LibJS+LibUnicode: Support ambiguous time zone transitions

For example, time zone transitions can result in repeated or skipped
wall times. Temporal wants us to handle these transitions.

Libraries/LibJS/Runtime/Date.cpp

@@ -403,13 +403,15 @@ Vector<Crypto::SignedBigInteger> get_named_time_zone_epoch_nanoseconds(StringVie
     auto local_nanoseconds = get_utc_epoch_nanoseconds(iso_date_time);
     auto local_time = UnixDateTime::from_nanoseconds_since_epoch(clip_bigint_to_sane_time(local_nanoseconds));
 
-    // FIXME: LibUnicode does not behave exactly as the spec expects. It does not consider repeated or skipped time points.
+    auto offsets = Unicode::disambiguated_time_zone_offsets(time_zone_identifier, local_time);
-    auto offset = Unicode::time_zone_offset(time_zone_identifier, local_time);
 
-    // Can only fail if the time zone identifier is invalid, which cannot be the case here.
+    Vector<Crypto::SignedBigInteger> result;
-    VERIFY(offset.has_value());
+    result.ensure_capacity(offsets.size());
 
-    return { local_nanoseconds.minus(Crypto::SignedBigInteger { offset->offset.to_nanoseconds() }) };
+    for (auto const& offset : offsets)
+        result.unchecked_append(local_nanoseconds.minus(Crypto::SignedBigInteger { offset.offset.to_nanoseconds() }));
+
+    return result;
 }
 
 // 21.4.1.21 GetNamedTimeZoneOffsetNanoseconds ( timeZoneIdentifier, epochNanoseconds ), https://tc39.es/ecma262/#sec-getnamedtimezoneoffsetnanoseconds

Libraries/LibJS/Tests/builtins/Temporal/Duration/Duration.compare.js

@@ -68,7 +68,7 @@ describe("correct behavior", () => {
         expect(result).toBe(-1);
     });
 
-    test("sub-minute time zone offset", () => {
+    test("sub-minute time zone offset (unambiguous time zone transition)", () => {
         const duration1 = new Temporal.Duration(0, 0, 0, 31);
         const duration2 = new Temporal.Duration(0, 1);
 
@@ -82,6 +82,26 @@ describe("correct behavior", () => {
         });
         expect(result).toBe(0);
     });
+
+    test("sub-minute time zone offset (ambiguous time zone transition)", () => {
+        const duration1 = new Temporal.Duration(0, 0, 0, 0, 24);
+        const duration2 = new Temporal.Duration(0, 0, 0, 1);
+
+        let result = Temporal.Duration.compare(duration1, duration2, {
+            relativeTo: "1952-10-15T23:59:59-11:19:40[Pacific/Niue]",
+        });
+        expect(result).toBe(-1);
+
+        result = Temporal.Duration.compare(duration1, duration2, {
+            relativeTo: "1952-10-15T23:59:59-11:20[Pacific/Niue]",
+        });
+        expect(result).toBe(-1);
+
+        result = Temporal.Duration.compare(duration1, duration2, {
+            relativeTo: "1952-10-15T23:59:59-11:20:00[Pacific/Niue]",
+        });
+        expect(result).toBe(0);
+    });
 });
 
 describe("errors", () => {
@@ -148,7 +168,7 @@ describe("errors", () => {
         );
     });
 
-    test("sub-minute time zone offset mismatch", () => {
+    test("sub-minute time zone offset mismatch (unambiguous time zone transition)", () => {
         const duration1 = new Temporal.Duration(0, 0, 0, 31);
         const duration2 = new Temporal.Duration(0, 1);
 
@@ -170,4 +190,18 @@ describe("errors", () => {
             "Invalid offset for the provided date and time in the current time zone"
         );
     });
+
+    test("sub-minute time zone offset mismatch (ambiguous time zone transition)", () => {
+        const duration1 = new Temporal.Duration(0, 0, 0, 0, 24);
+        const duration2 = new Temporal.Duration(0, 0, 0, 1);
+
+        expect(() => {
+            Temporal.Duration.compare(duration1, duration2, {
+                relativeTo: "1952-10-15T23:59:59-11:19:50[Pacific/Niue]",
+            });
+        }).toThrowWithMessage(
+            RangeError,
+            "Invalid offset for the provided date and time in the current time zone"
+        );
+    });
 });

Libraries/LibUnicode/TimeZone.cpp

@@ -10,6 +10,7 @@
 #include <LibUnicode/ICU.h>
 #include <LibUnicode/TimeZone.h>
 
+#include <unicode/basictz.h>
 #include <unicode/timezone.h>
 #include <unicode/ucal.h>
 
