package com.example.demo.service;

import com.example.demo.dto.UserStatsResponse;
import com.example.demo.exception.DuplicateEmailException;
import com.example.demo.exception.ResourceNotFoundException;
import com.example.demo.model.User;
import org.springframework.stereotype.Service;

import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.atomic.AtomicLong;
import java.util.stream.Collectors;

@Service
public class UserService {

    private final List<User> users = new CopyOnWriteArrayList<>();
    private final AtomicLong idGenerator = new AtomicLong(1);

    public UserService() {
        users.add(new User(idGenerator.getAndIncrement(), "Alice Chen", "alice@example.com", 25));
        users.add(new User(idGenerator.getAndIncrement(), "Brandon Li", "brandon@example.com", 30));
        users.add(new User(idGenerator.getAndIncrement(), "Carol Wang", "carol@example.com", 28));
    }

    public List<User> findAll() {
        return users.stream()
                .sorted((left, right) -> Long.compare(left.getId(), right.getId()))
                .collect(Collectors.toList());
    }

    public User findById(Long id) {
        return users.stream()
                .filter(user -> user.getId().equals(id))
                .findFirst()
                .orElseThrow(() -> new ResourceNotFoundException("User not found: id=" + id));
    }

    public List<User> search(String keyword) {
        if (keyword == null || keyword.isBlank()) {
            return findAll();
        }

        String normalizedKeyword = keyword.trim().toLowerCase();
        return users.stream()
                .filter(user -> user.getName().toLowerCase().contains(normalizedKeyword)
                        || user.getEmail().toLowerCase().contains(normalizedKeyword))
                .sorted((left, right) -> Long.compare(left.getId(), right.getId()))
                .collect(Collectors.toList());
    }

    public User create(User user) {
        ensureEmailAvailable(user.getEmail(), null);
        User normalized = normalize(user, idGenerator.getAndIncrement());
        users.add(normalized);
        return normalized;
    }

    public User update(Long id, User user) {
        findById(id);
        ensureEmailAvailable(user.getEmail(), id);

        User normalized = normalize(user, id);
        for (int index = 0; index < users.size(); index++) {
            if (users.get(index).getId().equals(id)) {
                users.set(index, normalized);
                return normalized;
            }
        }

        throw new ResourceNotFoundException("User not found: id=" + id);
    }

    public void delete(Long id) {
        boolean removed = users.removeIf(user -> user.getId().equals(id));
        if (!removed) {
            throw new ResourceNotFoundException("User not found: id=" + id);
        }
    }

    public UserStatsResponse getStats() {
        long totalUsers = users.size();
        long adults = users.stream().filter(user -> user.getAge() >= 18).count();
        long underThirty = users.stream().filter(user -> user.getAge() < 30).count();
        double averageAge = users.stream()
                .mapToInt(User::getAge)
                .average()
                .orElse(0.0);

        return new UserStatsResponse(totalUsers, adults, underThirty, averageAge);
    }

    private void ensureEmailAvailable(String email, Long currentUserId) {
        String normalizedEmail = email == null ? "" : email.trim().toLowerCase();
        boolean exists = users.stream()
                .anyMatch(user -> user.getEmail().equalsIgnoreCase(normalizedEmail)
                        && (currentUserId == null || !user.getId().equals(currentUserId)));

        if (exists) {
            throw new DuplicateEmailException("A user with email " + normalizedEmail + " already exists.");
        }
    }

    private User normalize(User user, Long id) {
        return new User(
                id,
                user.getName().trim(),
                user.getEmail().trim().toLowerCase(),
                user.getAge()
        );
    }
}