class StreamingStatsTest : public testing::Test {
  public: