1   /* Copyright 2002-2025 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   * <p>
25   * Note that only full bytes are supported. This means that for example
26   * the 300 bits message from GPS sub-frames must be completed with 4 zero
27   * bits to reach 304 bits = 38 bytes, even if only the first 300 bits
28   * will be decoded and the 4 extra bits in the last byte will be ignored.
29   * </p>
30   * @author Luc Maisonobe
31   * @since 11.0
32   */
33  public abstract class AbstractEncodedMessage implements EncodedMessage {
34  
35      /** Current byte (as an int). */
36      private int current;
37  
38      /** Remaining bits in current byte. */
39      private int remaining;
40  
41      /** Empty constructor.
42       * <p>
43       * This constructor is not strictly necessary, but it prevents spurious
44       * javadoc warnings with JDK 18 and later.
45       * </p>
46       * @since 12.0
47       */
48      public AbstractEncodedMessage() {
49          // nothing to do
50      }
51  
52      /** {@inheritDoc} */
53      @Override
54      public void start() {
55          this.remaining  = 0;
56      }
57  
58      /** Fetch the next byte from the message.
59       * @return next byte from the message, as a primitive integer,
60       * or -1 if end of data has been reached
61       */
62      protected abstract int fetchByte();
63  
64      /** {@inheritDoc} */
65      @Override
66      public long extractBits(final int n) {
67  
68          // safety check
69          if (n > 63) {
70              throw new OrekitException(OrekitMessages.TOO_LARGE_DATA_TYPE, n);
71          }
72  
73          // initialization
74          long value = 0l;
75  
76          // bits gathering loop
77          int needed = n;
78          while (needed > 0) {
79  
80              if (remaining == 0) {
81                  // we need to fetch one more byte
82                  final int read = fetchByte();
83                  if (read == -1) {
84                      // end was unexpected
85                      throw new OrekitException(OrekitMessages.END_OF_ENCODED_MESSAGE);
86                  }
87                  current   = read & 0xFF;
88                  remaining = 8;
89              }
90  
91              final int nbBits = FastMath.min(remaining, needed);
92              value      = (value << nbBits) | (current >>> (8 - nbBits));
93              current    = (current << nbBits) & 0xFF;
94              remaining -= nbBits;
95              needed    -= nbBits;
96  
97          }
98  
99          return value;
100 
101     }
102 
103 }