/*
    * Copyright 2016 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.smtp;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.handler.codec.DecoderException;
  import io.netty.handler.codec.LineBasedFrameDecoder;
  import io.netty.handler.codec.TooLongFrameException;
  import io.netty.util.CharsetUtil;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.UnstableApi;
  
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  
  /**
   * Decoder for SMTP responses.
   */
  @UnstableApi
  public final class SmtpResponseDecoder extends LineBasedFrameDecoder {
  
      private static final int DEFAULT_MAX_RESPONSE_SIZE = 64 * 1024;
  
      // Charged per accumulated line so the limit bounds retained heap rather than payload bytes alone.
      private static final int DETAIL_ENTRY_OVERHEAD = 48;
  
      private final int maxResponseSize;
  
      private List<CharSequence> details;
      private long responseSize;
  
      /**
       * Creates a new instance that enforces the given {@code maxLineLength} and a default limit of 64 KiB on the
       * accumulated size of a multi-line response.
       *
       * @param maxLineLength the maximum length of a single response line, in bytes.
       */
      public SmtpResponseDecoder(int maxLineLength) {
          this(maxLineLength, DEFAULT_MAX_RESPONSE_SIZE);
      }
  
      /**
       * Creates a new instance that enforces the given {@code maxLineLength} and {@code maxResponseSize}.
       * <p>
       * The lines of a multi-line response are buffered until its terminating line arrives.
       * {@code maxResponseSize} bounds that buffering. Once it is exceeded, decoding fails with a
       * {@link TooLongFrameException}. The limit is approximate and tracks retained memory rather than bytes
       * received, as it charges each buffered line a fixed object overhead in addition to its detail bytes, and
       * ignores the response code, the separator and the terminating line.
       *
       * @param maxLineLength   the maximum length of a single response line, in bytes.
       * @param maxResponseSize the maximum accumulated size of a multi-line response, in bytes.
       */
      public SmtpResponseDecoder(int maxLineLength, int maxResponseSize) {
          super(maxLineLength);
          this.maxResponseSize = ObjectUtil.checkPositive(maxResponseSize, "maxResponseSize");
      }
  
      @Override
      protected SmtpResponse decode(ChannelHandlerContext ctx, ByteBuf buffer) throws Exception {
          ByteBuf frame = (ByteBuf) super.decode(ctx, buffer);
          if (frame == null) {
              // No full line received yet.
              return null;
          }
          try {
              final int readable = frame.readableBytes();
              final int readerIndex = frame.readerIndex();
              if (readable < 4) {
                  reset();
                  throw newDecoderException(buffer, readerIndex, readable);
              }
              final int code = parseCode(frame);
              if (code < 0) {
                  reset();
                  throw newDecoderException(buffer, readerIndex, readable);
              }
              final int separator = frame.readByte();
              final CharSequence detail = frame.isReadable() ? frame.toString(CharsetUtil.US_ASCII) : null;
  
              List<CharSequence> details = this.details;
  
              switch (separator) {
              case ' ':
                 // Marks the end of a response.
                 reset();
                 if (details != null) {
                     if (detail != null) {
                         details.add(detail);
                     }
                 } else {
                     if (detail == null) {
                         details = Collections.emptyList();
                     } else {
                         details = Collections.singletonList(detail);
                     }
                 }
                 return new DefaultSmtpResponse(code, details);
             case '-':
                 // Multi-line response.
                 if (detail != null) {
                     responseSize += (long) detail.length() + DETAIL_ENTRY_OVERHEAD;
                     if (responseSize > maxResponseSize) {
                         reset();
                         throw new TooLongFrameException("SMTP response exceeds " + maxResponseSize + " bytes");
                     }
                     if (details == null) {
                         // Using initial capacity as it is very unlikely that we will receive a multi-line response
                         // with more then 3 lines.
                         this.details = details = new ArrayList<CharSequence>(4);
                     }
                     details.add(detail);
                 }
                 break;
             default:
                 reset();
                 throw newDecoderException(buffer, readerIndex, readable);
             }
         } finally {
             frame.release();
         }
         return null;
     }
 
     private void reset() {
         this.details = null;
         this.responseSize = 0;
     }
 
     private static DecoderException newDecoderException(ByteBuf buffer, int readerIndex, int readable) {
         return new DecoderException(
                 "Received invalid line: '" + buffer.toString(readerIndex, readable, CharsetUtil.US_ASCII) + '\'');
     }
 
     /**
      * Parses the io.netty.handler.codec.smtp code without any allocation, which is three digits.
      *
      * @return the code, or {@code -1} if one of the three bytes is not a digit.
      */
     private static int parseCode(ByteBuf buffer) {
         final int first = parseNumber(buffer.readByte());
         final int second = parseNumber(buffer.readByte());
         final int third = parseNumber(buffer.readByte());
         if (first < 0 || second < 0 || third < 0) {
             return -1;
         }
         return first * 100 + second * 10 + third;
     }
 
     private static int parseNumber(byte b) {
         return Character.digit((char) b, 10);
     }
 }