import { useState, useEffect, useCallback } from 'react';
import {
  View,
  Text,
  StyleSheet,
  ScrollView,
  RefreshControl,
  TouchableOpacity,
  Alert,
} from 'react-native';
import { Ionicons } from '@expo/vector-icons';
import { useAuthContext } from '../../lib/AuthContext';
import * as api from '../../lib/api';

const DAYS = ['Po', 'Ut', 'St', 'Ct', 'Pa', 'So', 'Ne'];
const MONTHS = [
  'Leden', 'Unor', 'Brezen', 'Duben', 'Kveten', 'Cerven',
  'Cervenec', 'Srpen', 'Zari', 'Rijen', 'Listopad', 'Prosinec',
];

function getDaysInMonth(year: number, month: number) {
  return new Date(year, month + 1, 0).getDate();
}

function getFirstDayOfMonth(year: number, month: number) {
  const day = new Date(year, month, 1).getDay();
  return day === 0 ? 6 : day - 1; // Monday = 0
}

export default function CalendarScreen() {
  const { token } = useAuthContext();
  const [tasks, setTasks] = useState<any[]>([]);
  const [refreshing, setRefreshing] = useState(false);
  const [currentDate, setCurrentDate] = useState(new Date());
  const [selectedDate, setSelectedDate] = useState<string | null>(null);

  const year = currentDate.getFullYear();
  const month = currentDate.getMonth();

  const loadTasks = useCallback(async () => {
    if (!token) return;
    try {
      const res = await api.getTasks(token);
      setTasks(res.data || []);
    } catch (err: any) {
      Alert.alert('Chyba', err.message);
    }
  }, [token]);

  useEffect(() => {
    loadTasks();
  }, [loadTasks]);

  const onRefresh = useCallback(async () => {
    setRefreshing(true);
    await loadTasks();
    setRefreshing(false);
  }, [loadTasks]);

  const prevMonth = () => {
    setCurrentDate(new Date(year, month - 1, 1));
    setSelectedDate(null);
  };

  const nextMonth = () => {
    setCurrentDate(new Date(year, month + 1, 1));
    setSelectedDate(null);
  };

  const daysInMonth = getDaysInMonth(year, month);
  const firstDay = getFirstDayOfMonth(year, month);

  const tasksByDate: Record<string, any[]> = {};
  tasks.forEach((t) => {
    if (t.due_date) {
      const d = t.due_date.substring(0, 10);
      if (!tasksByDate[d]) tasksByDate[d] = [];
      tasksByDate[d].push(t);
    }
  });

  const today = new Date().toISOString().substring(0, 10);

  const selectedTasks = selectedDate ? tasksByDate[selectedDate] || [] : [];

  return (
    <ScrollView
      style={styles.container}
      refreshControl={
        <RefreshControl refreshing={refreshing} onRefresh={onRefresh} />
      }
    >
      {/* Month navigator */}
      <View style={styles.monthNav}>
        <TouchableOpacity onPress={prevMonth} style={styles.navBtn}>
          <Ionicons name="chevron-back" size={24} color="#3B82F6" />
        </TouchableOpacity>
        <Text style={styles.monthTitle}>
          {MONTHS[month]} {year}
        </Text>
        <TouchableOpacity onPress={nextMonth} style={styles.navBtn}>
          <Ionicons name="chevron-forward" size={24} color="#3B82F6" />
        </TouchableOpacity>
      </View>

      {/* Day headers */}
      <View style={styles.dayHeaders}>
        {DAYS.map((d) => (
          <Text key={d} style={styles.dayHeader}>
            {d}
          </Text>
        ))}
      </View>