@@ -139,6 +140,15 @@ Optional<String> resolve_primary_time_zone(StringView time_zone)
     return icu_string_to_string(iana_id);
 }
 
+static UDate to_icu_time(UnixDateTime time)
+{
+    // We must clamp the time we provide to ICU such that the result of converting milliseconds to days fits in an i32.
+    // Further, that conversion must still be valid after applying DST offsets to the time we provide.
+    static constexpr auto min_time = (static_cast<UDate>(AK::NumericLimits<i32>::min()) + U_MILLIS_PER_DAY) * U_MILLIS_PER_DAY;
+    static constexpr auto max_time = (static_cast<UDate>(AK::NumericLimits<i32>::max()) - U_MILLIS_PER_DAY) * U_MILLIS_PER_DAY;
+    return clamp(static_cast<UDate>(time.milliseconds_since_epoch()), min_time, max_time);
+}
+
 Optional<TimeZoneOffset> time_zone_offset(StringView time_zone, UnixDateTime time)
 {
     UErrorCode status = U_ZERO_ERROR;
@@ -150,11 +160,7 @@ Optional<TimeZoneOffset> time_zone_offset(StringView time_zone, UnixDateTime tim
     i32 raw_offset = 0;
     i32 dst_offset = 0;
 
-    // We must clamp the time we provide to ICU such that the result of converting milliseconds to days fits in an i32.
+    auto icu_time = to_icu_time(time);
-    // Further, that conversion must still be valid after applying DST offsets to the time we provide.
-    static constexpr auto min_time = (static_cast<UDate>(AK::NumericLimits<i32>::min()) + U_MILLIS_PER_DAY) * U_MILLIS_PER_DAY;
-    static constexpr auto max_time = (static_cast<UDate>(AK::NumericLimits<i32>::max()) - U_MILLIS_PER_DAY) * U_MILLIS_PER_DAY;
-    auto icu_time = clamp(static_cast<UDate>(time.milliseconds_since_epoch()), min_time, max_time);
 
     time_zone_data->time_zone().getOffset(icu_time, 0, raw_offset, dst_offset, status);
     if (icu_failure(status))
@@ -166,4 +172,41 @@ Optional<TimeZoneOffset> time_zone_offset(StringView time_zone, UnixDateTime tim
     };
 }
 
+Vector<TimeZoneOffset> disambiguated_time_zone_offsets(StringView time_zone, UnixDateTime time)
+{
+    UErrorCode status = U_ZERO_ERROR;
+
+    auto time_zone_data = TimeZoneData::for_time_zone(time_zone);
+    if (!time_zone_data.has_value())
+        return {};
+
+    auto& basic_time_zone = as<icu::BasicTimeZone>(time_zone_data->time_zone());
+    auto icu_time = to_icu_time(time);
+
+    auto get_offset = [&](auto disambiguation_option) -> Optional<TimeZoneOffset> {
+        i32 raw_offset = 0;
+        i32 dst_offset = 0;
+
+        basic_time_zone.getOffsetFromLocal(icu_time, disambiguation_option, disambiguation_option, raw_offset, dst_offset, status);
+        if (icu_failure(status))
+            return {};
+
+        return TimeZoneOffset {
+            .offset = AK::Duration::from_milliseconds(raw_offset + dst_offset),
+            .in_dst = dst_offset == 0 ? TimeZoneOffset::InDST::No : TimeZoneOffset::InDST::Yes,
+        };
+    };
+
+    auto former = get_offset(UCAL_TZ_LOCAL_FORMER);
+    auto latter = get_offset(UCAL_TZ_LOCAL_LATTER);
+
+    Vector<TimeZoneOffset> offsets;
+    if (former.has_value())
+        offsets.append(*former);
+    if (latter.has_value() && latter->offset != former->offset)
+        offsets.append(*latter);
+
+    return offsets;
+}
+
 }

Libraries/LibUnicode/TimeZone.h

@@ -31,5 +31,6 @@ Vector<String> const& available_time_zones();
 Vector<String> available_time_zones_in_region(StringView region);
 Optional<String> resolve_primary_time_zone(StringView time_zone);
 Optional<TimeZoneOffset> time_zone_offset(StringView time_zone, UnixDateTime time);
+Vector<TimeZoneOffset> disambiguated_time_zone_offsets(StringView time_zone, UnixDateTime time);
 
 }