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/*
 * Copyright 2015 The gRPC Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.grpc.internal;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.base.Stopwatch;
import com.google.common.base.Throwables;
import com.google.common.base.Verify;
import io.grpc.Attributes;
import io.grpc.EquivalentAddressGroup;
import io.grpc.NameResolver;
import io.grpc.ProxiedSocketAddress;
import io.grpc.ProxyDetector;
import io.grpc.Status;
import io.grpc.internal.SharedResourceHolder.Resource;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.URI;
import java.net.UnknownHostException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.Nullable;
import javax.annotation.concurrent.GuardedBy;

/**
 * A DNS-based {@link NameResolver}.
 *
 * 
Each {@code A} or {@code AAAA} record emits an {@link EquivalentAddressGroup} in the list
 * passed to {@link NameResolver.Listener#onAddresses(List, Attributes)}
 *
 * @see DnsNameResolverProvider
 */
final class DnsNameResolver extends NameResolver {

  private static final Logger logger = Logger.getLogger(DnsNameResolver.class.getName());

  private static final String SERVICE_CONFIG_CHOICE_CLIENT_LANGUAGE_KEY = "clientLanguage";
  private static final String SERVICE_CONFIG_CHOICE_PERCENTAGE_KEY = "percentage";
  private static final String SERVICE_CONFIG_CHOICE_CLIENT_HOSTNAME_KEY = "clientHostname";
  private static final String SERVICE_CONFIG_CHOICE_SERVICE_CONFIG_KEY = "serviceConfig";

  // From https://github.com/grpc/proposal/blob/master/A2-service-configs-in-dns.md
  static final String SERVICE_CONFIG_PREFIX = "grpc_config=";
  private static final Set SERVICE_CONFIG_CHOICE_KEYS =
      Collections.unmodifiableSet(
          new HashSet<>(
              Arrays.asList(
                  SERVICE_CONFIG_CHOICE_CLIENT_LANGUAGE_KEY,
                  SERVICE_CONFIG_CHOICE_PERCENTAGE_KEY,
                  SERVICE_CONFIG_CHOICE_CLIENT_HOSTNAME_KEY,
                  SERVICE_CONFIG_CHOICE_SERVICE_CONFIG_KEY)));

  // From https://github.com/grpc/proposal/blob/master/A2-service-configs-in-dns.md
  private static final String SERVICE_CONFIG_NAME_PREFIX = "_grpc_config.";
  // From https://github.com/grpc/proposal/blob/master/A5-grpclb-in-dns.md
  private static final String GRPCLB_NAME_PREFIX = "_grpclb._tcp.";

  private static final String JNDI_PROPERTY =
      System.getProperty("io.grpc.internal.DnsNameResolverProvider.enable_jndi", "true");
  private static final String JNDI_LOCALHOST_PROPERTY =
      System.getProperty("io.grpc.internal.DnsNameResolverProvider.enable_jndi_localhost", "false");
  private static final String JNDI_SRV_PROPERTY =
      System.getProperty("io.grpc.internal.DnsNameResolverProvider.enable_grpclb", "false");
  private static final String JNDI_TXT_PROPERTY =
      System.getProperty("io.grpc.internal.DnsNameResolverProvider.enable_service_config", "false");

  /**
   * Java networking system properties name for caching DNS result.
   *
   * 
Default value is -1 (cache forever) if security manager is installed. If security manager is
   * not installed, the ttl value is {@code null} which falls back to {@link
   * #DEFAULT_NETWORK_CACHE_TTL_SECONDS gRPC default value}.
   *
   * 
For android, gRPC doesn't attempt to cache; this property value will be ignored.
   */
  @VisibleForTesting
  static final String NETWORKADDRESS_CACHE_TTL_PROPERTY = "networkaddress.cache.ttl";
  /** Default DNS cache duration if network cache ttl value is not specified ({@code null}). */
  @VisibleForTesting
  static final long DEFAULT_NETWORK_CACHE_TTL_SECONDS = 30;

