Implement Comparable time marks in a time source returned by Clock.asTimeSource()

core/common/src/Clock.kt

@@ -40,17 +40,50 @@ public fun Clock.todayIn(timeZone: TimeZone): LocalDate =
  * Returns a [TimeSource] that uses this [Clock] to mark a time instant and to find the amount of time elapsed since that mark.
  */
 @ExperimentalTime
-public fun Clock.asTimeSource(): TimeSource = object : TimeSource {
-    override fun markNow(): TimeMark = InstantTimeMark(now(), this@asTimeSource)
+public fun Clock.asTimeSource(): TimeSource.WithComparableMarks = object : TimeSource.WithComparableMarks {
+    override fun markNow(): ComparableTimeMark = InstantTimeMark(now(), this@asTimeSource)
 }
 
 @ExperimentalTime
-private class InstantTimeMark(private val instant: Instant, private val clock: Clock) : TimeMark {
-    override fun elapsedNow(): Duration = clock.now() - instant
+private class InstantTimeMark(private val instant: Instant, private val clock: Clock) : ComparableTimeMark {
+    override fun elapsedNow(): Duration = saturatingDiff(clock.now(), instant)
 
-    override fun plus(duration: Duration): TimeMark = InstantTimeMark(instant + duration, clock)
+    override fun plus(duration: Duration): ComparableTimeMark = InstantTimeMark(instant.saturatingAdd(duration), clock)
+    override fun minus(duration: Duration): ComparableTimeMark = InstantTimeMark(instant.saturatingAdd(-duration), clock)
 
-    override fun minus(duration: Duration): TimeMark = InstantTimeMark(instant - duration, clock)
+    override fun minus(other: ComparableTimeMark): Duration {
+        if (other !is InstantTimeMark || other.clock != this.clock) {
+            throw IllegalArgumentException("Subtracting or comparing time marks from different time sources is not possible: $this and $other")
+        }
+        return saturatingDiff(this.instant, other.instant)
+    }
+
+    override fun equals(other: Any?): Boolean {
+        return other is InstantTimeMark && this.clock == other.clock && this.instant == other.instant
+    }
+
+    override fun hashCode(): Int = instant.hashCode()
+
+    override fun toString(): String = "InstantTimeMark($instant, $clock)"
+
+    private fun Instant.isSaturated() = this == Instant.MAX || this == Instant.MIN
+    private fun Instant.saturatingAdd(duration: Duration): Instant {
+        if (isSaturated()) {
+            if (duration.isInfinite() && duration.isPositive() != this.isDistantFuture) {
+                throw IllegalArgumentException("Summing infinities of different signs")
+            }
+            return this
+        }
+        return this + duration
+    }
+    private fun saturatingDiff(instant1: Instant, instant2: Instant): Duration = when {
+        instant1 == instant2 ->
+            Duration.ZERO
+        instant1.isSaturated() || instant2.isSaturated() ->
+            (instant1 - instant2) * Double.POSITIVE_INFINITY
+        else ->
+            instant1 - instant2
+    }
 }
 
 @Deprecated("Use Clock.todayIn instead", ReplaceWith("this.todayIn(timeZone)"), DeprecationLevel.WARNING)

core/common/test/ClockTimeSourceTest.kt

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2019-2023 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.test
+
+import kotlinx.datetime.*
+import kotlin.test.*
+import kotlin.time.*
+import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.nanoseconds
+
+@OptIn(ExperimentalTime::class)
+class ClockTimeSourceTest {
+    @Test
+    fun arithmetic() {
+        val timeSource = Clock.System.asTimeSource()
+        val mark0 = timeSource.markNow()
+
+        val markPast = mark0 - 1.days
+        val markFuture = mark0 + 1.days
+
+        assertTrue(markPast < mark0)
+        assertTrue(markFuture > mark0)
+        assertEquals(mark0, markPast + 1.days)
+        assertEquals(2.days, markFuture - markPast)
+    }
+
+    @Test
+    fun elapsed() {
+        val clock = object : Clock {
+            var instant = Clock.System.now()
+            override fun now(): Instant = instant
+        }
+        val timeSource = clock.asTimeSource()
+        val mark = timeSource.markNow()
+        assertEquals(Duration.ZERO, mark.elapsedNow())
+
+        clock.instant += 1.days
+        assertEquals(1.days, mark.elapsedNow())
+
+        clock.instant -= 2.days
+        assertEquals(-1.days, mark.elapsedNow())
+
+        clock.instant = Instant.MAX
+        assertEquals(Duration.INFINITE, mark.elapsedNow())
+    }
+
+    @Test
+    fun differentSources() {
+        val mark1 = Clock.System.asTimeSource().markNow()
+        val mark2 = object : Clock {
+            override fun now(): Instant = Instant.DISTANT_FUTURE
+        }.asTimeSource().markNow()
+        assertNotEquals(mark1, mark2)
+        assertFailsWith<IllegalArgumentException> { mark1 - mark2 }
+        assertFailsWith<IllegalArgumentException> { mark1 compareTo mark2 }
+    }
+
+    @Test
+    fun saturation() {
+        val mark0 = Clock.System.asTimeSource().markNow()
+
+        val markFuture = mark0 + Duration.INFINITE
+        val markPast = mark0 - Duration.INFINITE
+
+        for (delta in listOf(Duration.ZERO, 1.nanoseconds, 1.days)) {
+            assertEquals(markFuture, markFuture - delta)
+            assertEquals(markFuture, markFuture + delta)
+
+            assertEquals(markPast, markPast - delta)
+            assertEquals(markPast, markPast + delta)
+        }
+        val infinitePairs = listOf(markFuture to markPast, markFuture to mark0, mark0 to markPast)
+        for ((later, earlier) in infinitePairs) {
+            assertEquals(Duration.INFINITE, later - earlier)
+            assertEquals(-Duration.INFINITE, earlier - later)
+        }
+        assertEquals(Duration.ZERO, markFuture - markFuture)
+        assertEquals(Duration.ZERO, markPast - markPast)
+
+        assertFailsWith<IllegalArgumentException> { markFuture - Duration.INFINITE }
+        assertFailsWith<IllegalArgumentException> { markPast + Duration.INFINITE }
+    }
+}