package wristband

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
)

// randomBase64URL returns n cryptographically random bytes, base64url
// (RawURLEncoding, no padding) encoded, matching PHP's
// rtrim(strtr(base64_encode(random_bytes(n)), '+/', '-_'), '=') byte for
// byte (D-01/D-04).
func randomBase64URL(n int) (string, error) {
	buf := make([]byte, n)
	if _, err := rand.Read(buf); err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(buf), nil
}

// sha256Hex is the fixed transform every opaque secret (client secret, code,
// refresh token) is compared and persisted through: never the raw variable-
// length secret (D-04).
func sha256Hex(raw string) string {
	sum := sha256.Sum256([]byte(raw))
	return hex.EncodeToString(sum[:])
}

// constantEqual compares two fixed-transform strings (sha256 hex digests or
// PKCE S256 challenges) in constant time (D-04; RFC 7636 verifier compare).
func constantEqual(a, b string) bool {
	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
}

// s256Challenge is the RFC 7636 S256 transform: base64url(sha256(verifier)).
func s256Challenge(verifier string) string {
	sum := sha256.Sum256([]byte(verifier))
	return base64.RawURLEncoding.EncodeToString(sum[:])
}