fix: drop occurrences via EXDATE on calendars with no _sync_id (#47)

The cancelled-exception fix works on synced calendars but not local ones. A
cancelled exception only attaches to its parent through ORIGINAL_SYNC_ID; a
local event has no _sync_id, so the link never forms and the provider's
expansion of the *parent* collapses — every other occurrence disappears, which
is the original #47 corruption, just on a different calendar type. Verified
on-device both ways: a DAVx5 series survives a single-occurrence delete, the
same series on a LOCAL calendar vanishes entirely.

deleteOccurrence now branches on _sync_id. Synced events keep the (verified)
exception path. Events without one — local calendars, and synced events not yet
pushed — add the occurrence to the master's EXDATE, which needs no parent link
and is the canonical iCalendar way to drop one; a sync adapter carries it
upstream unchanged if the calendar later syncs.

Two provider quirks shape the write (both observed on a Pixel):
- An EXDATE-only update is not treated as a recurrence change: the expanded
  Instances rows are left alone, so the occurrence stays visible. The
  time/recurrence set has to ride along to force re-expansion.
- DTSTART alone is worse — the provider then recomputes lastDate as if the event
  were a single instance and collapses the series to its first occurrence.
  DTSTART + DURATION + RRULE + zone together re-expand it correctly.

This path is reached in normal use: Calendula's own contact special-date
calendars are local and hold all-day yearly series, so deleting one birthday
occurrence went through it. All-day series take the VALUE=DATE EXDATE form.

Adds pure buildOccurrenceExdateValues + JVM tests (timed, append, duplicate
fold, all-day).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarDataSource.kt

@@ -979,6 +979,8 @@ class AndroidCalendarDataSource @Inject constructor(
             CalendarContract.Events.EVENT_TIMEZONE,
             CalendarContract.Events.DURATION,
             CalendarContract.Events.ALL_DAY,
+            CalendarContract.Events._SYNC_ID,
+            CalendarContract.Events.EXDATE,
         ),
         null, null, null,
     )?.use { c ->
@@ -989,6 +991,8 @@ class AndroidCalendarDataSource @Inject constructor(
                 timezone = c.getString(2),
                 duration = c.getString(3),
                 allDay = c.getInt(4),
+                syncId = c.getString(5),
+                exdate = c.getString(6),
             )
         } else {
             null
@@ -1001,6 +1005,9 @@ class AndroidCalendarDataSource @Inject constructor(
         val timezone: String?,
         val duration: String?,
         val allDay: Int,
+        /** Null on a local calendar (and before a synced event's first push). */
+        val syncId: String? = null,
+        val exdate: String? = null,
     ) {
         /** UNTIL cutoff for ending the series before the occurrence at [beginMillis]. */
         fun truncationCutoff(beginMillis: Long): Long = previousLocalDayEndUtcMillis(
@@ -1191,15 +1198,38 @@ class AndroidCalendarDataSource @Inject constructor(
     }
 
     override fun deleteOccurrence(eventId: Long, beginMillis: Long) {
-        // A cancelled exception row hides exactly this occurrence; the sync
-        // adapter turns it into an EXDATE/cancelled VEVENT upstream. It has to
-        // carry the full time set (DTSTART + DURATION + zone), not just STATUS:
-        // the provider only demotes the cloned exception to a single instance —
-        // clearing the inherited RRULE — when it can derive that instance from
-        // those columns. A STATUS-only cancel left the RRULE standing and
-        // cancelled the whole series, wiping every other occurrence (#47), the
-        // same trap the edit path documents (Codeberg #16).
         val row = querySeriesRow(eventId)
+        if (row.syncId == null) {
+            // No _sync_id — a local calendar, or a synced event not pushed yet.
+            // A cancelled exception can only attach to its parent through
+            // ORIGINAL_SYNC_ID, so with none the link never forms and the
+            // provider's expansion of the *parent* collapses, taking every other
+            // occurrence with it (#47 on a local calendar). EXDATE needs no link.
+            // Calendula's own contact special-date calendars are local and hold
+            // yearly series, so this path is reached in normal use.
+            val values = buildOccurrenceExdateValues(
+                existingExdate = row.exdate,
+                occurrenceMillis = beginMillis,
+                dtStartMillis = row.dtStartMillis,
+                rrule = row.rrule,
+                duration = row.duration,
+                timezone = row.timezone,
+                allDay = row.allDay,
+            )
+            val updated = resolver.update(
+                ContentUris.withAppendedId(CalendarContract.Events.CONTENT_URI, eventId),
+                values.toContentValues(), null, null,
+            )
+            if (updated == 0) {
+                throw WriteFailedException("exdate occurrence event id=$eventId begin=$beginMillis")
+            }
+            return
+        }
+        // A cancelled exception row hides exactly this occurrence; the sync
+        // adapter turns it into an EXDATE/cancelled VEVENT upstream. It carries
+        // the full time set (DTSTART + DURATION + zone) so the provider derives a
+        // single instance rather than cloning the master's RRULE — the same trap
+        // the edit path documents (Codeberg #16).
         val values = buildOccurrenceCancelValues(
             originalInstanceMillis = beginMillis,
             dtStartMillis = beginMillis,

