feat(model): Add model classes for calendar functionality

- Implement model classes to support calendar feature integration
- Define necessary structures for handling iCal data and event processing

Author: Othmane Bencheqroun

app/src/main/java/com/github/lookupgroup27/lookup/model/calendar/CalendarViewModel.kt

@@ -0,0 +1,148 @@
+package com.github.lookupgroup27.lookup.model.calendar
+
+import android.util.Log
+import androidx.compose.runtime.mutableStateListOf
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import java.io.StringReader
+import java.util.Calendar as JavaCalendar
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+import net.fortuna.ical4j.data.CalendarBuilder
+import net.fortuna.ical4j.model.Calendar
+import net.fortuna.ical4j.model.Date
+import net.fortuna.ical4j.model.DateTime
+import net.fortuna.ical4j.model.Period
+import net.fortuna.ical4j.model.Property
+import net.fortuna.ical4j.model.component.VEvent
+import net.fortuna.ical4j.model.property.DtEnd
+import net.fortuna.ical4j.model.property.DtStart
+import net.fortuna.ical4j.model.property.RRule
+import net.fortuna.ical4j.util.MapTimeZoneCache
+import okhttp3.OkHttpClient
+import okhttp3.Request
+
+class CalendarViewModel : ViewModel() {
+
+  private val client = OkHttpClient()
+  private val _icalEvents = mutableStateListOf<VEvent>()
+  val icalEvents: List<VEvent> = _icalEvents
+
+  init {
+    System.setProperty("net.fortuna.ical4j.timezone.cache.impl", MapTimeZoneCache::class.java.name)
+    fetchICalData()
+  }
+
+  fun fetchICalData() {
+    viewModelScope.launch {
+      val icalData =
+          fetchIcalFromUrl(
+              "https://p127-caldav.icloud.com/published/2/MTE0OTM4OTk2MTExNDkzOIzDRaBjEGa9_1mlmgjzcdlka5HK6EzMiIdOswU-0rZBZMDBibtH_M7CDyMpDQRQJPdGOSM0hTsS2qFNGOObsTc")
+      icalData?.let { parseICalData(it) }
+    }
+  }
+
+  private suspend fun fetchIcalFromUrl(url: String): String? =
+      withContext(Dispatchers.IO) {
+        return@withContext try {
+          val request = Request.Builder().url(url).build()
+          val response = client.newCall(request).execute()
+          response.body?.string()
+        } catch (e: Exception) {
+          Log.e("CalendarViewModel", "Error fetching iCal data: ${e.localizedMessage}", e)
+          null
+        }
+      }
+
+  private fun parseICalData(icalData: String) {
+    try {
+      val reader = StringReader(icalData)
+      val calendar: Calendar = CalendarBuilder().build(reader)
+      val start = DateTime(System.currentTimeMillis())
+      val end = DateTime(System.currentTimeMillis() + 365L * 24 * 60 * 60 * 1000)
+      val period = Period(start, end)
+
+      val allEvents = mutableListOf<VEvent>()
+
+      for (component in calendar.getComponents<VEvent>(VEvent.VEVENT)) {
+        val dtStart = component.getProperty<DtStart>(Property.DTSTART)
+        val dtEnd = component.getProperty<DtEnd>(Property.DTEND)
+
+        dtStart?.let { startDate ->
+          val endDate = dtEnd?.date ?: startDate.date
+          val rrule = component.getProperty<RRule>(Property.RRULE)
+
+          if (rrule != null) {
+            handleRecurringEvents(component, period, startDate.date, endDate, allEvents)
+          } else if (startDate.date.before(end) && endDate.after(start)) {
+            handleNonRecurringEvents(component, startDate.date, endDate, allEvents)
+          }
+        }
+      }
+
+      _icalEvents.clear()
+      _icalEvents.addAll(allEvents)
+      Log.d("CalendarViewModel", "Total events parsed: ${_icalEvents.size}")
+    } catch (e: Exception) {
+      Log.e("CalendarViewModel", "Error parsing iCal data: ${e.localizedMessage}", e)
+    }
+  }
+
+  private fun handleRecurringEvents(
+      component: VEvent,
+      period: Period,
+      eventStartDate: Date,
+      eventEndDate: Date,
+      allEvents: MutableList<VEvent>
+  ) {
+    val recurrenceSet = component.calculateRecurrenceSet(period)
+    for (recurrence in recurrenceSet) {
+      val eventInstance = component.copy() as VEvent
+      eventInstance.getProperty<DtStart>(Property.DTSTART)?.date = recurrence.start
+      if (eventEndDate.after(recurrence.start)) {
+        var current = recurrence.start
+        while (current.before(eventEndDate)) {
+          val multiDayEventInstance = eventInstance.copy() as VEvent
+          multiDayEventInstance.getProperty<DtStart>(Property.DTSTART)?.date =
+              DateTime(current.time)
+          allEvents.add(multiDayEventInstance)
+
+          val calendarInstance =
+              JavaCalendar.getInstance().apply {
+                time = current
+                add(JavaCalendar.DATE, 1)
+              }
+          current = DateTime(calendarInstance.time)
+        }
+      } else {
+        allEvents.add(eventInstance)
+      }
+    }
+  }
+
+  private fun handleNonRecurringEvents(
+      component: VEvent,
+      eventStartDate: Date,
+      eventEndDate: Date,
+      allEvents: MutableList<VEvent>
+  ) {
+    if (eventEndDate.after(eventStartDate)) {
+      var current = eventStartDate
+      while (current.before(eventEndDate)) {
+        val eventInstance = component.copy() as VEvent
+        eventInstance.getProperty<DtStart>(Property.DTSTART)?.date = DateTime(current.time)
+        allEvents.add(eventInstance)
+
+        val calendarInstance =
+            JavaCalendar.getInstance().apply {
+              time = current
+              add(JavaCalendar.DATE, 1)
+            }
+        current = DateTime(calendarInstance.time)
+      }
+    } else {
+      allEvents.add(component)
+    }
+  }
+}