folly::chrono::abs, backporting std::chrono::abs

Summary: [Folly] `folly::chrono::abs`, backporting `std::chrono::abs`.

Reviewed By: nbronson

Differential Revision: D13584287

fbshipit-source-id: 92adbfcdac34d89674e6494af4ff3329d2f6974a

folly/Chrono.h

@@ -36,6 +36,7 @@
 namespace folly {
 namespace chrono {
 
+/* using override */ using std::chrono::abs;
 /* using override */ using std::chrono::ceil;
 /* using override */ using std::chrono::floor;
 /* using override */ using std::chrono::round;
@@ -81,6 +82,18 @@ constexpr To round_impl(Duration const& d, To const& t0) {
 }
 } // namespace detail
 
+//  mimic: std::chrono::abs, C++17
+template <
+    typename Rep,
+    typename Period,
+    typename = typename std::enable_if<
+        std::chrono::duration<Rep, Period>::min() <
+        std::chrono::duration<Rep, Period>::zero()>::type>
+constexpr std::chrono::duration<Rep, Period> abs(
+    std::chrono::duration<Rep, Period> const& d) {
+  return d < std::chrono::duration<Rep, Period>::zero() ? -d : d;
+}
+
 //  mimic: std::chrono::ceil, C++17
 //  from: http://en.cppreference.com/w/cpp/chrono/duration/ceil, CC-BY-SA
 template <

folly/test/ChronoTest.cpp

@@ -25,6 +25,11 @@ namespace {
 class ChronoTest : public testing::Test {};
 } // namespace
 
+TEST_F(ChronoTest, abs_duration) {
+  EXPECT_EQ(seconds(7), abs(seconds(7)));
+  EXPECT_EQ(seconds(7), abs(seconds(-7)));
+}
+
 TEST_F(ChronoTest, ceil_duration) {
   EXPECT_EQ(seconds(7), ceil<seconds>(seconds(7)));
   EXPECT_EQ(seconds(7), ceil<seconds>(milliseconds(7000)));