package logic

import (
	"billit/internal/models"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"os"
	"time"

	"github.com/golang-jwt/jwt/v5"
	"golang.org/x/crypto/bcrypt"
)

var (
	ErrInvalidCredentials = errors.New("invalid email or password")
	ErrInvalidToken       = errors.New("invalid or expired token")
	ErrUserNotFound       = errors.New("user not found")
	ErrUserExists         = errors.New("user already exists")
)

// AuthConfig holds auth configuration
type AuthConfig struct {
	JWTSecret     []byte
	TokenDuration time.Duration
}

// Claims represents JWT claims
type Claims struct {
	UserID string `json:"user_id"`
	Email  string `json:"email"`
	jwt.RegisteredClaims
}

// AuthService handles authentication logic
type AuthService struct {
	config AuthConfig
	users  UserStore
}

// UserStore interface for user persistence (subset of database.Service)
type UserStore interface {
	CreateUser(email, passwordHash string) (*models.User, error)
	GetUserByEmail(email string) (*models.User, error)
	GetUserByID(id string) (*models.User, error)
}

// NewAuthService creates a new auth service
func NewAuthService(users UserStore) *AuthService {
	secret := os.Getenv("JWT_SECRET")
	if secret == "" {
		// Generate a random secret if not provided (not recommended for production)
		b := make([]byte, 32)
		rand.Read(b)
		secret = hex.EncodeToString(b)
	}

	return &AuthService{
		config: AuthConfig{
			JWTSecret:     []byte(secret),
			TokenDuration: 24 * time.Hour,
		},
		users: users,
	}
}

// HashPassword hashes a password using bcrypt with high cost
func HashPassword(password string) (string, error) {
	// Use cost of 12 for good security/performance balance
	hash, err := bcrypt.GenerateFromPassword([]byte(password), 12)
	if err != nil {
		return "", err
	}
	return string(hash), nil
}

// CheckPassword verifies a password against a hash
func CheckPassword(password, hash string) bool {
	err := bcrypt.CompareHashAndPassword([]byte(hash), []byte(password))
	return err == nil
}

// Register creates a new user account
func (s *AuthService) Register(email, password string) (*models.User, error) {
	// Check if user exists
	existing, _ := s.users.GetUserByEmail(email)
	if existing != nil {
		return nil, ErrUserExists
	}

	// Validate password strength
	if len(password) < 8 {
		return nil, errors.New("password must be at least 8 characters")
	}

	// Hash password
	hash, err := HashPassword(password)
	if err != nil {
		return nil, err
	}

	return s.users.CreateUser(email, hash)
}

// Login authenticates a user and returns a JWT token
func (s *AuthService) Login(email, password string) (string, error) {
	user, err := s.users.GetUserByEmail(email)
	if err != nil || user == nil {
		return "", ErrInvalidCredentials
	}

	if !CheckPassword(password, user.Password) {
		return "", ErrInvalidCredentials
	}

	return s.generateToken(user)
}

// generateToken creates a new JWT token for a user
func (s *AuthService) generateToken(user *models.User) (string, error) {
	now := time.Now()
	claims := &Claims{
		UserID: user.ID,
		Email:  user.Email,
		RegisteredClaims: jwt.RegisteredClaims{
			ExpiresAt: jwt.NewNumericDate(now.Add(s.config.TokenDuration)),
			IssuedAt:  jwt.NewNumericDate(now),
			NotBefore: jwt.NewNumericDate(now),
			Issuer:    "billit",
			Subject:   user.ID,
		},
	}

	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	return token.SignedString(s.config.JWTSecret)
}

// ValidateToken validates a JWT token and returns the claims
func (s *AuthService) ValidateToken(tokenString string) (*Claims, error) {
	token, err := jwt.ParseWithClaims(tokenString, &Claims{}, func(token *jwt.Token) (interface{}, error) {
		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
			return nil, ErrInvalidToken
		}
		return s.config.JWTSecret, nil
	})

	if err != nil {
		return nil, ErrInvalidToken
	}

	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
		return claims, nil
	}

	return nil, ErrInvalidToken
}

// GetUserFromToken retrieves the user from a valid token
func (s *AuthService) GetUserFromToken(tokenString string) (*models.User, error) {
	claims, err := s.ValidateToken(tokenString)
	if err != nil {
		return nil, err
	}

	return s.users.GetUserByID(claims.UserID)
}