fix(worldclock): fix Etc/GMT names at the fallback, not per caller

The previous rule named every non-place id by its current offset, which
renamed legacy ids such as Japan, called Etc/UTC "GMT" and made a CET
row's name change at DST. Only the Etc/GMT+-n ids have a misleading
last segment, so ZoneCatalog.cityFallbackFor now flips just those to
their real offset. That also reaches the zone picker's device row,
which takes its name from the same fallback.

app/src/main/java/de/jeanlucmakiola/clockula/domain/worldclock/ZoneCatalog.kt

@@ -60,24 +60,31 @@ object ZoneCatalog {
      * input trimmed when that is blank. Never throws.
      */
     fun cityFallbackFor(zoneId: String): String {
+        etcOffsetName(zoneId)?.let { return it }
         val segment = zoneId.substringAfterLast('/').replace('_', ' ').trim()
         return segment.ifBlank { zoneId.trim() }
     }
 
+    /**
+     * `Etc/GMT-9` is UTC**+**9: POSIX signs these ids the other way round, so
+     * their last segment would name the zone with the wrong sign. A device set
+     * by UTC offset rather than by city (Android 13+ offers it) reports one, so
+     * the face and the local row name it by its real offset ("GMT+9").
+     */
+    private fun etcOffsetName(zoneId: String): String? {
+        val match = ETC_OFFSET.matchEntire(zoneId.trim()) ?: return null
+        val hours = match.groupValues[2].toInt()
+        if (hours == 0) return "GMT"
+        val sign = if (match.groupValues[1] == "-") "+" else "-"
+        return "GMT$sign$hours"
+    }
+
+    private val ETC_OFFSET = Regex("""Etc/GMT([+-])(\d{1,2})""")
+
     /** By `ZoneSearch.fold(city)` ascending, then [ZoneEntry.zoneId] ascending. */
     fun sorted(entries: List<ZoneEntry>): List<ZoneEntry> =
         entries.sortedWith(compareBy({ ZoneSearch.fold(it.city) }, { it.zoneId }))
 
-    /**
-     * True when [cityFallbackFor] gives a fair name for [zoneId]: a place id,
-     * current (`Asia/Tokyo`) or legacy (`US/Pacific`), or [UTC]. False for the
-     * `Etc/` offsets and the bare abbreviations tzdata also lists (`Etc/GMT-9`,
-     * `CET`), whose last segment is no place and, for `Etc/GMT±n`, has the sign
-     * reversed.
-     */
-    fun hasPlaceName(zoneId: String): Boolean =
-        zoneId == UTC || ('/' in zoneId && !zoneId.startsWith("Etc/"))
-
     private fun isRegionId(zoneId: String): Boolean =
         zoneId.contains('/') && zoneId.substringBefore('/') in REGIONS
 }

app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRowState.kt

@@ -1,7 +1,6 @@
 package de.jeanlucmakiola.clockula.ui.worldclock
 
 import de.jeanlucmakiola.clockula.domain.WorldClock
-import de.jeanlucmakiola.clockula.domain.Zones
 import de.jeanlucmakiola.clockula.domain.isKnownZone
 import de.jeanlucmakiola.clockula.domain.worldclock.DayLabel
 import de.jeanlucmakiola.clockula.domain.worldclock.DayNight
@@ -89,7 +88,7 @@ internal object WorldClockRows {
         WorldClockRowState(
             id = clock.id,
             zoneId = clock.zoneId,
-            name = clock.label ?: cityOf(clock.zoneId, entries, now),
+            name = clock.label ?: cityOf(clock.zoneId, entries),
             time = comparison?.localDateTime?.toLocalTime()?.truncatedTo(ChronoUnit.MINUTES),
             offset = comparison?.let { ZoneOffsetFormat.labelFor(it.offsetMinutes) },
             day = comparison?.let { ZoneOffsetFormat.dayLabelFor(it.dayDifference) },
@@ -133,22 +132,13 @@ internal object WorldClockRows {
         val local = now.toJavaInstant().atZone(homeZone).toLocalTime().truncatedTo(ChronoUnit.SECONDS)
         return HomeFaceState(
             zoneId = homeZone.id,
-            city = cityOf(homeZone.id, entries, now),
+            city = cityOf(homeZone.id, entries),
             zoneName = zoneNames[homeZone.id],
             time = local,
             dayFraction = DayNight.fractionAt(local),
         )
     }
 
-    /**
-     * A device set by UTC offset rather than by city (Android 13+ offers it)
-     * reports an id like `Etc/GMT-9`, whose last segment is no place and has
-     * its sign reversed, so such a zone is named by its real offset ("GMT+9").
-     */
-    private fun cityOf(zoneId: String, entries: Map<String, ZoneEntry>, now: Instant): String =
-        if (!ZoneCatalog.hasPlaceName(zoneId) && Zones.isValid(zoneId)) {
-            ZoneOffsetFormat.gmtLabel(ZoneComparisons.utcOffsetMinutes(ZoneId.of(zoneId), now))
-        } else {
-            entries[zoneId]?.city ?: ZoneCatalog.cityFallbackFor(zoneId)
-        }
+    private fun cityOf(zoneId: String, entries: Map<String, ZoneEntry>): String =
+        entries[zoneId]?.city ?: ZoneCatalog.cityFallbackFor(zoneId)
 }

app/src/test/java/de/jeanlucmakiola/clockula/domain/worldclock/ZoneCatalogTest.kt

@@ -226,11 +226,11 @@ class ZoneCatalogTest {
     }
 
     @Test
-    fun `a place id or UTC has a place name, an offset or abbreviation does not`() {
-        assertThat(ZoneCatalog.hasPlaceName("Asia/Tokyo")).isTrue()
-        assertThat(ZoneCatalog.hasPlaceName("US/Pacific")).isTrue()
-        assertThat(ZoneCatalog.hasPlaceName("UTC")).isTrue()
-        assertThat(ZoneCatalog.hasPlaceName("Etc/GMT-9")).isFalse()
-        assertThat(ZoneCatalog.hasPlaceName("CET")).isFalse()
+    fun `an Etc offset id falls back to its real offset, not its reversed sign`() {
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT-9")).isEqualTo("GMT+9")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT+5")).isEqualTo("GMT-5")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT+0")).isEqualTo("GMT")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/UTC")).isEqualTo("UTC")
+        assertThat(ZoneCatalog.cityFallbackFor("CET")).isEqualTo("CET")
     }
 }

docs/ARCHITECTURE.md

@@ -2407,7 +2407,7 @@ differ from it, so with no stored home the rule is always null, and Settings ›
 Clock says so under the switch rather than hiding it. A device set by UTC
 offset rather than by city reports an id like `Etc/GMT-9`, whose last segment
 has its sign reversed, so that zone is named by its real offset ("GMT+9")
-instead, on the local row and on the face alike (`ZoneCatalog.hasPlaceName`).
+instead, wherever a zone falls back to its id for a name (`ZoneCatalog.cityFallbackFor`).
 
 The row is not stored. It is `WorldClockUiState.Loaded.local`, built per tick
 by the source from the same `zones.current()` read the face uses, so a