  @VisibleForTesting
  static boolean enableJndi = Boolean.parseBoolean(JNDI_PROPERTY);
  @VisibleForTesting
  static boolean enableJndiLocalhost = Boolean.parseBoolean(JNDI_LOCALHOST_PROPERTY);
  @VisibleForTesting
  static boolean enableSrv = Boolean.parseBoolean(JNDI_SRV_PROPERTY);
  @VisibleForTesting
  static boolean enableTxt = Boolean.parseBoolean(JNDI_TXT_PROPERTY);

  private static final ResourceResolverFactory resourceResolverFactory =
      getResourceResolverFactory(DnsNameResolver.class.getClassLoader());

  @VisibleForTesting
  final ProxyDetector proxyDetector;

  /** Access through {@link #getLocalHostname}. */
  private static String localHostname;

  private final Random random = new Random();

  private volatile AddressResolver addressResolver = JdkAddressResolver.INSTANCE;
  private final AtomicReference resourceResolver = new AtomicReference<>();

  private final String authority;
  private final String host;
  private final int port;
  private final Resource executorResource;
  @GuardedBy("this")
  private boolean shutdown;
  @GuardedBy("this")
  private Executor executor;
  @GuardedBy("this")
  private boolean resolving;
  @GuardedBy("this")
  private Listener listener;

  private final Runnable resolveRunnable;

  DnsNameResolver(@Nullable String nsAuthority, String name, Helper helper,
      Resource executorResource, Stopwatch stopwatch, boolean isAndroid) {
    // TODO: if a DNS server is provided as nsAuthority, use it.
    // https://www.captechconsulting.com/blogs/accessing-the-dusty-corners-of-dns-with-java
    this.executorResource = executorResource;
    // Must prepend a "//" to the name when constructing a URI, otherwise it will be treated as an
    // opaque URI, thus the authority and host of the resulted URI would be null.
    URI nameUri = URI.create("//" + checkNotNull(name, "name"));
    Preconditions.checkArgument(nameUri.getHost() != null, "Invalid DNS name: %s", name);
    authority = Preconditions.checkNotNull(nameUri.getAuthority(),
        "nameUri (%s) doesn't have an authority", nameUri);
    host = nameUri.getHost();
    if (nameUri.getPort() == -1) {
      port = helper.getDefaultPort();
    } else {
      port = nameUri.getPort();
    }
    this.proxyDetector = Preconditions.checkNotNull(helper.getProxyDetector(), "proxyDetector");
    this.resolveRunnable = new Resolve(this, stopwatch, getNetworkAddressCacheTtlNanos(isAndroid));
  }

  @Override
  public final String getServiceAuthority() {
    return authority;
  }

  @Override
  public final synchronized void start(Listener listener) {
    Preconditions.checkState(this.listener == null, "already started");
    executor = SharedResourceHolder.get(executorResource);
    this.listener = Preconditions.checkNotNull(listener, "listener");
    resolve();
  }

  @Override
  public final synchronized void refresh() {
    Preconditions.checkState(listener != null, "not started");
    resolve();
  }

  @VisibleForTesting
  static final class Resolve implements Runnable {

    private final DnsNameResolver resolver;
    private final Stopwatch stopwatch;
    private final long cacheTtlNanos;
    private ResolutionResults cachedResolutionResults = null;

    Resolve(DnsNameResolver resolver, Stopwatch stopwatch, long cacheTtlNanos) {
      this.resolver = resolver;
      this.stopwatch = Preconditions.checkNotNull(stopwatch, "stopwatch");
      this.cacheTtlNanos = cacheTtlNanos;
    }

    @Override
    public void run() {
      if (logger.isLoggable(Level.FINER)) {
        logger.finer("Attempting DNS resolution of " + resolver.host);
      }
      Listener savedListener;
      synchronized (resolver) {
        if (resolver.shutdown || !cacheRefreshRequired()) {
          return;
        }
        savedListener = resolver.listener;
        resolver.resolving = true;
      }
      try {
        resolveInternal(savedListener);
      } finally {
        synchronized (resolver) {
          resolver.resolving = false;
        }
      }
    }

    private boolean cacheRefreshRequired() {
      return cachedResolutionResults == null
          || cacheTtlNanos == 0
          || (cacheTtlNanos > 0 && stopwatch.elapsed(TimeUnit.NANOSECONDS) > cacheTtlNanos);
    }

