Change Duration.toString(unit, decimals) formatting

- switch to scientific notation when value is too big (greater than 1e14)
- allow at most 12 decimals, larger values are coerced to 12
- use scientific format with exactly two decimals in mantissa in JVM

libraries/stdlib/jvm/src/kotlin/time/formatToDecimals.kt

@@ -34,4 +34,4 @@ internal actual fun formatUpToDecimals(value: Double, decimals: Int): String =
 
 private val scientificFormat = ThreadLocal<DecimalFormat>()
 internal actual fun formatScientific(value: Double): String =
-    scientificFormat.getOrSet { DecimalFormat("0E0", rootFormatSymbols).apply { maximumFractionDigits = 2 } }.format(value)
+    scientificFormat.getOrSet { DecimalFormat("0E0", rootFormatSymbols).apply { minimumFractionDigits = 2 } }.format(value)

libraries/stdlib/src/kotlin/time/Duration.kt

@@ -279,7 +279,11 @@ public inline class Duration internal constructor(internal val value: Double) :
     public fun toString(unit: DurationUnit, decimals: Int = 0): String {
         require(decimals >= 0) { "decimals must be not negative, but was $decimals" }
         if (isInfinite()) return value.toString()
-        return formatToExactDecimals(toDouble(unit), decimals) + unit.shortName()
+        val number = toDouble(unit)
+        return when {
+            abs(number) < 1e14 -> formatToExactDecimals(number, decimals.coerceAtMost(12))
+            else -> formatScientific(number)
+        } + unit.shortName()
     }
 
 

libraries/stdlib/test/time/DurationTest.kt

@@ -308,12 +308,13 @@ class DurationTest {
         d = (d - 516.nanoseconds) / 17
         test(DurationUnit.NANOSECONDS, "0ns", "0.0ns", "0.02ns", "0.020ns", "0.0202ns")
 
-        testOnJvm {
-            d = Double.MAX_VALUE.nanoseconds
-            // are such long representations ok?
-            test(DurationUnit.DAYS, "20806633505350875${"0".repeat(278)}d")
-            test(DurationUnit.NANOSECONDS, "17976931348623157${"0".repeat(292)}ns")
-        }
+        d = Double.MAX_VALUE.nanoseconds
+        test(DurationUnit.DAYS, "2.08e294d")
+        test(DurationUnit.NANOSECONDS, "1.80e308ns")
+
+        assertEquals("0.500000000000s", 0.5.seconds.toString(DurationUnit.SECONDS, 100))
+        assertEquals("99999000000000.000000000000ns", 99_999.seconds.toString(DurationUnit.NANOSECONDS, 15))
+        assertEquals("1.00e14ns", 100_000.seconds.toString(DurationUnit.NANOSECONDS, 9))
 
         d = Duration.INFINITE
         test(DurationUnit.DAYS, "Infinity", "Infinity")