added utility function for saturated add

3 files changed, 37 insertions, 0 deletions

diff --git a/vespalib/src/tests/time/time_test.cpp b/vespalib/src/tests/time/time_test.cpp
index 0ca583c8326..f5738611a41 100644
--- a/vespalib/src/tests/time/time_test.cpp
+++ b/vespalib/src/tests/time/time_test.cpp
@@ -5,6 +5,7 @@
 #include <cinttypes>
 #include <thread>
 #include <atomic>
+#include <iostream>
 
 using namespace vespalib;
 
@@ -83,4 +84,25 @@ TEST(TimeTest, timeout_is_relative_to_frequency) {
     EXPECT_EQ(2000ms, adjustTimeoutByHz(20ms, 10));
 }
 
+TEST(TimeTest, print_limits) {
+    std::cerr << "steady_time::min().time_since_epoch(): " << steady_time::min().time_since_epoch() << "\n";
+    std::cerr << "steady_time::max().time_since_epoch(): " << steady_time::max().time_since_epoch() << "\n";
+    std::cerr << "duration::min(): " << duration::min() << "\n";
+    std::cerr << "duration::max(): " << duration::max() << "\n";
+}
+
+TEST(TimeTest, saturated_add_without_overflow) {
+    steady_time time_10ms(10ms);
+    steady_time time_20ms(20ms);
+    EXPECT_EQ(saturated_add(time_10ms, 10ms), time_20ms);
+    EXPECT_EQ(saturated_add(time_20ms, -10ms), time_10ms);
+}
+
+TEST(TimeTest, saturated_add_with_overflow) {
+    steady_time future(1000000000s);
+    steady_time  past(-1000000000s);
+    EXPECT_EQ(saturated_add(future, duration::max()), steady_time::max());
+    EXPECT_EQ(saturated_add(past, duration::min()), steady_time::min());
+}
+
 GTEST_MAIN_RUN_ALL_TESTS()
diff --git a/vespalib/src/vespa/vespalib/util/time.cpp b/vespalib/src/vespa/vespalib/util/time.cpp
index 3737a25f268..2dbaade8c0a 100644
--- a/vespalib/src/vespa/vespalib/util/time.cpp
+++ b/vespalib/src/vespa/vespalib/util/time.cpp
@@ -62,6 +62,19 @@ to_string(system_time time) {
     return to_string(time.time_since_epoch());
 }
 
+steady_time saturated_add(steady_time time, duration diff) {
+    auto td = time.time_since_epoch();
+    using dur_t = decltype(td);
+    using val_t = dur_t::rep;
+    val_t a = td.count();
+    val_t b = std::chrono::duration_cast<dur_t>(diff).count();
+    val_t res;
+    if (__builtin_add_overflow(a, b, &res)) {
+        return (b > 0) ? steady_time::max() : steady_time::min();
+    }
+    return steady_time(dur_t(res));
+}
+
 Timer::~Timer() = default;
 
 void
diff --git a/vespalib/src/vespa/vespalib/util/time.h b/vespalib/src/vespa/vespalib/util/time.h
index 2355b269fa5..ce914b097d4 100644
--- a/vespalib/src/vespa/vespalib/util/time.h
+++ b/vespalib/src/vespa/vespalib/util/time.h
@@ -73,6 +73,8 @@ constexpr duration from_timespec(const timespec & ts) {
 
 vespalib::string to_string(system_time time);
 
+steady_time saturated_add(steady_time time, duration diff);
+
 /**
  * Simple utility class used to measure how much time has elapsed
  * since it was constructed.