app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapper.kt

@@ -209,6 +209,73 @@ internal fun buildOccurrenceCancelValues(
     put(CalendarContract.Events.STATUS, CalendarContract.Events.STATUS_CANCELED)
 }
 
+/**
+ * The master-row columns that drop the occurrence at [occurrenceMillis] from a
+ * series by adding it to `EXDATE` — the path for events that have **no
+ * `_sync_id`** (a local calendar, or a synced event not yet pushed).
+ *
+ * A cancelled exception row (see [buildOccurrenceCancelValues]) only attaches to
+ * its parent through `ORIGINAL_SYNC_ID`. Without a `_sync_id` the link never
+ * forms, and the provider's expansion of the *parent* collapses — every other
+ * occurrence disappears (Codeberg #47, reproduced on a local calendar). EXDATE
+ * needs no link, and is the canonical iCalendar way to drop an occurrence, so a
+ * sync adapter carries it upstream unchanged if the calendar later syncs.
+ *
+ * The whole time/recurrence set is rewritten alongside it on purpose. The
+ * provider does **not** treat an EXDATE-only update as a recurrence change: it
+ * leaves the expanded `Instances` rows untouched, so the occurrence stays visible
+ * (and, symmetrically, un-excluding one leaves it hidden). Writing DTSTART with
+ * it forces the re-expansion — but DTSTART *alone* makes the provider recompute
+ * `lastDate` as if the event were a single instance, collapsing the series to its
+ * first occurrence. Passing DTSTART + DURATION + RRULE + zone together is what
+ * re-expands it correctly. All observed on a Pixel; see the #47 notes.
+ *
+ * EXDATE is a comma-separated list, so an existing one is appended to (a repeat
+ * of the same occurrence is folded away). All-day series take the `VALUE=DATE`
+ * form (`yyyyMMdd`), timed ones the UTC date-time form (`yyyyMMddTHHmmssZ`).
+ */
+internal fun buildOccurrenceExdateValues(
+    existingExdate: String?,
+    occurrenceMillis: Long,
+    dtStartMillis: Long,
+    rrule: String?,
+    duration: String?,
+    timezone: String?,
+    allDay: Int,
+): Map<String, Any?> {
+    val stamp = formatExdateStamp(occurrenceMillis, isAllDay = allDay != 0)
+    val existing = existingExdate?.split(',')
+        ?.map { it.trim() }
+        ?.filter { it.isNotEmpty() }
+        .orEmpty()
+    val merged = (existing + stamp).distinct().joinToString(",")
+    return mapOf(
+        CalendarContract.Events.EXDATE to merged,
+        CalendarContract.Events.DTSTART to dtStartMillis,
+        CalendarContract.Events.RRULE to rrule,
+        CalendarContract.Events.DURATION to duration,
+        CalendarContract.Events.EVENT_TIMEZONE to timezone,
+        CalendarContract.Events.ALL_DAY to allDay,
+    )
+}
+
+/**
+ * One EXDATE entry for the occurrence starting at [occurrenceMillis]. Both forms
+ * are UTC: the provider stores an all-day DTSTART at UTC midnight, so its date
+ * reads off the UTC calendar day.
+ */
+private fun formatExdateStamp(occurrenceMillis: Long, isAllDay: Boolean): String {
+    val utc = Instant.ofEpochMilli(occurrenceMillis).atZone(ZoneOffset.UTC)
+    return if (isAllDay) {
+        "%04d%02d%02d".format(utc.year, utc.monthValue, utc.dayOfMonth)
+    } else {
+        "%04d%02d%02dT%02d%02d%02dZ".format(
+            utc.year, utc.monthValue, utc.dayOfMonth,
+            utc.hour, utc.minute, utc.second,
+        )
+    }
+}
+
 /**
  * The `EVENT_COLOR` / `EVENT_COLOR_KEY` columns for a colour selection. A
  * [colorKey] writes the key alone (the provider derives `EVENT_COLOR` from the

