tesseract  5.0.0
qrsequence.h
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1 // File: qrsequence.h
3 // Description: Quasi-random sequence generator class.
4 // Author: Ranjith Unnikrishnan
5 //
6 // Class to generate a (deterministic) quasi-random Van der Corput sequence that
7 // covers the interval [0,N) without repetition.
8 //
9 // The sequence is generated by reversing the base-2 representation of the
10 // sequence of natural numbers {0, 1,... M-1}, where M is 2^{num_bits_} and
11 // num_bits is the minimum number of bits required to represent N. If a reversed
12 // numbers is >= N it is rejected and the next natural number is considered
13 // until a valid output number is found.
14 //
15 // (C) Copyright 2009, Google Inc.
16 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
17 // use this file except in compliance with the License. You may obtain a copy
18 // of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required
19 // by applicable law or agreed to in writing, software distributed under the
20 // License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
21 // OF ANY KIND, either express or implied. See the License for the specific
22 // language governing permissions and limitations under the License.
23 //
25 
26 #ifndef TESSERACT_CCUTIL_QRSEQUENCE_H_
27 #define TESSERACT_CCUTIL_QRSEQUENCE_H_
28 
29 #include <cmath>
30 
32 public:
33  // Object is initialized with the size of the output range.
34  explicit QRSequenceGenerator(int N) : N_(N), next_num_(0) {
35  num_bits_ = static_cast<int>(ceil(log(static_cast<double>(N)) / log(2.0)));
36  }
37 
38  // Main worker method that retrieves the next number in the sequence.
39  // Returns kInvalidVal if called more than N times after object initialization
40  int GetVal() {
41  const int kInvalidVal = -1;
42  const int kMaxNaturalNumberValue = 1 << num_bits_;
43  if (next_num_ >= kMaxNaturalNumberValue) {
44  return kInvalidVal;
45  }
46  int n = next_num_;
47 
48  while (next_num_ < kMaxNaturalNumberValue) {
50  if (n < N_) {
51  break;
52  }
53  }
54  return (next_num_ > kMaxNaturalNumberValue) ? kInvalidVal : n;
55  }
56 
57 protected:
58  // Outputs the integer formed by reversing the bits of the input integer. Only
59  // the lowest num_bits_ bits of the input integer are reversed.
60  int GetBinaryReversedInteger(int in_val) const {
61  int bit_pos = num_bits_;
62  int out_val = 0;
63  while (bit_pos--) {
64  // Set the value of the last bit.
65  out_val |= (in_val & 0x1);
66  if (bit_pos > 0) {
67  // Left-shift output value to prepare for storing the next bit.
68  out_val <<= 1;
69  }
70  // Right-shift input value to prepare for retrieving the next bit.
71  in_val >>= 1;
72  }
73  return out_val;
74  }
75  int N_;
76  // Next number to be considered for reversal and output.
77  int next_num_;
78  // number of bits required to represent the numbers of the sequence
79  int num_bits_;
80 };
81 
82 #endif // TESSERACT_CCUTIL_QRSEQUENCE_H_
int GetBinaryReversedInteger(int in_val) const
Definition: qrsequence.h:60
QRSequenceGenerator(int N)
Definition: qrsequence.h:34