package auth

import (
	"crypto/hmac"
	"crypto/sha256"
	"errors"
	"fmt"
	"time"

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

// ErrInvalidCredentials indicates that the supplied username/password is wrong.
var ErrInvalidCredentials = errors.New("invalid credentials")

// ErrInvalidToken indicates that the JWT is invalid or expired.
var ErrInvalidToken = errors.New("invalid or expired token")

// TokenExpiry is the lifetime of a JWT session token.
const TokenExpiry = 24 * time.Hour

// jwtClaims extends jwt.RegisteredClaims with application-specific fields.
type jwtClaims struct {
	jwt.RegisteredClaims
	UserID   string `json:"user_id"`
	Username string `json:"username"`
	Role     string `json:"role"`
}

// LocalAuth handles password hashing and JWT token management for local auth mode.
type LocalAuth struct {
	jwtSecret []byte
}

// NewLocalAuth creates a LocalAuth deriving the JWT signing key from the encryption key
// using HMAC-SHA256.
func NewLocalAuth(encKey [32]byte) *LocalAuth {
	mac := hmac.New(sha256.New, encKey[:])
	mac.Write([]byte("docker-watcher-jwt-secret"))
	return &LocalAuth{
		jwtSecret: mac.Sum(nil),
	}
}

// HashPassword hashes a plaintext password using bcrypt.
func HashPassword(password string) (string, error) {
	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
	if err != nil {
		return "", fmt.Errorf("hash password: %w", err)
	}
	return string(hash), nil
}

// CheckPassword compares a plaintext password against a bcrypt hash.
func CheckPassword(hash, password string) error {
	if err := bcrypt.CompareHashAndPassword([]byte(hash), []byte(password)); err != nil {
		return ErrInvalidCredentials
	}
	return nil
}

// GenerateToken creates a signed JWT for the given user claims.
func (la *LocalAuth) GenerateToken(claims Claims) (SessionToken, error) {
	expiresAt := time.Now().Add(TokenExpiry)

	token := jwt.NewWithClaims(jwt.SigningMethodHS256, jwtClaims{
		RegisteredClaims: jwt.RegisteredClaims{
			ExpiresAt: jwt.NewNumericDate(expiresAt),
			IssuedAt:  jwt.NewNumericDate(time.Now()),
			Issuer:    "docker-watcher",
		},
		UserID:   claims.UserID,
		Username: claims.Username,
		Role:     claims.Role,
	})

	signed, err := token.SignedString(la.jwtSecret)
	if err != nil {
		return SessionToken{}, fmt.Errorf("sign token: %w", err)
	}

	return SessionToken{
		Token:     signed,
		ExpiresAt: expiresAt,
	}, nil
}

// ValidateToken parses and validates a JWT, returning the embedded claims.
func (la *LocalAuth) ValidateToken(tokenString string) (Claims, error) {
	token, err := jwt.ParseWithClaims(tokenString, &jwtClaims{}, func(token *jwt.Token) (any, error) {
		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
		}
		return la.jwtSecret, nil
	})
	if err != nil {
		return Claims{}, ErrInvalidToken
	}

	claims, ok := token.Claims.(*jwtClaims)
	if !ok || !token.Valid {
		return Claims{}, ErrInvalidToken
	}

	return Claims{
		UserID:   claims.UserID,
		Username: claims.Username,
		Role:     claims.Role,
	}, nil
}