report zero values for unexpected overflows in MonotonicCounterUint

Resolves #98.

spectator/monotonic_counter_uint.cc

@@ -3,6 +3,7 @@
 
 namespace spectator {
 
+static constexpr auto kOverflow = 9.223372e+18;  // 2^63
 static constexpr auto kNaN = std::numeric_limits<double>::quiet_NaN();
 static constexpr auto kMax = std::numeric_limits<uint64_t>::max();
 
@@ -36,7 +37,12 @@ void MonotonicCounterUint::Measure(Measurements* results) const noexcept {
     if (!count_id_) {
       count_id_ = std::make_unique<Id>(MeterId().WithDefaultStat(refs().count()));
     }
-    results->emplace_back(*count_id_, delta);
+    if (delta > kOverflow) {
+      // deltas that appear to be unexpected overflows will be reported as zeroes instead
+      results->emplace_back(*count_id_, 0);
+    } else {
+      results->emplace_back(*count_id_, delta);
+    }
   }
 }
 

spectator/monotonic_counter_uint_test.cc

@@ -59,6 +59,38 @@ TEST(MonotonicCounterUint, Overflow) {
   EXPECT_EQ(expected3, ms.back());
 }
 
+TEST(MonotonicCounterUint, UnexpectedOverflow) {
+  auto c = getMonotonicCounterUint("overflow");
+  EXPECT_TRUE(std::isnan(c.Delta()));
+
+  // initialize overflow condition
+  spectator::Measurements ms;
+  c.Set(0);
+  c.Measure(&ms);
+  EXPECT_TRUE(ms.empty());
+
+  c.Set(100000);
+  c.Measure(&ms);
+  EXPECT_EQ(ms.size(), 1);
+  auto id = c.MeterId().WithStat(refs().count());
+  auto expected1 = spectator::Measurement{id, 100000};
+  EXPECT_EQ(expected1, ms.back());
+
+  // trigger overflow condition
+  c.Set(0);
+  c.Measure(&ms);
+  EXPECT_EQ(ms.size(), 2);
+  auto expected2 = spectator::Measurement{id, 0};
+  EXPECT_EQ(expected2, ms.back());
+
+  // normal increment condition
+  c.Set(5);
+  c.Measure(&ms);
+  EXPECT_EQ(ms.size(), 3);
+  auto expected3 = spectator::Measurement{id, 5};
+  EXPECT_EQ(expected3, ms.back());
+}
+
 TEST(MonotonicCounterUint, Id) {
   auto c = getMonotonicCounterUint("id");
   auto id = spectator::Id("id", spectator::Tags{});