/*
    * Copyright 2021 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;
  
  import io.netty.util.NetUtil;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.io.BufferedReader;
  import java.io.File;
  import java.io.FileInputStream;
  import java.io.IOException;
  import java.io.InputStreamReader;
  import java.io.Reader;
  import java.net.Inet4Address;
  import java.net.InetAddress;
  import java.nio.charset.Charset;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Locale;
  import java.util.Map;
  import java.util.regex.Pattern;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * A container of hosts file entries
   */
  public final class HostsFileEntriesProvider {
  
      public interface Parser {
  
          /**
           * Parses the hosts file at standard OS location using the system default {@link Charset} for decoding.
           *
           * @return a new {@link HostsFileEntriesProvider}
           * @throws IOException file could not be read
           */
          HostsFileEntriesProvider parse() throws IOException;
  
          /**
           * Parses the hosts file at standard OS location using the given {@link Charset}s one after another until
           * parse something or none is left.
           *
           * @param charsets the {@link Charset}s to try as file encodings when parsing
           * @return a new {@link HostsFileEntriesProvider}
           * @throws IOException file could not be read
           */
          HostsFileEntriesProvider parse(Charset... charsets) throws IOException;
  
          /**
           * Parses the provided hosts file using the given {@link Charset}s one after another until
           * parse something or none is left. In case {@link Charset}s are not provided,
           * the system default {@link Charset} is used for decoding.
           *
           * @param file the file to be parsed
           * @param charsets the {@link Charset}s to try as file encodings when parsing, in case {@link Charset}s
           * are not provided, the system default {@link Charset} is used for decoding
           * @return a new {@link HostsFileEntriesProvider}
           * @throws IOException file could not be read
           */
          HostsFileEntriesProvider parse(File file, Charset... charsets) throws IOException;
  
          /**
           * Performs the parsing operation using the provided reader of hosts file format.
           *
           * @param reader the reader of hosts file format
           * @return a new {@link HostsFileEntriesProvider}
           */
          HostsFileEntriesProvider parse(Reader reader) throws IOException;
  
          /**
           * Parses the hosts file at standard OS location using the system default {@link Charset} for decoding.
           *
           * @return a new {@link HostsFileEntriesProvider}
           */
          HostsFileEntriesProvider parseSilently();
  
          /**
           * Parses the hosts file at standard OS location using the given {@link Charset}s one after another until
           * parse something or none is left.
           *
           * @param charsets the {@link Charset}s to try as file encodings when parsing
          * @return a new {@link HostsFileEntriesProvider}
          */
         HostsFileEntriesProvider parseSilently(Charset... charsets);
 
         /**
          * Parses the provided hosts file using the given {@link Charset}s one after another until
          * parse something or none is left. In case {@link Charset}s are not provided,
          * the system default {@link Charset} is used for decoding.
          *
          * @param file the file to be parsed
          * @param charsets the {@link Charset}s to try as file encodings when parsing, in case {@link Charset}s
          * are not provided, the system default {@link Charset} is used for decoding
          * @return a new {@link HostsFileEntriesProvider}
          */
         HostsFileEntriesProvider parseSilently(File file, Charset... charsets);
     }
 
     /**
      * Creates a parser for {@link HostsFileEntriesProvider}.
      *
      * @return a new {@link HostsFileEntriesProvider.Parser}
      */
     public static Parser parser() {
         return ParserImpl.INSTANCE;
     }
 
     static final HostsFileEntriesProvider EMPTY =
             new HostsFileEntriesProvider(
                     Collections.<String, List<InetAddress>>emptyMap(),
                     Collections.<String, List<InetAddress>>emptyMap());
 
     private final Map<String, List<InetAddress>> ipv4Entries;
     private final Map<String, List<InetAddress>> ipv6Entries;
 
     HostsFileEntriesProvider(Map<String, List<InetAddress>> ipv4Entries, Map<String, List<InetAddress>> ipv6Entries) {
         this.ipv4Entries = Collections.unmodifiableMap(new HashMap<String, List<InetAddress>>(ipv4Entries));
         this.ipv6Entries = Collections.unmodifiableMap(new HashMap<String, List<InetAddress>>(ipv6Entries));
     }
 
     /**
      * The IPv4 entries.
      *
      * @return the IPv4 entries
      */
     public Map<String, List<InetAddress>> ipv4Entries() {
         return ipv4Entries;
     }
 
     /**
      * The IPv6 entries.
      *
      * @return the IPv6 entries
      */
     public Map<String, List<InetAddress>> ipv6Entries() {
         return ipv6Entries;
     }
 
     private static final class ParserImpl implements Parser {
 
         private static final String WINDOWS_DEFAULT_SYSTEM_ROOT = "C:\\Windows";
         private static final String WINDOWS_HOSTS_FILE_RELATIVE_PATH = "\\system32\\drivers\\etc\\hosts";
         private static final String X_PLATFORMS_HOSTS_FILE_PATH = "/etc/hosts";
 
         private static final Pattern WHITESPACES = Pattern.compile("[ \t]+");
 
         private static final InternalLogger logger = InternalLoggerFactory.getInstance(Parser.class);
 
         static final ParserImpl INSTANCE = new ParserImpl();
 
         private ParserImpl() {
             // singleton
         }
 
         @Override
         public HostsFileEntriesProvider parse() throws IOException {
             return parse(locateHostsFile(), Charset.defaultCharset());
         }
 
