package auth

import (
	"errors"
	"fmt"
	"time"

	"github.com/golang-jwt/jwt/v5"
)

// SessionLifetime is how long an issued JWT remains valid.
const SessionLifetime = 24 * time.Hour

// ErrInvalidJWT is returned when a JWT fails parsing, signature
// verification, or claim validation.
var ErrInvalidJWT = errors.New("invalid jwt")

// Claims is the JWT payload we sign for authenticated sessions.
type Claims struct {
	UserID string `json:"sub"`
	Email  string `json:"email"`
	jwt.RegisteredClaims
}

// Signer issues and verifies session JWTs using HS256.
type Signer struct {
	secret []byte
}

// NewSigner builds a Signer from a shared secret (>= 32 bytes recommended).
func NewSigner(secret []byte) *Signer { return &Signer{secret: secret} }

// Issue creates a signed JWT for the given user.
func (s *Signer) Issue(userID, email string) (string, error) {
	now := time.Now().UTC()
	claims := Claims{
		UserID: userID,
		Email:  email,
		RegisteredClaims: jwt.RegisteredClaims{
			IssuedAt:  jwt.NewNumericDate(now),
			ExpiresAt: jwt.NewNumericDate(now.Add(SessionLifetime)),
			NotBefore: jwt.NewNumericDate(now),
			Issuer:    "librenotes",
			Subject:   userID,
		},
	}
	tok := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	signed, err := tok.SignedString(s.secret)
	if err != nil {
		return "", fmt.Errorf("sign: %w", err)
	}
	return signed, nil
}

// Verify parses and validates a JWT, returning the claims on success.
func (s *Signer) Verify(token string) (*Claims, error) {
	claims := &Claims{}
	parsed, err := jwt.ParseWithClaims(token, claims, func(t *jwt.Token) (any, error) {
		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
		}
		return s.secret, nil
	}, jwt.WithValidMethods([]string{"HS256"}))
	if err != nil || !parsed.Valid {
		return nil, ErrInvalidJWT
	}
	if claims.UserID == "" {
		return nil, ErrInvalidJWT
	}
	return claims, nil
}