import com.sun.net.httpserver.HttpServer;
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.time.Instant;

/**
 * A minimal, dependency-free stand-in for an OTLP collector. It accepts any POST to any path,
 * logs the path, content-length and arrival time to a file (one line per request), and returns
 * 200 OK with an empty OTLP-shaped protobuf response body. Used only to answer one factual
 * question: did this JVM actually attempt to POST metrics to the configured OTLP endpoint within
 * the observation window? Not a real collector -- doesn't parse the protobuf payload.
 *
 * Usage: java StubOtlpReceiver.java <port> <logFile>
 */
public class StubOtlpReceiver {
    public static void main(String[] args) throws Exception {
        int port = Integer.parseInt(args[0]);
        Path logFile = Path.of(args[1]);
        Files.deleteIfExists(logFile);
        Files.createFile(logFile);

        HttpServer server = HttpServer.create(new InetSocketAddress("0.0.0.0", port), 0);
        server.createContext("/", exchange -> {
            String line = Instant.now() + " " + exchange.getRequestMethod() + " " + exchange.getRequestURI()
                    + " content-length=" + exchange.getRequestHeaders().getFirst("Content-Length") + System.lineSeparator();
            Files.writeString(logFile, line, StandardOpenOption.APPEND);
            exchange.getRequestBody().readAllBytes(); // drain
            byte[] resp = new byte[0];
            exchange.sendResponseHeaders(200, resp.length);
            try (OutputStream os = exchange.getResponseBody()) {
                os.write(resp);
            }
        });
        server.setExecutor(null);
        server.start();
        System.out.println("StubOtlpReceiver listening on :" + port + ", logging to " + logFile);
        // Run until killed externally.
        Thread.currentThread().join();
    }
}