random: Add FastRandomContext::rand_uniform_delay

Author: MarcoFalke

src/random.h

@@ -223,6 +223,17 @@ class FastRandomContext
     /** Generate a random boolean. */
     bool randbool() noexcept { return randbits(1); }
 
+    /** Return the time point advanced by a uniform random duration. */
+    template <typename Tp>
+    Tp rand_uniform_delay(const Tp& time, typename Tp::duration range)
+    {
+        using Dur = typename Tp::duration;
+        Dur dur{range.count() > 0 ? /* interval [0..range) */ Dur{randrange(range.count())} :
+                range.count() < 0 ? /* interval (range..0] */ -Dur{randrange(-range.count())} :
+                                    /* interval [0..0] */ Dur{0}};
+        return time + dur;
+    }
+
     // Compatibility with the C++11 UniformRandomBitGenerator concept
     typedef uint64_t result_type;
     static constexpr uint64_t min() { return 0; }

src/test/random_tests.cpp

@@ -31,6 +31,16 @@ BOOST_AUTO_TEST_CASE(fastrandom_tests)
         BOOST_CHECK_EQUAL(GetRandMicros(std::chrono::hours{1}).count(), 2917185654);
         BOOST_CHECK_EQUAL(GetRandMillis(std::chrono::hours{1}).count(), 2144374);
     }
+    {
+        constexpr SteadySeconds time_point{1s};
+        FastRandomContext ctx{true};
+        BOOST_CHECK_EQUAL(7, ctx.rand_uniform_delay(time_point, 9s).time_since_epoch().count());
+        BOOST_CHECK_EQUAL(-6, ctx.rand_uniform_delay(time_point, -9s).time_since_epoch().count());
+        BOOST_CHECK_EQUAL(1, ctx.rand_uniform_delay(time_point, 0s).time_since_epoch().count());
+        BOOST_CHECK_EQUAL(1467825113502396065, ctx.rand_uniform_delay(time_point, 9223372036854775807s).time_since_epoch().count());
+        BOOST_CHECK_EQUAL(-970181367944767837, ctx.rand_uniform_delay(time_point, -9223372036854775807s).time_since_epoch().count());
+        BOOST_CHECK_EQUAL(24761, ctx.rand_uniform_delay(time_point, 9h).time_since_epoch().count());
+    }
     BOOST_CHECK_EQUAL(ctx1.rand32(), ctx2.rand32());
     BOOST_CHECK_EQUAL(ctx1.rand32(), ctx2.rand32());
     BOOST_CHECK_EQUAL(ctx1.rand64(), ctx2.rand64());