    @VisibleForTesting
    void resolveInternal(Listener savedListener) {
      InetSocketAddress destination =
          InetSocketAddress.createUnresolved(resolver.host, resolver.port);
      ProxiedSocketAddress proxiedAddr;
      try {
        proxiedAddr = resolver.proxyDetector.proxyFor(destination);
      } catch (IOException e) {
        savedListener.onError(
            Status.UNAVAILABLE.withDescription("Unable to resolve host " + resolver.host)
                .withCause(e));
        return;
      }
      if (proxiedAddr != null) {
        if (logger.isLoggable(Level.FINER)) {
          logger.finer("Using proxy address " + proxiedAddr);
        }
        EquivalentAddressGroup server = new EquivalentAddressGroup(proxiedAddr);
        savedListener.onAddresses(Collections.singletonList(server), Attributes.EMPTY);
        return;
      }

      ResolutionResults resolutionResults;
      try {
        ResourceResolver resourceResolver = null;
        if (shouldUseJndi(enableJndi, enableJndiLocalhost, resolver.host)) {
          resourceResolver = resolver.getResourceResolver();
        }
        resolutionResults = resolveAll(
            resolver.addressResolver,
            resourceResolver,
            enableSrv,
            enableTxt,
            resolver.host);
        cachedResolutionResults = resolutionResults;
        if (cacheTtlNanos > 0) {
          stopwatch.reset().start();
        }
        if (logger.isLoggable(Level.FINER)) {
          logger.finer("Found DNS results " + resolutionResults + " for " + resolver.host);
        }
      } catch (Exception e) {
        savedListener.onError(
            Status.UNAVAILABLE.withDescription("Unable to resolve host " + resolver.host)
                .withCause(e));
        return;
      }
      // Each address forms an EAG
      List servers = new ArrayList<>();
      for (InetAddress inetAddr : resolutionResults.addresses) {
        servers.add(new EquivalentAddressGroup(new InetSocketAddress(inetAddr, resolver.port)));
      }
      servers.addAll(resolutionResults.balancerAddresses);
      if (servers.isEmpty()) {
        savedListener.onError(Status.UNAVAILABLE.withDescription(
            "No DNS backend or balancer addresses found for " + resolver.host));
        return;
      }

      Attributes.Builder attrs = Attributes.newBuilder();
      if (!resolutionResults.txtRecords.isEmpty()) {
        Map serviceConfig = null;
        try {
          for (Map possibleConfig :
              parseTxtResults(resolutionResults.txtRecords)) {
            try {
              serviceConfig =
                  maybeChooseServiceConfig(possibleConfig, resolver.random, getLocalHostname());
            } catch (RuntimeException e) {
              logger.log(Level.WARNING, "Bad service config choice " + possibleConfig, e);
            }
            if (serviceConfig != null) {
              break;
            }
          }
        } catch (RuntimeException e) {
          logger.log(Level.WARNING, "Can't parse service Configs", e);
        }
        if (serviceConfig != null) {
          attrs.set(GrpcAttributes.NAME_RESOLVER_SERVICE_CONFIG, serviceConfig);
        }
      } else {
        logger.log(Level.FINE, "No TXT records found for {0}", new Object[]{resolver.host});
      }
      savedListener.onAddresses(servers, attrs.build());
    }
  }

  @GuardedBy("this")
  private void resolve() {
    if (resolving || shutdown) {
      return;
    }
    executor.execute(resolveRunnable);
  }

