1 | ///////////////////////////////////////////////////////////////////////////////
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2 | // rolling_variance.hpp
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3 | // Copyright (C) 2005 Eric Niebler
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4 | // Copyright (C) 2011 Pieter Bastiaan Ober (Integricom). Distributed under the Boost
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5 | // Software License, Version 1.0. (See accompanying file
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6 | // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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7 |
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8 | #ifndef BOOST_ACCUMULATORS_STATISTICS_ROLLING_VARIANCE_HPP_EAN_15_11_2011
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9 | #define BOOST_ACCUMULATORS_STATISTICS_ROLLING_VARIANCE_HPP_EAN_15_11_2011
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10 |
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11 | #include <boost/accumulators/accumulators.hpp>
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12 | #include <boost/accumulators/statistics/stats.hpp>
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13 |
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14 | #include <boost/mpl/placeholders.hpp>
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15 | #include <boost/accumulators/framework/accumulator_base.hpp>
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16 | #include <boost/accumulators/framework/extractor.hpp>
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17 | #include <boost/accumulators/numeric/functional.hpp>
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18 | #include <boost/accumulators/framework/parameters/sample.hpp>
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19 | #include <boost/accumulators/framework/depends_on.hpp>
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20 | #include <boost/accumulators/statistics_fwd.hpp>
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21 | #include <boost/accumulators/statistics/rolling_window.hpp>
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22 | #include <boost/accumulators/statistics/rolling_count.hpp>
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23 |
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24 | #include <RollingMean.hpp>
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25 |
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26 | // See: Chan, Tony F.; Golub, Gene H.; LeVeque, Randall J. (1983).
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27 | // Algorithms for Computing the Sample Variance: Analysis and Recommendations.
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28 | // The American Statistician 37, 242-247.
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29 | //
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30 | // Variance(x[1:N]):
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31 | // {
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32 | // M[1] = x[1]
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33 | // S[1] = 0.0
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34 | //
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35 | // j=2:N
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36 | // {
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37 | // M[j] = M[j-1] + (1/j)*(x[j]-M[j]-1)
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38 | // S[j] = S[j-1] + ((j-1)/j)*(x[j]-M[j-1])^2
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39 | // Variance = S[j]/j or
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40 | // Variance = S[j]/(j-1)
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41 | // }
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42 | // }
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43 | //
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44 | // When the buffer of size N is full, not only points are added but points are dropped as well.
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45 | // from the window over which the variance is computed. Adaptation to a rolling window gives
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46 | // for j=N+1,...:
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47 | // {
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48 | // M[j] = M[j-1] + (1/N)*(x[j]-x[j-N])
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49 | // S[j] = S[j-1] + (N/(N+1))*(x[j]-M[j-1])^2 - (N/(N+1))*(x[j-N]-M[j])^2
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50 | // Variance = S[j]/N or
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51 | // Variance = S[j]/(N-1)
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52 | // }
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53 |
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54 |
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55 | namespace boost { namespace accumulators
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56 | {
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57 | namespace impl
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58 | {
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59 | ///////////////////////////////////////////////////////////////////////////////
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60 | // rolling_variance_impl
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61 | //
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62 | template<typename Sample>
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63 | struct rolling_variance_impl
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64 | : accumulator_base
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65 | {
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66 | typedef Sample result_type;
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67 |
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68 | template<typename Args>
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69 | rolling_variance_impl(Args const &args)
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70 | : previous_mean_(0.0)
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71 | , sum_of_squares_(0.0)
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72 | {
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73 | // VOID;
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74 | }
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75 |
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76 | template<typename Args>
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77 | void operator()(Args const &args)
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78 | {
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79 | Sample added_sample = args[sample];
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80 | size_t nr_samples = rolling_count(args);
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81 |
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82 | Sample mean = rolling_mean(args);
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83 |
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84 | if(is_rolling_window_plus1_full(args))
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85 | {
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86 | Sample weight = static_cast<Sample>(nr_samples)/static_cast<Sample>(nr_samples+1);
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87 | Sample removed_sample = rolling_window_plus1(args).front();
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88 | sum_of_squares_ += weight*(
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89 | (added_sample-previous_mean_)*(added_sample-previous_mean_) -
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90 | (removed_sample-mean)*(removed_sample-mean)
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91 | );
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92 | if (sum_of_squares_ < 0.0) sum_of_squares_ = 0.0;
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93 | }
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94 | else
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95 | {
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96 | Sample weight = static_cast<Sample>(nr_samples-1)/static_cast<Sample>(nr_samples);
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97 | sum_of_squares_ += weight*(added_sample-previous_mean_)*(added_sample-previous_mean_);
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98 | }
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99 | previous_mean_ = mean;
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100 | }
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101 |
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102 | template<typename Args>
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103 | result_type result(Args const &args) const
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104 | {
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105 | size_t nr_samples = rolling_count(args);
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106 | if (nr_samples < 2) return 0;
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107 | return sum_of_squares_/(nr_samples-1);
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108 | }
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109 |
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110 | private:
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111 |
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112 | Sample previous_mean_;
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113 | Sample sum_of_squares_;
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114 | };
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115 | } // namespace impl
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116 |
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117 | ///////////////////////////////////////////////////////////////////////////////
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118 | // tag:: rolling_variance
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119 | //
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120 | namespace tag
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121 | {
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122 | struct rolling_variance
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123 | : depends_on< rolling_window_plus1, rolling_count, rolling_mean>
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124 | {
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125 | /// INTERNAL ONLY
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126 | ///
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127 | typedef accumulators::impl::rolling_variance_impl< mpl::_1 > impl;
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128 |
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129 | #ifdef BOOST_ACCUMULATORS_DOXYGEN_INVOKED
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130 | /// tag::rolling_window::window_size named parameter
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131 | static boost::parameter::keyword<tag::rolling_window_size> const window_size;
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132 | #endif
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133 | };
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134 | } // namespace tag
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135 |
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136 | ///////////////////////////////////////////////////////////////////////////////
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137 | // extract::rolling_variance
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138 | //
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139 | namespace extract
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140 | {
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141 | extractor<tag::rolling_variance> const rolling_variance = {};
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142 |
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143 | BOOST_ACCUMULATORS_IGNORE_GLOBAL(rolling_variance)
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144 | }
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145 |
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146 | using extract::rolling_variance;
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147 |
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148 | }} // namespace boost::accumulators
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149 |
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150 | #endif
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