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81 lines
3.0 KiB
C
81 lines
3.0 KiB
C
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///////////////////////////////////////////////////////////////////////
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// File: qrsequence.h
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// Description: Quasi-random sequence generator class.
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// Author: Ranjith Unnikrishnan
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// Created: Wed May 20 2009
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//
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// Class to generate a (deterministic) quasi-random Van der Corput sequence that
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// covers the interval [0,N) without repetition.
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//
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// The sequence is generated by reversing the base-2 representation of the
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// sequence of natural numbers {0, 1,... M-1}, where M is 2^{num_bits_} and
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// num_bits is the minimum number of bits required to represent N. If a reversed
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// numbers is >= N it is rejected and the next natural number is considered
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// until a valid output number is found.
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//
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// (C) Copyright 2009, Google Inc.
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// Licensed under the Apache License, Version 2.0 (the "License"); you may not
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// use this file except in compliance with the License. You may obtain a copy
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// of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required
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// by applicable law or agreed to in writing, software distributed under the
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// License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS
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// OF ANY KIND, either express or implied. See the License for the specific
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// language governing permissions and limitations under the License.
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//
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///////////////////////////////////////////////////////////////////////
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#ifndef TESSERACT_CCUTIL_QRSEQUENCE_H_
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#define TESSERACT_CCUTIL_QRSEQUENCE_H_
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#include <math.h>
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class QRSequenceGenerator {
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public:
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// Object is initalized with the size of the output range.
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explicit QRSequenceGenerator(int N) : N_(N), next_num_(0) {
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num_bits_ = ceil(log(static_cast<double>(N)) / log(2.0));
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}
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// Main worker method that retrieves the next number in the sequence.
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// Returns kInvalidVal if called more than N times after object initialization
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int GetVal() {
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const int kInvalidVal = -1;
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const int kMaxNaturalNumberValue = 1 << num_bits_;
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if (next_num_ >= kMaxNaturalNumberValue)
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return kInvalidVal;
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int n = next_num_;
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while (next_num_ < kMaxNaturalNumberValue) {
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n = GetBinaryReversedInteger(next_num_++);
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if (n < N_) break;
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}
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return (next_num_ > kMaxNaturalNumberValue) ? kInvalidVal : n;
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}
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protected:
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// Outputs the integer formed by reversing the bits of the input integer. Only
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// the lowest num_bits_ bits of the input integer are reversed.
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int GetBinaryReversedInteger(int in_val) const {
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int bit_pos = num_bits_;
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int out_val = 0;
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while(bit_pos--) {
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// Set the value of the last bit.
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out_val |= (in_val & 0x1);
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if (bit_pos > 0) {
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// Left-shift output value to prepare for storing the next bit.
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out_val <<= 1;
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}
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// Right-shift input value to prepare for retrieving the next bit.
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in_val >>= 1;
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}
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return out_val;
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}
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int N_;
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// Next number to be considered for reversal and output.
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int next_num_;
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// number of bits required to represent the numbers of the sequence
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int num_bits_;
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};
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#endif // TESSERACT_CCUTIL_QRSEQUENCE_H_
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