package authjwt

import (
	"crypto/ecdsa"
	"crypto/x509"
	"encoding/pem"
	"fmt"

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

var _ Verifier = (*ecPublicVerifier)(nil)

type ecPublicVerifier struct{ public *ecdsa.PublicKey }

// NewECPublicKeyVerifier returns a Verifier backed by an ECDSA public key.
// Use this in services that verify tokens but never issue them.
func NewECPublicKeyVerifier(publicKey *ecdsa.PublicKey) Verifier {
	return &ecPublicVerifier{public: publicKey}
}

// NewECPublicKeyVerifierFromPEM parses a PKIX PEM-encoded ECDSA public key.
func NewECPublicKeyVerifierFromPEM(pemKey []byte) (Verifier, error) {
	block, _ := pem.Decode(pemKey)
	if block == nil {
		return nil, fmt.Errorf("no PEM block found")
	}
	pub, err := x509.ParsePKIXPublicKey(block.Bytes)
	if err != nil {
		return nil, fmt.Errorf("parse EC public key: %w", err)
	}
	ecPub, ok := pub.(*ecdsa.PublicKey)
	if !ok {
		return nil, fmt.Errorf("PEM key is not an EC public key")
	}
	return NewECPublicKeyVerifier(ecPub), nil
}

func (v *ecPublicVerifier) Verify(tokenString string) (*jwt.Token, error) {
	return jwt.Parse(tokenString, func(t *jwt.Token) (any, error) {
		if _, ok := t.Method.(*jwt.SigningMethodECDSA); !ok {
			return nil, fmt.Errorf("unexpected signing method %q", t.Header["alg"])
		}
		return v.public, nil
	}, jwt.WithJSONNumber())
}