/*
    * Copyright 2019 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.handler.codec.dns;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  import io.netty.handler.codec.CorruptedFrameException;
  import io.netty.util.CharsetUtil;
  
  import static io.netty.handler.codec.dns.DefaultDnsRecordDecoder.*;
  
  final class DnsCodecUtil {
      private DnsCodecUtil() {
          // Util class
      }
  
      static void encodeDomainName(String name, ByteBuf buf) {
          if (ROOT.equals(name)) {
              // Root domain
              buf.writeByte(0);
              return;
          }
  
          final String[] labels = name.split("\\.");
          for (String label : labels) {
              final int labelLen = label.length();
              if (labelLen == 0) {
                  // zero-length label means the end of the name.
                  break;
              }
  
              buf.writeByte(labelLen);
              ByteBufUtil.writeAscii(buf, label);
          }
  
          buf.writeByte(0); // marks end of name field
      }
  
      static String decodeDomainName(ByteBuf in) {
          int position = -1;
          int checked = 0;
          final int end = in.writerIndex();
          final int readable = in.readableBytes();
  
          // Looking at the spec we should always have at least enough readable bytes to read a byte here but it seems
          // some servers do not respect this for empty names. So just workaround this and return an empty name in this
          // case.
          //
          // See:
          // - https://github.com/netty/netty/issues/5014
          // - https://www.ietf.org/rfc/rfc1035.txt , Section 3.1
          if (readable == 0) {
              return ROOT;
          }
  
          final StringBuilder name = new StringBuilder(readable << 1);
          while (in.isReadable()) {
              final int len = in.readUnsignedByte();
              final boolean pointer = (len & 0xc0) == 0xc0;
              if (pointer) {
                  if (position == -1) {
                      position = in.readerIndex() + 1;
                  }
  
                  if (!in.isReadable()) {
                      throw new CorruptedFrameException("truncated pointer in a name");
                  }
  
                  final int next = (len & 0x3f) << 8 | in.readUnsignedByte();
                  if (next >= end) {
                      throw new CorruptedFrameException("name has an out-of-range pointer");
                  }
                  in.readerIndex(next);
  
                  // check for loops
                  checked += 2;
                  if (checked >= end) {
                      throw new CorruptedFrameException("name contains a loop.");
                  }
              } else if (len != 0) {
                  if (!in.isReadable(len)) {
                      throw new CorruptedFrameException("truncated label in a name");
                  }
                  name.append(in.toString(in.readerIndex(), len, CharsetUtil.UTF_8)).append('.');
                  in.skipBytes(len);
             } else { // len == 0
                 break;
             }
         }
 
         if (position != -1) {
             in.readerIndex(position);
         }
 
         if (name.length() == 0) {
             return ROOT;
         }
 
         if (name.charAt(name.length() - 1) != '.') {
             name.append('.');
         }
 
         return name.toString();
     }
 
     /**
      * Decompress pointer data.
      * @param compression compressed data
      * @return decompressed data
      */
     static ByteBuf decompressDomainName(ByteBuf compression) {
         String domainName = decodeDomainName(compression);
         ByteBuf result = compression.alloc().buffer(domainName.length() << 1);
         encodeDomainName(domainName, result);
         return result;
     }
 }