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt

@@ -260,6 +260,77 @@ class EventWriteMapperTest {
             .isEqualTo(CalendarContract.Events.STATUS_CANCELED)
     }
 
+    // --- buildOccurrenceExdateValues ("delete only this event", no _sync_id) ---
+
+    @Test
+    fun `exdate drop excludes the occurrence and rewrites the recurrence set`() {
+        // 2026-07-15T08:00:00Z.
+        val values = buildOccurrenceExdateValues(
+            existingExdate = null,
+            occurrenceMillis = 1_784_102_400_000L,
+            dtStartMillis = 1_783_929_600_000L,
+            rrule = "FREQ=DAILY;COUNT=5",
+            duration = "PT1H",
+            timezone = "Europe/Berlin",
+            allDay = 0,
+        )
+        assertThat(values[CalendarContract.Events.EXDATE]).isEqualTo("20260715T080000Z")
+        // The whole time/recurrence set rides along: an EXDATE-only update is not
+        // treated as a recurrence change, so the provider would leave the expanded
+        // instances (and the occurrence) in place. DTSTART alone is worse — it
+        // makes the provider recompute lastDate as a single instance and collapse
+        // the series to its first occurrence.
+        assertThat(values[CalendarContract.Events.DTSTART]).isEqualTo(1_783_929_600_000L)
+        assertThat(values[CalendarContract.Events.RRULE]).isEqualTo("FREQ=DAILY;COUNT=5")
+        assertThat(values[CalendarContract.Events.DURATION]).isEqualTo("PT1H")
+        assertThat(values[CalendarContract.Events.EVENT_TIMEZONE]).isEqualTo("Europe/Berlin")
+    }
+
+    @Test
+    fun `exdate drop appends to an existing exdate list`() {
+        val values = buildOccurrenceExdateValues(
+            existingExdate = "20260714T080000Z",
+            occurrenceMillis = 1_784_102_400_000L,
+            dtStartMillis = 1_783_929_600_000L,
+            rrule = "FREQ=DAILY;COUNT=5",
+            duration = "PT1H",
+            timezone = "Europe/Berlin",
+            allDay = 0,
+        )
+        assertThat(values[CalendarContract.Events.EXDATE])
+            .isEqualTo("20260714T080000Z,20260715T080000Z")
+    }
+
+    @Test
+    fun `exdate drop folds away a repeated occurrence`() {
+        val values = buildOccurrenceExdateValues(
+            existingExdate = "20260715T080000Z",
+            occurrenceMillis = 1_784_102_400_000L,
+            dtStartMillis = 1_783_929_600_000L,
+            rrule = "FREQ=DAILY;COUNT=5",
+            duration = "PT1H",
+            timezone = "Europe/Berlin",
+            allDay = 0,
+        )
+        assertThat(values[CalendarContract.Events.EXDATE]).isEqualTo("20260715T080000Z")
+    }
+
+    @Test
+    fun `all-day exdate drop uses the date-only form`() {
+        // An all-day DTSTART sits at UTC midnight, so the date reads off UTC.
+        val values = buildOccurrenceExdateValues(
+            existingExdate = null,
+            occurrenceMillis = 1_784_073_600_000L, // 2026-07-15T00:00:00Z
+            dtStartMillis = 1_783_900_800_000L,
+            rrule = "FREQ=YEARLY",
+            duration = "P1D",
+            timezone = "UTC",
+            allDay = 1,
+        )
+        assertThat(values[CalendarContract.Events.EXDATE]).isEqualTo("20260715")
+        assertThat(values[CalendarContract.Events.ALL_DAY]).isEqualTo(1)
+    }
+
     // --- per-event colour ---
 
     @Test