1   /* Copyright 2002-2022 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.hipparchus.util.FastMath;
20  import org.orekit.errors.OrekitException;
21  import org.orekit.errors.OrekitMessages;
22  
23  /** Encoded messages as a sequence of bytes.
24   * @author Luc Maisonobe
25   * @since 11.0
26   */
27  public abstract class AbstractEncodedMessages implements EncodedMessage {
28  
29      /** Current byte (as an int). */
30      private int current;
31  
32      /** Remaining bits in current byte. */
33      private int remaining;
34  
35      /** {@inheritDoc} */
36      @Override
37      public void start() {
38          this.remaining  = 0;
39      }
40  
41      /** Fetch the next byte from the message.
42       * @return next byte from the message, as a primitive integer,
43       * or -1 if end of data has been reached
44       */
45      protected abstract int fetchByte();
46  
47      /** {@inheritDoc} */
48      @Override
49      public long extractBits(final int n) {
50  
51          // safety check
52          if (n > 63) {
53              throw new OrekitException(OrekitMessages.TOO_LARGE_DATA_TYPE, n);
54          }
55  
56          // initialization
57          long value = 0l;
58  
59          // bits gathering loop
60          int needed = n;
61          while (needed > 0) {
62  
63              if (remaining == 0) {
64                  // we need to fetch one more byte
65                  final int read = fetchByte();
66                  if (read == -1) {
67                      // end was unexpected
68                      throw new OrekitException(OrekitMessages.END_OF_ENCODED_MESSAGE);
69                  }
70                  current   = read & 0xFF;
71                  remaining = 8;
72              }
73  
74              final int nbBits = FastMath.min(remaining, needed);
75              value      = (value << nbBits) | (current >>> (8 - nbBits));
76              current    = (current << nbBits) & 0xFF;
77              remaining -= nbBits;
78              needed    -= nbBits;
79  
80          }
81  
82          return value;
83  
84      }
85  
86  }