// Package auth provides JWT issuance + verification for the BFF.
//
// Tokens carry these claims:
//
//	sub  user id (UUID string)
//	tv   token_version snapshot — middleware rejects if it doesn't match the
//	     current value on the user row. Bumping the user's token_version
//	     invalidates every JWT ever issued to that user (used on password
//	     change and forced logout).
//	iat  issued-at
//	exp  expiry
package auth

import (
	"errors"
	"fmt"
	"time"

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

// Claims is the JWT payload we sign + verify.
type Claims struct {
	UserID       string `json:"sub"`
	TokenVersion int    `json:"tv"`
	jwt.RegisteredClaims
}

// Issuer issues and verifies tokens with a single shared HMAC secret.
type Issuer struct {
	secret []byte
	ttl    time.Duration
}

// NewIssuer returns an Issuer configured with the given HMAC secret and TTL.
// The TTL is applied to every token issued via Issue().
func NewIssuer(secret string, ttl time.Duration) *Issuer {
	return &Issuer{secret: []byte(secret), ttl: ttl}
}

// Issue mints a signed JWT for the given user.
func (i *Issuer) Issue(userID string, tokenVersion int) (string, time.Time, error) {
	exp := time.Now().Add(i.ttl)
	claims := &Claims{
		UserID:       userID,
		TokenVersion: tokenVersion,
		RegisteredClaims: jwt.RegisteredClaims{
			IssuedAt:  jwt.NewNumericDate(time.Now()),
			ExpiresAt: jwt.NewNumericDate(exp),
		},
	}
	tok := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
	signed, err := tok.SignedString(i.secret)
	if err != nil {
		return "", time.Time{}, fmt.Errorf("sign jwt: %w", err)
	}
	return signed, exp, nil
}

// ErrInvalidToken is returned by Parse for any verification failure.
var ErrInvalidToken = errors.New("invalid or expired token")

// Parse validates the token's signature + expiry and returns the claims.
// Callers must additionally compare claims.TokenVersion against the user's
// current token_version (the JWT alone can't tell us whether the user has
// since bumped their version).
func (i *Issuer) Parse(raw string) (*Claims, error) {
	claims := &Claims{}
	tok, err := jwt.ParseWithClaims(raw, 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 i.secret, nil
	})
	if err != nil || !tok.Valid {
		return nil, ErrInvalidToken
	}
	return claims, nil
}

// TTL returns the configured token lifetime. Handlers use it to set cookie
// Max-Age so the cookie and the JWT expire in lockstep.
func (i *Issuer) TTL() time.Duration { return i.ttl }