package customerauth

import (
	"crypto/hmac"
	"encoding/base64"
	"strconv"
	"strings"
	"time"
)

// Purpose-scoped tokens (email verification, password reset) reuse the session
// Signer's HMAC secret but bind a purpose + subject, so a token minted for one
// flow cannot be replayed in another. Format:
//
//	base64url(<purpose> NUL <subject> NUL <expUnix>) "." base64url(hmac)
//
// subject is the normalized email. Like session tokens these are signed, not
// encrypted: they carry no secret, only a purpose, an email and an expiry, and
// the HMAC makes them tamper-evident.
const (
	purposeVerifyEmail   = "verify"
	purposePasswordReset = "reset"
)

// IssuePurpose returns a signed, purpose-bound token for subject that expires
// after ttl.
func (s *Signer) IssuePurpose(purpose, subject string, ttl time.Duration) string {
	exp := time.Now().Add(ttl).Unix()
	payload := purpose + "\x00" + subject + "\x00" + strconv.FormatInt(exp, 10)
	b := base64.RawURLEncoding.EncodeToString([]byte(payload))
	return b + "." + s.sign(b)
}

// ParsePurpose verifies a purpose token and returns its subject. It requires the
// embedded purpose to match want and the token to be unexpired, returning
// ErrInvalidToken for a bad signature/format/purpose and ErrExpiredToken when
// the token has expired.
func (s *Signer) ParsePurpose(want, token string) (string, error) {
	b, sig, ok := strings.Cut(token, ".")
	if !ok || b == "" || sig == "" {
		return "", ErrInvalidToken
	}
	if !hmac.Equal([]byte(sig), []byte(s.sign(b))) {
		return "", ErrInvalidToken
	}
	raw, err := base64.RawURLEncoding.DecodeString(b)
	if err != nil {
		return "", ErrInvalidToken
	}
	parts := strings.Split(string(raw), "\x00")
	if len(parts) != 3 || parts[0] != want {
		return "", ErrInvalidToken
	}
	exp, err := strconv.ParseInt(parts[2], 10, 64)
	if err != nil {
		return "", ErrInvalidToken
	}
	if time.Now().Unix() >= exp {
		return "", ErrExpiredToken
	}
	return parts[1], nil
}