  @Override
  public final synchronized void shutdown() {
    if (shutdown) {
      return;
    }
    shutdown = true;
    if (executor != null) {
      executor = SharedResourceHolder.release(executorResource, executor);
    }
  }

  final int getPort() {
    return port;
  }

  @VisibleForTesting
  static ResolutionResults resolveAll(
      AddressResolver addressResolver,
      @Nullable ResourceResolver resourceResolver,
      boolean requestSrvRecords,
      boolean requestTxtRecords,
      String name) {
    List addresses = Collections.emptyList();
    Exception addressesException = null;
    List balancerAddresses = Collections.emptyList();
    Exception balancerAddressesException = null;
    List txtRecords = Collections.emptyList();
    Exception txtRecordsException = null;

    try {
      addresses = addressResolver.resolveAddress(name);
    } catch (Exception e) {
      addressesException = e;
    }
    if (resourceResolver != null) {
      if (requestSrvRecords) {
        try {
          balancerAddresses =
              resourceResolver.resolveSrv(addressResolver, GRPCLB_NAME_PREFIX + name);
        } catch (Exception e) {
          balancerAddressesException = e;
        }
      }
      if (requestTxtRecords) {
        boolean balancerLookupFailedOrNotAttempted =
            !requestSrvRecords || balancerAddressesException != null;
        boolean dontResolveTxt =
            (addressesException != null) && balancerLookupFailedOrNotAttempted;
        // Only do the TXT record lookup if one of the above address resolutions succeeded.
        if (!dontResolveTxt) {
          try {
            txtRecords = resourceResolver.resolveTxt(SERVICE_CONFIG_NAME_PREFIX + name);
          } catch (Exception e) {
            txtRecordsException = e;
          }
        }
      }
    }
    try {
      if (addressesException != null
          && (balancerAddressesException != null || balancerAddresses.isEmpty())) {
        Throwables.throwIfUnchecked(addressesException);
        throw new RuntimeException(addressesException);
      }
    } finally {
      if (addressesException != null) {
        logger.log(Level.FINE, "Address resolution failure", addressesException);
      }
      if (balancerAddressesException != null) {
        logger.log(Level.FINE, "Balancer resolution failure", balancerAddressesException);
      }
      if (txtRecordsException != null) {
        logger.log(Level.FINE, "ServiceConfig resolution failure", txtRecordsException);
      }
    }
    return new ResolutionResults(addresses, txtRecords, balancerAddresses);
  }

  @SuppressWarnings("unchecked")
  @VisibleForTesting
  static List> parseTxtResults(List txtRecords) {
    List> serviceConfigs = new ArrayList<>();
    for (String txtRecord : txtRecords) {
      if (txtRecord.startsWith(SERVICE_CONFIG_PREFIX)) {
        List> choices;
        try {
          Object rawChoices = JsonParser.parse(txtRecord.substring(SERVICE_CONFIG_PREFIX.length()));
          if (!(rawChoices instanceof List)) {
            throw new IOException("wrong type " + rawChoices);
          }
          List listChoices = (List) rawChoices;
          for (Object obj : listChoices) {
            if (!(obj instanceof Map)) {
              throw new IOException("wrong element type " + rawChoices);
            }
          }
          choices = (List>) (List) listChoices;
        } catch (IOException e) {
          logger.log(Level.WARNING, "Bad service config: " + txtRecord, e);
          continue;
        }
        serviceConfigs.addAll(choices);
      } else {
        logger.log(Level.FINE, "Ignoring non service config {0}", new Object[]{txtRecord});
      }
    }
    return serviceConfigs;
  }

  @Nullable
  private static final Double getPercentageFromChoice(
      Map serviceConfigChoice) {
    if (!serviceConfigChoice.containsKey(SERVICE_CONFIG_CHOICE_PERCENTAGE_KEY)) {
      return null;
    }
    return ServiceConfigUtil.getDouble(serviceConfigChoice, SERVICE_CONFIG_CHOICE_PERCENTAGE_KEY);
  }

  @Nullable
  private static final List getClientLanguagesFromChoice(
      Map serviceConfigChoice) {
    if (!serviceConfigChoice.containsKey(SERVICE_CONFIG_CHOICE_CLIENT_LANGUAGE_KEY)) {
      return null;
    }
    return ServiceConfigUtil.checkStringList(
        ServiceConfigUtil.getList(serviceConfigChoice, SERVICE_CONFIG_CHOICE_CLIENT_LANGUAGE_KEY));
  }

