@Slf4j
@Service
@RequiredArgsConstructor
public class FivePostApiTokenService {
    private final static Duration TOKEN_UPDATE_THRESHOLD = Duration.ofMinutes(2);

    private final FivePostAuthApiClient fivePostAuthApiClient;
    private final ObjectMapper objectMapper;

    private TokenHolder tokenHolder = new TokenHolder();
    private Lock tokenUpdateLock = new ReentrantLock();

    public Mono<String> getToken() {
        return Mono.defer(() -> {
            //double check (first check)
            if (needRequestNewToken()) {
                return updateToken()
                        .subscribeOn(Schedulers.boundedElastic())
                        .then(Mono.fromCallable(() -> tokenHolder.raw));
            } else {
                return Mono.just(tokenHolder.raw);
            }
        });
    }

    private boolean needRequestNewToken() {
        return tokenHolder.isNotInitialized() || tokenExpired();
    }

    private boolean tokenExpired() {
        return tokenHolder
                .expiresAt
                .minus(TOKEN_UPDATE_THRESHOLD)
                .isBefore(Instant.now());
    }

    private Mono<Void> updateToken() {
        return Mono.defer(() -> {
            tokenUpdateLock.lock();

            try {
                //double check (second check)
                if (needRequestNewToken()) {

                    //noinspection BlockingMethodInNonBlockingContext
                    fivePostAuthApiClient.requestToken()
                            .doOnSuccess(tokenHolder::update)
                            .then()
                            .block();
                }

                return Mono.empty();
            } finally {
                tokenUpdateLock.unlock();
            }
        });
    }

    private class TokenHolder {
        private volatile String raw = "";
        private volatile Instant expiresAt = Instant.MIN;

        boolean isNotInitialized() {
            return raw.isBlank();
        }

        void update(FivePostCredentials tokenResponse) {
            FivePostJwtToken tokenBody = JwtTokenParser.getBody(tokenResponse.getJwt(), FivePostJwtToken.class, objectMapper);

            if (tokenBody != null) {
                expiresAt = tokenBody.getExpiresAt();
                raw = tokenResponse.getJwt();
            } else {
                log.error("Token update failed");
            }
        }
    }
}