Introduce TestClock.plusAssign(Duration) and hide implementation details

TestClock.plusAssign(Duration) is for advancing TestClock time.
Hide reading value, do not allow to provide initial reading,
fix clock unit to nanoseconds at construction time.

libraries/stdlib/test/time/ClockMarkTest.kt

@@ -12,7 +12,7 @@ class ClockMarkTest {
 
     @Test
     fun adjustment() {
-        val clock = TestClock(unit = DurationUnit.NANOSECONDS)
+        val clock = TestClock()
 
         val mark = clock.mark()
         val markFuture1 = mark + 1.milliseconds
@@ -21,7 +21,7 @@ class ClockMarkTest {
         val markPast1 = mark - 1.milliseconds
         val markPast2 = markFuture1 + (-2).milliseconds
 
-        clock.reading = 500_000L
+        clock += 500_000.nanoseconds
 
         val elapsed = mark.elapsed()
         val elapsedFromFuture = elapsed - 1.milliseconds

libraries/stdlib/test/time/MeasureTimeTest.kt

@@ -47,11 +47,12 @@ class MeasureTimeTest {
     }
 
 
-    @Test fun measureTimeTestClock() {
-        val clock = TestClock(unit = DurationUnit.NANOSECONDS)
+    @Test
+    fun measureTimeTestClock() {
+        val clock = TestClock()
         val expectedNs = Random.nextLong(1_000_000_000L)
         val elapsed = clock.measureTime {
-            clock.reading += expectedNs
+            clock += expectedNs.nanoseconds
         }
 
         assertEquals(expectedNs.nanoseconds, elapsed)
@@ -59,7 +60,7 @@ class MeasureTimeTest {
         val expectedResult: Long
 
         val (result, elapsed2) = clock.measureTimedValue {
-            clock.reading += expectedNs
+            clock += expectedNs.nanoseconds
             expectedResult = expectedNs
             expectedNs
         }

libraries/stdlib/test/time/TestClockTest.kt

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+@file:UseExperimental(ExperimentalTime::class)
+package test.time
+
+import kotlin.test.*
+import kotlin.time.*
+
+class TestClockTest {
+
+    @Test
+    fun overflows() {
+        for (enormousDuration in listOf(Duration.INFINITE, Double.MAX_VALUE.nanoseconds, Long.MAX_VALUE.nanoseconds * 2)) {
+            assertFailsWith<IllegalStateException>(enormousDuration.toString()) { TestClock() += enormousDuration }
+            assertFailsWith<IllegalStateException>((-enormousDuration).toString()) { TestClock() += -enormousDuration }
+        }
+
+        val moderatePositiveDuration = Long.MAX_VALUE.takeHighestOneBit().nanoseconds
+        val borderlinePositiveDuration = Long.MAX_VALUE.nanoseconds // rounded to 2.0^63, which is slightly more than Long.MAX_VALUE
+        val borderlineNegativeDuration = Long.MIN_VALUE.nanoseconds
+        run {
+            val clock = TestClock()
+            clock += moderatePositiveDuration
+            assertFailsWith<IllegalStateException>("Should overflow positive") { clock += moderatePositiveDuration }
+        }
+        run {
+            val clock = TestClock()
+            clock += borderlinePositiveDuration
+            assertFailsWith<IllegalStateException>("Should overflow positive") { clock += 1.nanoseconds }
+        }
+        run {
+            val clock = TestClock()
+            clock += borderlineNegativeDuration
+            assertFailsWith<IllegalStateException>("Should overflow negative") { clock += -1.nanoseconds }
+        }
+
+        run {
+            val clock = TestClock()
+            clock += moderatePositiveDuration
+            // does not overflow event if duration doesn't fit in long
+            clock += -moderatePositiveDuration + borderlineNegativeDuration
+        }
+    }
+
+    @Test
+    fun nanosecondRounding() {
+        val clock = TestClock()
+        val mark = clock.mark()
+
+        repeat(10_000) {
+            clock += 0.9.nanoseconds
+
+            assertEquals(Duration.ZERO, mark.elapsed())
+        }
+
+        clock += 1.9.nanoseconds
+        assertEquals(1.nanoseconds, mark.elapsed())
+    }
+}