// Package auth provides cryptographic core for authentication.
package auth

import (
	"errors"
	"strconv"
	"time"

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

// ErrInvalidToken is returned when a token is invalid or cannot be parsed.
var ErrInvalidToken = errors.New("auth: invalid token")

// TokenManager handles issuing and parsing JWT access tokens.
type TokenManager struct {
	secret []byte
}

// NewTokenManager creates a TokenManager using the provided secret key.
func NewTokenManager(secret string) *TokenManager {
	return &TokenManager{secret: []byte(secret)}
}

// IssueAccessToken issues a signed JWT access token for the given userID with TTL.
func (t *TokenManager) IssueAccessToken(userID int64, ttl time.Duration) (string, error) {
	now := time.Now()
	claims := jwt.RegisteredClaims{
		Subject:   strconv.FormatInt(userID, 10),
		IssuedAt:  jwt.NewNumericDate(now),
		ExpiresAt: jwt.NewNumericDate(now.Add(ttl)),
	}
	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	return token.SignedString(t.secret)
}

// ParseAccessToken parses and validates the JWT access token, returning the userID.
func (t *TokenManager) ParseAccessToken(tokenStr string) (int64, error) {
	token, err := jwt.ParseWithClaims(tokenStr, &jwt.RegisteredClaims{}, func(token *jwt.Token) (interface{}, error) {
		if token.Method != jwt.SigningMethodHS256 {
			return nil, ErrInvalidToken
		}
		return t.secret, nil
	})
	if err != nil {
		return 0, ErrInvalidToken
	}
	claims, ok := token.Claims.(*jwt.RegisteredClaims)
	if !ok || !token.Valid {
		return 0, ErrInvalidToken
	}
	userID, err := strconv.ParseInt(claims.Subject, 10, 64)
	if err != nil {
		return 0, ErrInvalidToken
	}
	return userID, nil
}