         @Override
         public HostsFileEntriesProvider parse(Charset... charsets) throws IOException {
             return parse(locateHostsFile(), charsets);
         }
 
         @Override
         public HostsFileEntriesProvider parse(File file, Charset... charsets) throws IOException {
             checkNotNull(file, "file");
             checkNotNull(charsets, "charsets");
             if (charsets.length == 0) {
                 charsets = new Charset[]{Charset.defaultCharset()};
             }
             if (file.exists() && file.isFile()) {
                 for (Charset charset : charsets) {
                     try (BufferedReader reader = new BufferedReader(
                             new InputStreamReader(new FileInputStream(file), charset))) {
                         HostsFileEntriesProvider entries = parse(reader);
                         if (entries != HostsFileEntriesProvider.EMPTY) {
                             return entries;
                         }
                     }
                 }
             }
             return HostsFileEntriesProvider.EMPTY;
         }
 
         @Override
         public HostsFileEntriesProvider parse(Reader reader) throws IOException {
             checkNotNull(reader, "reader");
             try (BufferedReader buff = new BufferedReader(reader)) {
                 Map<String, List<InetAddress>> ipv4Entries = new HashMap<>();
                 Map<String, List<InetAddress>> ipv6Entries = new HashMap<>();
                 String line;
                 while ((line = buff.readLine()) != null) {
                     // remove comment
                     int commentPosition = line.indexOf('#');
                     if (commentPosition != -1) {
                         line = line.substring(0, commentPosition);
                     }
                     // skip empty lines
                     line = line.trim();
                     if (line.isEmpty()) {
                         continue;
                     }
 
                     // split
                     List<String> lineParts = new ArrayList<>();
                     for (String s : WHITESPACES.split(line)) {
                         if (!s.isEmpty()) {
                             lineParts.add(s);
                         }
                     }
 
                     // a valid line should be [IP, hostname, alias*]
                     if (lineParts.size() < 2) {
                         // skip invalid line
                         continue;
                     }
 
                     byte[] ipBytes = NetUtil.createByteArrayFromIpAddressString(lineParts.get(0));
 
                     if (ipBytes == null) {
                         // skip invalid IP
                         continue;
                     }
 
                     // loop over hostname and aliases
                     for (int i = 1; i < lineParts.size(); i++) {
                         String hostname = lineParts.get(i);
                         String hostnameLower = hostname.toLowerCase(Locale.ENGLISH);
                         InetAddress address = InetAddress.getByAddress(hostname, ipBytes);
                         List<InetAddress> addresses;
                         if (address instanceof Inet4Address) {
                             addresses = ipv4Entries.get(hostnameLower);
                             if (addresses == null) {
                                 addresses = new ArrayList<>();
                                 ipv4Entries.put(hostnameLower, addresses);
                             }
                         } else {
                             addresses = ipv6Entries.get(hostnameLower);
                             if (addresses == null) {
                                 addresses = new ArrayList<>();
                                 ipv6Entries.put(hostnameLower, addresses);
                             }
                         }
                         addresses.add(address);
                     }
                 }
                 return ipv4Entries.isEmpty() && ipv6Entries.isEmpty() ?
                         HostsFileEntriesProvider.EMPTY :
                         new HostsFileEntriesProvider(ipv4Entries, ipv6Entries);
             }
         }
 
         @Override
         public HostsFileEntriesProvider parseSilently() {
             return parseSilently(locateHostsFile(), Charset.defaultCharset());
         }
 
         @Override
         public HostsFileEntriesProvider parseSilently(Charset... charsets) {
             return parseSilently(locateHostsFile(), charsets);
         }
 
         @Override
         public HostsFileEntriesProvider parseSilently(File file, Charset... charsets) {
             try {
                 return parse(file, charsets);
             } catch (IOException e) {
                 if (logger.isWarnEnabled()) {
                     logger.warn("Failed to load and parse hosts file at " + file.getPath(), e);
                 }
                 return HostsFileEntriesProvider.EMPTY;
             }
         }
 
         private static File locateHostsFile() {
             File hostsFile;
             if (PlatformDependent.isWindows()) {
                 hostsFile = new File(System.getenv("SystemRoot") + WINDOWS_HOSTS_FILE_RELATIVE_PATH);
                 if (!hostsFile.exists()) {
                     hostsFile = new File(WINDOWS_DEFAULT_SYSTEM_ROOT + WINDOWS_HOSTS_FILE_RELATIVE_PATH);
                 }
             } else {
                 hostsFile = new File(X_PLATFORMS_HOSTS_FILE_PATH);
             }
             return hostsFile;
         }
     }
 }