More diagnostic info

Author: dkhalanskyjb

core/windows/src/internal/TzdbInRegistry.kt

@@ -60,19 +60,16 @@ internal class TzdbInRegistry: TimeZoneDatabase {
                         }
                     }
                 }
-                val lastOffset = offsets.lastOrNull()
-                if (lastOffset == null) {
-                    offsets.add(recurring.offsetAtYearStart())
-                } else {
+                offsets.lastOrNull()?.let { lastOffset ->
                     /* If there are already some offsets, we can not add a new offset without defining a transition to
                     it. The moment when we start using the recurring rules is the first year that does not have any
                     historic data provided. */
                     val firstYearWithRecurringRules = historic.last().first + 1
                     val newYearInLastOffset = LocalDate(firstYearWithRecurringRules, Month.JANUARY, 1).atTime(0, 0)
                         .toInstant(lastOffset)
                     transitionEpochSeconds.add(newYearInLastOffset.epochSeconds)
-                    offsets.add(offsets.last())
                 }
+                offsets.add(recurring.offsetAtYearStart())
                 TimeZoneRules(transitionEpochSeconds, offsets, recurringRules)
             }
             put(name, rules)
@@ -335,7 +332,9 @@ private class PerYearZoneRulesDataWithTransitions(
     val standardTransition get() =
         RecurringZoneRules.Rule(standardTransitionTime, offsetBefore = daylightOffset, offsetAfter = standardOffset)
 
-    override fun offsetAtYearStart(): UtcOffset = standardOffset // TODO: not true in all years + all zones
+    override fun offsetAtYearStart(): UtcOffset =
+        if (daylightTransitionTime.toLocalDateTime(2030) < standardTransitionTime.toLocalDateTime(2030))
+            standardOffset else daylightOffset
 
     override fun toString(): String = "standard offset is $standardOffset" +
             ", daylight offset is $daylightOffset" +

core/windows/test/TimeZoneRulesCompleteTest.kt

@@ -42,12 +42,24 @@ class TimeZoneRulesCompleteTest {
                             val ldt = instant.toLocalDateTime(dtzi, inputSystemtime.ptr, outputSystemtime.ptr)
                             val offset = rules.infoAtInstant(instant)
                             val ourLdt = instant.toLocalDateTime(offset)
-                            assertEquals(
-                                ldt,
-                                ourLdt,
-                                "in zone $windowsName at $instant (our guess at the offset is $offset). " +
-                                        "The rules are $rules"
-                            )
+                            if (ldt != ourLdt) {
+                                val offsetsAccordingToWindows = buildList {
+                                    var date = LocalDate(ldt.year, Month.JANUARY, 1)
+                                    while (date.year == ldt.year) {
+                                        val instant = date.atTime(0, 0).toInstant(UtcOffset.ZERO)
+                                        val ldtAccordingToWindows =
+                                            instant.toLocalDateTime(dtzi, inputSystemtime.ptr, outputSystemtime.ptr)
+                                        val offsetAccordingToWindows = UtcOffset(seconds =
+                                            (ldtAccordingToWindows.toInstant(UtcOffset.ZERO) - instant).inWholeSeconds
+                                        )
+                                        add(instant to offsetAccordingToWindows)
+                                    }
+                                }
+                                throw AssertionError(
+                                    "Expected $ldt, got $ourLdt in zone $windowsName at $instant (our guess at the offset is $offset)." +
+                                    "The rules are $rules, and the offsets throughout the year according to Windows are: "
+                                )
+                            }
                         }
                         fun checkTransition(instant: Instant) {
                             checkAtInstant(instant - 2.milliseconds)