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qrsequence.h
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1
2 // File: qrsequence.h
3 // Description: Quasi-random sequence generator class.
4 // Author: Ranjith Unnikrishnan
5 // Created: Wed May 20 2009
6 //
7 // Class to generate a (deterministic) quasi-random Van der Corput sequence that
8 // covers the interval [0,N) without repetition.
9 //
10 // The sequence is generated by reversing the base-2 representation of the
11 // sequence of natural numbers {0, 1,... M-1}, where M is 2^{num_bits_} and
12 // num_bits is the minimum number of bits required to represent N. If a reversed
13 // numbers is >= N it is rejected and the next natural number is considered
14 // until a valid output number is found.
15 //
16 // (C) Copyright 2009, Google Inc.
17 // Licensed under the Apache License, Version 2.0 (the "License"); you may not
18 // use this file except in compliance with the License. You may obtain a copy
19 // of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required
20 // by applicable law or agreed to in writing, software distributed under the
21 // License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
22 // OF ANY KIND, either express or implied. See the License for the specific
23 // language governing permissions and limitations under the License.
24 //
26
27 #ifndef TESSERACT_CCUTIL_QRSEQUENCE_H_
28 #define TESSERACT_CCUTIL_QRSEQUENCE_H_
29
30 #include <math.h>
31
32 class QRSequenceGenerator {
33 public:
34 // Object is initalized with the size of the output range.
35 explicit QRSequenceGenerator(int N) : N_(N), next_num_(0) {
36 num_bits_ = static_cast<int>(ceil(log(static_cast<double>(N)) / log(2.0)));
37 }
38
39 // Main worker method that retrieves the next number in the sequence.
40 // Returns kInvalidVal if called more than N times after object initialization
41 int GetVal() {
42 const int kInvalidVal = -1;
43 const int kMaxNaturalNumberValue = 1 << num_bits_;
44 if (next_num_ >= kMaxNaturalNumberValue)
45 return kInvalidVal;
46 int n = next_num_;
47
48 while (next_num_ < kMaxNaturalNumberValue) {
49 n = GetBinaryReversedInteger(next_num_++);
50 if (n < N_) break;
51 }
52 return (next_num_ > kMaxNaturalNumberValue) ? kInvalidVal : n;
53 }
54
55 protected:
56 // Outputs the integer formed by reversing the bits of the input integer. Only
57 // the lowest num_bits_ bits of the input integer are reversed.
58 int GetBinaryReversedInteger(int in_val) const {
59 int bit_pos = num_bits_;
60 int out_val = 0;
61 while(bit_pos--) {
62 // Set the value of the last bit.
63 out_val |= (in_val & 0x1);
64 if (bit_pos > 0) {
65 // Left-shift output value to prepare for storing the next bit.
66 out_val <<= 1;
67 }
68 // Right-shift input value to prepare for retrieving the next bit.
69 in_val >>= 1;
70 }
71 return out_val;
72 }
73 int N_;
74 // Next number to be considered for reversal and output.
75 int next_num_;
76 // number of bits required to represent the numbers of the sequence
77 int num_bits_;
78 };
79
80 #endif // TESSERACT_CCUTIL_QRSEQUENCE_H_