package auth

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/hex"
)

// SetupToken guards the first-admin registration. Until the first account
// exists, register makes whoever calls it the admin, so without this a fresh
// instance on a public address would belong to whoever found it first. The
// call must carry this token, which is generated at startup and written only
// to the server log: proof that the caller can read the server's logs.
//
// The token lives in memory. A restart mints a new one and logs it again,
// which is the behaviour wanted: an old token from a log line someone else
// saw stops working.
type SetupToken struct {
	value string
}

// NewSetupToken mints a 128-bit token.
func NewSetupToken() (*SetupToken, error) {
	b := make([]byte, 16)
	if _, err := rand.Read(b); err != nil {
		return nil, err
	}
	return &SetupToken{value: hex.EncodeToString(b)}, nil
}

// Value returns the token for logging.
func (t *SetupToken) Value() string {
	if t == nil {
		return ""
	}
	return t.value
}

// Matches reports whether supplied is the token, in constant time. A nil
// token never matches anything, so a missing token fails closed.
func (t *SetupToken) Matches(supplied string) bool {
	if t == nil || t.value == "" || supplied == "" {
		return false
	}
	return subtle.ConstantTimeCompare([]byte(t.value), []byte(supplied)) == 1
}