  @Nullable
  private static final List getHostnamesFromChoice(
      Map serviceConfigChoice) {
    if (!serviceConfigChoice.containsKey(SERVICE_CONFIG_CHOICE_CLIENT_HOSTNAME_KEY)) {
      return null;
    }
    return ServiceConfigUtil.checkStringList(
        ServiceConfigUtil.getList(serviceConfigChoice, SERVICE_CONFIG_CHOICE_CLIENT_HOSTNAME_KEY));
  }

  /**
   * Returns value of network address cache ttl property if not Android environment. For android,
   * DnsNameResolver does not cache the dns lookup result.
   */
  private static long getNetworkAddressCacheTtlNanos(boolean isAndroid) {
    if (isAndroid) {
      // on Android, ignore dns cache.
      return 0;
    }

    String cacheTtlPropertyValue = System.getProperty(NETWORKADDRESS_CACHE_TTL_PROPERTY);
    long cacheTtl = DEFAULT_NETWORK_CACHE_TTL_SECONDS;
    if (cacheTtlPropertyValue != null) {
      try {
        cacheTtl = Long.parseLong(cacheTtlPropertyValue);
      } catch (NumberFormatException e) {
        logger.log(
            Level.WARNING,
            "Property({0}) valid is not valid number format({1}), fall back to default({2})",
            new Object[] {NETWORKADDRESS_CACHE_TTL_PROPERTY, cacheTtlPropertyValue, cacheTtl});
      }
    }
    return cacheTtl > 0 ? TimeUnit.SECONDS.toNanos(cacheTtl) : cacheTtl;
  }

  /**
   * Determines if a given Service Config choice applies, and if so, returns it.
   *
   * @see 
   *   Service Config in DNS
   * @param choice The service config choice.
   * @return The service config object or {@code null} if this choice does not apply.
   */
  @Nullable
  @VisibleForTesting
  static Map maybeChooseServiceConfig(
      Map choice, Random random, String hostname) {
    for (Entry entry : choice.entrySet()) {
      Verify.verify(SERVICE_CONFIG_CHOICE_KEYS.contains(entry.getKey()), "Bad key: %s", entry);
    }

    List clientLanguages = getClientLanguagesFromChoice(choice);
    if (clientLanguages != null && !clientLanguages.isEmpty()) {
      boolean javaPresent = false;
      for (String lang : clientLanguages) {
        if ("java".equalsIgnoreCase(lang)) {
          javaPresent = true;
          break;
        }
      }
      if (!javaPresent) {
        return null;
      }
    }
    Double percentage = getPercentageFromChoice(choice);
    if (percentage != null) {
      int pct = percentage.intValue();
      Verify.verify(pct >= 0 && pct <= 100, "Bad percentage: %s", percentage);
      if (random.nextInt(100) >= pct) {
        return null;
      }
    }
    List clientHostnames = getHostnamesFromChoice(choice);
    if (clientHostnames != null && !clientHostnames.isEmpty()) {
      boolean hostnamePresent = false;
      for (String clientHostname : clientHostnames) {
        if (clientHostname.equals(hostname)) {
          hostnamePresent = true;
          break;
        }
      }
      if (!hostnamePresent) {
        return null;
      }
    }
    return ServiceConfigUtil.getObject(choice, SERVICE_CONFIG_CHOICE_SERVICE_CONFIG_KEY);
  }

  /**
   * Describes the results from a DNS query.
   */
  @VisibleForTesting
  static final class ResolutionResults {
    final List addresses;
    final List txtRecords;
    final List balancerAddresses;

    ResolutionResults(
        List addresses,
        List txtRecords,
        List balancerAddresses) {
      this.addresses = Collections.unmodifiableList(checkNotNull(addresses, "addresses"));
      this.txtRecords = Collections.unmodifiableList(checkNotNull(txtRecords, "txtRecords"));
      this.balancerAddresses =
          Collections.unmodifiableList(checkNotNull(balancerAddresses, "balancerAddresses"));
    }

    @Override
    public String toString() {
      return MoreObjects.toStringHelper(this)
          .add("addresses", addresses)
          .add("txtRecords", txtRecords)
          .add("balancerAddresses", balancerAddresses)
          .toString();
    }
  }

  @VisibleForTesting
  void setAddressResolver(AddressResolver addressResolver) {
    this.addressResolver = addressResolver;
  }

