Limit calendar import to 100 events

app/src/main/java/net/moasdawiki/app/CalendarSyncAdapter.java

@@ -48,8 +48,10 @@ import net.moasdawiki.util.PathUtils;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
+import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 import java.util.TimeZone;
 
@@ -71,8 +73,21 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
     private static final Uri EVENT_URI    = CalendarContract.Events.   CONTENT_URI;
     private static final Uri REMINDER_URI = CalendarContract.Reminders.CONTENT_URI;
 
+    /**
+     * Alert event 4h before it starts, i.e. at 8pm in the evening before the event day.
+     */
     private static final int REMINDER_BEFORE_EVENT_MINUTES = 4 * 60;
 
+    /**
+     * Import maximum 100 events. If there are more events found, take the 100 upcoming events
+     * sorted by date.
+     *
+     * Android has a global event limitation of 500 events. If more than 500 events are created,
+     * Android throws errors with message "Maximum limit of concurrent alarms 500 reached" and
+     * ignores some events.
+     */
+    private static final int MAX_EVENT_COUNT = 100;
+
     private final ContentResolver contentResolver;
 
     @SuppressWarnings("SameParameterValue")
@@ -84,13 +99,19 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
     @Override
     public void onPerformSync(Account account, Bundle extras, String authority, ContentProviderClient provider, SyncResult syncResult) {
         Log.d(TAG, "Begin of onPerformSync()");
-        List<TerminTransformer.Event> events = getWikiEvents();
+        // collect and filter events
+        List<TerminTransformer.Event> rawEvents = getWikiEvents();
+        fillEmptyDateFields(rawEvents);
+        List<TerminTransformer.Event> events = filterEvents(rawEvents);
+
+        // update Android calendar
         String calendarId = createCalendar();
         if (calendarId != null) {
             deleteAllEvents(calendarId);
             addEvents(calendarId, events);
         }
 
+        // inform user about completed update
         Handler handler = new Handler(Looper.getMainLooper());
         handler.post(() -> {
             Toast toast = Toast.makeText(getContext(), R.string.calendar_sync_finished, Toast.LENGTH_SHORT);
@@ -116,6 +137,65 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
         return events;
     }
 
+    /**
+     * Fills empty day, month, and year fields in all events.
+     */
+    private void fillEmptyDateFields(@NotNull List<TerminTransformer.Event> events) {
+        TimeZone utc = TimeZone.getTimeZone("UTC");
+        Calendar calendar = Calendar.getInstance(utc);
+        for (TerminTransformer.Event event : events) {
+            if (event.dateFields.day == null) {
+                event.dateFields.day = 1;
+            }
+            if (event.dateFields.month == null) {
+                event.dateFields.month = 1;
+            }
+            if (event.dateFields.year == null) {
+                event.dateFields.year = calendar.get(Calendar.YEAR);
+            }
+        }
+    }
+
+    /**
+     * Reduces the number of events if more than MAX_EVENT_COUNT, only keep events in the near future.
+     */
+    @NotNull
+    private List<TerminTransformer.Event> filterEvents(@NotNull List<TerminTransformer.Event> rawEvents) {
+        if (rawEvents.size() <= MAX_EVENT_COUNT) {
+            return rawEvents;
+        }
+
+        // sort by month and day;
+        // ignore year because events repeat every year
+        List<TerminTransformer.Event> sortedEvents = new ArrayList<>(rawEvents);
+        sortedEvents.sort(Comparator.comparingInt((TerminTransformer.Event event) -> event.dateFields.month).thenComparingInt(event -> event.dateFields.day));
+
+        // keep next MAX_EVENT_COUNT events from today onwards
+        List<TerminTransformer.Event> result = new ArrayList<>(MAX_EVENT_COUNT);
+        // find first event in future
+        TimeZone utc = TimeZone.getTimeZone("UTC");
+        Calendar calendar = Calendar.getInstance(utc);
+        int index = 0;
+        while (index < sortedEvents.size() - 1
+                && (sortedEvents.get(index).dateFields.month < calendar.get(Calendar.MONTH) + 1
+                    || sortedEvents.get(index).dateFields.month == calendar.get(Calendar.MONTH) + 1
+                        && sortedEvents.get(index).dateFields.day < calendar.get(Calendar.DAY_OF_MONTH))) {
+            index++;
+        }
+        // copy events until end of year
+        while (result.size() < MAX_EVENT_COUNT && index < sortedEvents.size() - 1) {
+            result.add(sortedEvents.get(index));
+            index++;
+        }
+        // copy events in new year
+        index = 0;
+        while (result.size() < MAX_EVENT_COUNT && index < sortedEvents.size() - 1) {
+            result.add(sortedEvents.get(index));
+            index++;
+        }
+        return result;
+    }
+
     @NotNull
     private Uri buildUri(@NotNull Uri uri) {
         return uri.buildUpon()
@@ -155,7 +235,6 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
         cv.put(CalendarContract.Calendars.NAME,                  CALENDAR_NAME);
         String displayName = getContext().getString(R.string.calendar_display_name);
         cv.put(CalendarContract.Calendars.CALENDAR_DISPLAY_NAME, displayName);
-        //cv.put(CalendarContract.Calendars.CALENDAR_COLOR,        0xEA8561);
         cv.put(CalendarContract.Calendars.CALENDAR_ACCESS_LEVEL, CalendarContract.Calendars.CAL_ACCESS_READ);
         cv.put(CalendarContract.Calendars.OWNER_ACCOUNT,         ACCOUNT_NAME);
         cv.put(CalendarContract.Calendars.VISIBLE,               1);
@@ -191,8 +270,8 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
     }
 
     /**
-     * Adds all events from the event list. The first occurrence is in the current year, the events
+     * Adds up to 100 events from the event list.
-     * are repeated every year.
+     * The first occurrence is in the current year, the events are repeated every year.
      */
     private void addEvents(@NotNull String calendarId, @NotNull List<TerminTransformer.Event> events) {
         Log.d(TAG, "Create calendar events");
@@ -214,7 +293,7 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
      * Adds a single event to the calendar.
      */
     @Nullable
-    private String addEvent(@NotNull String calendarId, @Nullable Integer day, @Nullable Integer month, @Nullable Integer year, @NotNull String title, @NotNull String description) {
+    private String addEvent(@NotNull String calendarId, int day, int month, int year, @NotNull String title, @NotNull String description) {
         Log.d(TAG, "Create calendar event: day=" + day + ", month=" + month + ", year=" + year
                 + ", title=" + title + ", description=" + description);
         ContentValues cv = new ContentValues();
@@ -224,18 +303,8 @@ public class CalendarSyncAdapter extends AbstractThreadedSyncAdapter {
 
         TimeZone utc = TimeZone.getTimeZone("UTC");
         Calendar beginTime = Calendar.getInstance(utc);
-        if (day == null) {
-            day = 1;
-        }
-        if (month == null) {
-            month = 1;
-        }
-        if (year == null) {
-            year = beginTime.get(Calendar.YEAR);
-        }
         beginTime.clear();
         beginTime.set(year, month - 1, day);
-
         cv.put(CalendarContract.Events.DTSTART, beginTime.getTimeInMillis());
         cv.put(CalendarContract.Events.DURATION, "PT1D");
         cv.put(CalendarContract.Events.ALL_DAY, 1);