1   /* Copyright 2002-2021 CS GROUP
2    * Licensed to CS GROUP (CS) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * CS licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *   http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.orekit.gnss.metric.parser;
18  
19  import org.orekit.errors.OrekitException;
20  import org.orekit.errors.OrekitMessages;
21  
22  /** Encoded messages as a sequence of bytes.
23   * @author Luc Maisonobe
24   * @since 11.0
25   */
26  public abstract class AbstractEncodedMessages implements EncodedMessage {
27  
28      /** Current byte (as an int). */
29      private int current;
30  
31      /** Remaining bits in current byte. */
32      private int remaining;
33  
34      /** {@inheritDoc} */
35      @Override
36      public void start() {
37          this.remaining  = 0;
38      }
39  
40      /** Fetch the next byte from the message.
41       * @return next byte from the message, as a primitive integer,
42       * or -1 if end of data has been reached
43       */
44      protected abstract int fetchByte();
45  
46      /** {@inheritDoc} */
47      @Override
48      public long extractBits(final int n) {
49  
50          // safety check
51          if (n > 63) {
52              throw new OrekitException(OrekitMessages.TOO_LARGE_DATA_TYPE, n);
53          }
54  
55          // initialization
56          long value = 0l;
57  
58          // bits gathering loop
59          int needed = n;
60          while (needed > 0) {
61  
62              if (remaining == 0) {
63                  // we need to fetch one more byte
64                  final int read = fetchByte();
65                  if (read == -1) {
66                      // end was unexpected
67                      throw new OrekitException(OrekitMessages.END_OF_ENCODED_MESSAGE);
68                  }
69                  current   = read & 0xFF;
70                  remaining = 8;
71              }
72  
73              final int nbBits = Math.min(remaining, needed);
74              value      = (value << nbBits) | (current >>> (8 - nbBits));
75              current    = (current << nbBits) & 0xFF;
76              remaining -= nbBits;
77              needed    -= nbBits;
78  
79          }
80  
81          return value;
82  
83      }
84  
85  }