      {/* Calendar grid */}
      <View style={styles.grid}>
        {Array.from({ length: firstDay }).map((_, i) => (
          <View key={`empty-${i}`} style={styles.dayCell} />
        ))}
        {Array.from({ length: daysInMonth }).map((_, i) => {
          const day = i + 1;
          const dateStr = `${year}-${String(month + 1).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
          const hasTasks = !!tasksByDate[dateStr];
          const isToday = dateStr === today;
          const isSelected = dateStr === selectedDate;

          return (
            <TouchableOpacity
              key={day}
              style={[
                styles.dayCell,
                isToday && styles.todayCell,
                isSelected && styles.selectedCell,
              ]}
              onPress={() => setSelectedDate(dateStr)}
            >
              <Text
                style={[
                  styles.dayText,
                  isToday && styles.todayText,
                  isSelected && styles.selectedText,
                ]}
              >
                {day}
              </Text>
              {hasTasks && (
                <View
                  style={[
                    styles.dot,
                    isSelected && { backgroundColor: '#fff' },
                  ]}
                />
              )}
            </TouchableOpacity>
          );
        })}
      </View>

      {/* Selected date tasks */}
      {selectedDate && (
        <View style={styles.tasksSection}>
          <Text style={styles.tasksSectionTitle}>
            Ukoly na {selectedDate}
          </Text>
          {selectedTasks.length === 0 ? (
            <Text style={styles.noTasks}>Zadne ukoly</Text>
          ) : (
            selectedTasks.map((t) => (
              <View key={t.id} style={styles.taskItem}>
                <Ionicons
                  name={t.status === 'done' ? 'checkmark-circle' : 'ellipse-outline'}
                  size={20}
                  color={t.status === 'done' ? '#22C55E' : '#94A3B8'}
                />
                <Text
                  style={[
                    styles.taskItemText,
                    t.status === 'done' && { textDecorationLine: 'line-through', color: '#94A3B8' },
                  ]}
                >
                  {t.title}
                </Text>
              </View>
            ))
          )}
        </View>
      )}
    </ScrollView>
  );
}

const styles = StyleSheet.create({
  container: { flex: 1, backgroundColor: '#F8FAFC' },
  monthNav: {
    flexDirection: 'row',
    alignItems: 'center',
    justifyContent: 'space-between',
    padding: 16,
    backgroundColor: '#fff',
  },
  navBtn: { padding: 8 },
  monthTitle: { fontSize: 18, fontWeight: '700', color: '#1E293B' },
  dayHeaders: {
    flexDirection: 'row',
    backgroundColor: '#fff',
    paddingBottom: 8,
    borderBottomWidth: 1,
    borderBottomColor: '#E2E8F0',
  },
  dayHeader: {
    flex: 1,
    textAlign: 'center',
    fontSize: 12,
    fontWeight: '600',
    color: '#64748B',
  },
  grid: {
    flexDirection: 'row',
    flexWrap: 'wrap',
    backgroundColor: '#fff',
    padding: 4,
  },
  dayCell: {
    width: '14.28%',
    aspectRatio: 1,
    alignItems: 'center',
    justifyContent: 'center',
  },
  todayCell: {
    backgroundColor: '#EFF6FF',
    borderRadius: 20,
  },
  selectedCell: {
    backgroundColor: '#3B82F6',
    borderRadius: 20,
  },
  dayText: { fontSize: 15, color: '#1E293B' },
  todayText: { color: '#3B82F6', fontWeight: '700' },
  selectedText: { color: '#fff', fontWeight: '700' },
  dot: {
    width: 5,
    height: 5,
    borderRadius: 3,
    backgroundColor: '#3B82F6',
    marginTop: 2,
  },
  tasksSection: {
    margin: 12,
    padding: 16,
    backgroundColor: '#fff',
    borderRadius: 12,
  },
  tasksSectionTitle: { fontSize: 16, fontWeight: '600', color: '#1E293B', marginBottom: 12 },
  noTasks: { color: '#94A3B8', fontSize: 14 },
  taskItem: {
    flexDirection: 'row',
    alignItems: 'center',
    gap: 10,
    paddingVertical: 8,
    borderBottomWidth: 1,
    borderBottomColor: '#F1F5F9',
  },
  taskItemText: { fontSize: 14, color: '#1E293B', flex: 1 },
});