// Env.java -- prints everything a GC measurement needs in order to be reproducible.
//
// Run it (JDK 25 single-file source launch, no compile step needed):
//
//     java env/Env.java
//     java -XX:+UseZGC -Xms2g -Xmx2g env/Env.java
//     java -XX:+UseG1GC -Xms2g -Xmx2g env/Env.java
//
// Why this file exists: a GC benchmark without the environment printed next to it
// is an anecdote. Heap size, CPU count, GC selection and the *ergonomic* values the
// JVM picked for you (SoftMaxHeapSize, region size, GC thread counts) all move the
// numbers more than the code under test does.

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryPoolMXBean;
import java.lang.management.RuntimeMXBean;

public final class Env {

    public static void main(String[] args) {
        RuntimeMXBean runtime = ManagementFactory.getRuntimeMXBean();

        System.out.println("=== Runtime ===");
        System.out.printf("java.version        : %s%n", System.getProperty("java.version"));
        System.out.printf("java.vm.name        : %s%n", System.getProperty("java.vm.name"));
        System.out.printf("java.vm.version     : %s%n", System.getProperty("java.vm.version"));
        System.out.printf("java.vendor         : %s%n", System.getProperty("java.vendor"));
        System.out.printf("os.name / os.arch   : %s / %s%n",
                System.getProperty("os.name"), System.getProperty("os.arch"));
        System.out.printf("availableProcessors : %d%n", Runtime.getRuntime().availableProcessors());
        System.out.printf("maxMemory (-Xmx)    : %s%n", mb(Runtime.getRuntime().maxMemory()));
        System.out.printf("totalMemory (now)   : %s%n", mb(Runtime.getRuntime().totalMemory()));

        System.out.println();
        System.out.println("=== Collector in use ===");
        // On JDK 25 the ZGC bean names are "ZGC Major Cycles" / "ZGC Minor Cycles" (and the
        // matching "... Pauses" beans). The presence of BOTH a Minor and a Major bean is the
        // cheapest programmatic proof that you are running *generational* ZGC.
        boolean sawMinor = false, sawMajor = false;
        for (GarbageCollectorMXBean gc : ManagementFactory.getGarbageCollectorMXBeans()) {
            System.out.printf("  bean: %-24s count=%-6d timeMs=%d%n",
                    gc.getName(), gc.getCollectionCount(), gc.getCollectionTime());
            String n = gc.getName().toLowerCase();
            if (n.contains("minor") || n.contains("young")) sawMinor = true;
            if (n.contains("major") || n.contains("old") || n.contains("full")) sawMajor = true;
        }
        System.out.printf("  generational?     : %s%n",
                (sawMinor && sawMajor) ? "yes (separate young + old cycles reported)" : "no / single cycle");

        System.out.println();
        System.out.println("=== Memory pools ===");
        for (MemoryPoolMXBean pool : ManagementFactory.getMemoryPoolMXBeans()) {
            System.out.printf("  %-28s type=%-10s max=%s%n",
                    pool.getName(), pool.getType(), mb(pool.getUsage().getMax()));
        }

        System.out.println();
        System.out.println("=== JVM arguments actually in effect ===");
        if (runtime.getInputArguments().isEmpty()) {
            System.out.println("  (none -- everything below is ergonomic)");
        }
        runtime.getInputArguments().forEach(a -> System.out.println("  " + a));

        System.out.println();
        System.out.println("Tip: dump every ergonomic value the JVM chose with");
        System.out.println("  java -XX:+UseZGC -Xms2g -Xmx2g -XX:+PrintFlagsFinal -version");
    }

    private static String mb(long bytes) {
        if (bytes < 0) return "unbounded";
        return String.format("%,d bytes (%.0f MB)", bytes, bytes / 1024.0 / 1024.0);
    }
}