/*
    * Copyright 2024 The Netty Project
    *
    * The Netty Project licenses this file to you 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:
    *
    *   https://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.netty.resolver.dns;
  
  import io.netty.util.internal.BoundedInputStream;
  
  import java.io.BufferedReader;
  import java.io.FileInputStream;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.net.InetSocketAddress;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  
  /**
   * Looks up the {@code nameserver}s from the {@code /etc/resolv.conf} file, intended for Linux and macOS.
   */
  final class ResolvConf {
      private final List<InetSocketAddress> nameservers;
  
      /**
       * Reads from the given reader and extracts the {@code nameserver}s using the syntax of the
       * {@code /etc/resolv.conf} file, see {@code man resolv.conf}.
       *
       * @param reader contents of {@code resolv.conf} are read from this {@link BufferedReader},
       *              up to the caller to close it
       */
      static ResolvConf fromReader(BufferedReader reader) throws IOException {
          return new ResolvConf(reader);
      }
  
      /**
       * Reads the given file and extracts the {@code nameserver}s using the syntax of the
       * {@code /etc/resolv.conf} file, see {@code man resolv.conf}.
       */
      static ResolvConf fromFile(String file) throws IOException {
          // Use 1 MB to be a bit conservative
          try (BufferedReader reader = new BufferedReader(new InputStreamReader(
                  new BoundedInputStream(new FileInputStream(file), 1024 * 1024)))) {
              return fromReader(reader);
          }
      }
  
      /**
       * Returns the {@code nameserver}s from the {@code /etc/resolv.conf} file. The file is only read once
       * during the lifetime of this class.
       */
      static ResolvConf system() {
          ResolvConf resolvConv = ResolvConfLazy.machineResolvConf;
          if (resolvConv != null) {
              return resolvConv;
          }
          throw new IllegalStateException("/etc/resolv.conf could not be read");
      }
  
      private ResolvConf(BufferedReader reader) throws IOException {
          List<InetSocketAddress> nameservers = new ArrayList<>();
          String ln;
          while ((ln = reader.readLine()) != null) {
              ln = ln.trim();
              if (ln.isEmpty()) {
                  continue;
              }
  
              if (ln.startsWith("nameserver")) {
                  ln = ln.substring("nameserver".length());
                  int cIndex = ln.indexOf('#');
                  if (cIndex != -1) {
                      ln = ln.substring(0, cIndex);
                  }
                  ln = ln.trim();
                  if (ln.isEmpty()) {
                      continue;
                  }
                  nameservers.add(new InetSocketAddress(ln, 53));
              }
          }
          this.nameservers = Collections.unmodifiableList(nameservers);
      }
  
      List<InetSocketAddress> getNameservers() {
          return nameservers;
      }
  
      private static final class ResolvConfLazy {
         static final ResolvConf machineResolvConf;
 
         static {
             ResolvConf resolvConf;
             try {
                 resolvConf = ResolvConf.fromFile("/etc/resolv.conf");
             } catch (IOException | SecurityException e) {
                 resolvConf = null;
             }
             machineResolvConf = resolvConf;
         }
     }
 }