  @VisibleForTesting
  void setResourceResolver(ResourceResolver resourceResolver) {
    this.resourceResolver.set(resourceResolver);
  }

  /**
   * {@link ResourceResolverFactory} is a factory for making resource resolvers.  It supports
   * optionally checking if the factory is available.
   */
  interface ResourceResolverFactory {

    /**
     * Creates a new resource resolver.  The return value is {@code null} iff
     * {@link #unavailabilityCause()} is not null;
     */
    @Nullable ResourceResolver newResourceResolver();

    /**
     * Returns the reason why the resource resolver cannot be created.  The return value is
     * {@code null} if {@link #newResourceResolver()} is suitable for use.
     */
    @Nullable Throwable unavailabilityCause();
  }

  /**
   * AddressResolver resolves a hostname into a list of addresses.
   */
  interface AddressResolver {
    List resolveAddress(String host) throws Exception;
  }

  private enum JdkAddressResolver implements AddressResolver {
    INSTANCE;

    @Override
    public List resolveAddress(String host) throws UnknownHostException {
      return Collections.unmodifiableList(Arrays.asList(InetAddress.getAllByName(host)));
    }
  }

  /**
   * {@link ResourceResolver} is a Dns ResourceRecord resolver.
   */
  interface ResourceResolver {
    List resolveTxt(String host) throws Exception;

    List resolveSrv(
        AddressResolver addressResolver, String host) throws Exception;
  }

  @Nullable
  private ResourceResolver getResourceResolver() {
    ResourceResolver rr;
    if ((rr = resourceResolver.get()) == null) {
      if (resourceResolverFactory != null) {
        assert resourceResolverFactory.unavailabilityCause() == null;
        rr = resourceResolverFactory.newResourceResolver();
      }
    }
    return rr;
  }

  @Nullable
  @VisibleForTesting
  static ResourceResolverFactory getResourceResolverFactory(ClassLoader loader) {
    Class jndiClazz;
    try {
      jndiClazz =
          Class.forName("io.grpc.internal.JndiResourceResolverFactory", true, loader)
              .asSubclass(ResourceResolverFactory.class);
    } catch (ClassNotFoundException e) {
      logger.log(Level.FINE, "Unable to find JndiResourceResolverFactory, skipping.", e);
      return null;
    }
    Constructor jndiCtor;
    try {
      jndiCtor = jndiClazz.getConstructor();
    } catch (Exception e) {
      logger.log(Level.FINE, "Can't find JndiResourceResolverFactory ctor, skipping.", e);
      return null;
    }
    ResourceResolverFactory rrf;
    try {
      rrf = jndiCtor.newInstance();
    } catch (Exception e) {
      logger.log(Level.FINE, "Can't construct JndiResourceResolverFactory, skipping.", e);
      return null;
    }
    if (rrf.unavailabilityCause() != null) {
      logger.log(
          Level.FINE,
          "JndiResourceResolverFactory not available, skipping.",
          rrf.unavailabilityCause());
    }
    return rrf;
  }

  private static String getLocalHostname() {
    if (localHostname == null) {
      try {
        localHostname = InetAddress.getLocalHost().getHostName();
      } catch (UnknownHostException e) {
        throw new RuntimeException(e);
      }
    }
    return localHostname;
  }

  @VisibleForTesting
  static boolean shouldUseJndi(boolean jndiEnabled, boolean jndiLocalhostEnabled, String target) {
    if (!jndiEnabled) {
      return false;
    }
    if ("localhost".equalsIgnoreCase(target)) {
      return jndiLocalhostEnabled;
    }
    // Check if this name looks like IPv6
    if (target.contains(":")) {
      return false;
    }
    // Check if this might be IPv4.  Such addresses have no alphabetic characters.  This also
    // checks the target is empty.
    boolean alldigits = true;
    for (int i = 0; i < target.length(); i++) {
      char c = target.charAt(i);
      if (c != '.') {
        alldigits &= (c >= '0' && c <= '9');
      }
    }
    return !alldigits;
  }
}
    

    

    

                
 
            

    

                
 
            



    
    






    
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