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1 <html lang="en">
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2 <head>
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3 <title>Planner Flags - FFTW 3.3.4</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.4">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Basic-Interface.html#Basic-Interface" title="Basic Interface">
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9 <link rel="prev" href="Complex-DFTs.html#Complex-DFTs" title="Complex DFTs">
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10 <link rel="next" href="Real_002ddata-DFTs.html#Real_002ddata-DFTs" title="Real-data DFTs">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.4, 20 September 2013).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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33 -->
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34 <meta http-equiv="Content-Style-Type" content="text/css">
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35 <style type="text/css"><!--
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="Planner-Flags"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Real_002ddata-DFTs.html#Real_002ddata-DFTs">Real-data DFTs</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Complex-DFTs.html#Complex-DFTs">Complex DFTs</a>,
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53 Up: <a rel="up" accesskey="u" href="Basic-Interface.html#Basic-Interface">Basic Interface</a>
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54 <hr>
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55 </div>
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56
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57 <h4 class="subsection">4.3.2 Planner Flags</h4>
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58
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59 <p>All of the planner routines in FFTW accept an integer <code>flags</code>
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60 argument, which is a bitwise OR (‘<samp><span class="samp">|</span></samp>’) of zero or more of the flag
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61 constants defined below. These flags control the rigor (and time) of
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62 the planning process, and can also impose (or lift) restrictions on the
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63 type of transform algorithm that is employed.
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64
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65 <p><em>Important:</em> the planner overwrites the input array during
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66 planning unless a saved plan (see <a href="Wisdom.html#Wisdom">Wisdom</a>) is available for that
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67 problem, so you should initialize your input data after creating the
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68 plan. The only exceptions to this are the <code>FFTW_ESTIMATE</code> and
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69 <code>FFTW_WISDOM_ONLY</code> flags, as mentioned below.
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70
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71 <p>In all cases, if wisdom is available for the given problem that was
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72 created with equal-or-greater planning rigor, then the more rigorous
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73 wisdom is used. For example, in <code>FFTW_ESTIMATE</code> mode any available
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74 wisdom is used, whereas in <code>FFTW_PATIENT</code> mode only wisdom created
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75 in patient or exhaustive mode can be used. See <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>.
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76
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77 <h5 class="subsubheading">Planning-rigor flags</h5>
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78
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79 <ul>
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80 <li><a name="index-FFTW_005fESTIMATE-171"></a><code>FFTW_ESTIMATE</code> specifies that, instead of actual measurements of
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81 different algorithms, a simple heuristic is used to pick a (probably
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82 sub-optimal) plan quickly. With this flag, the input/output arrays are
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83 not overwritten during planning.
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84
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85 <li><a name="index-FFTW_005fMEASURE-172"></a><code>FFTW_MEASURE</code> tells FFTW to find an optimized plan by actually
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86 <em>computing</em> several FFTs and measuring their execution time.
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87 Depending on your machine, this can take some time (often a few
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88 seconds). <code>FFTW_MEASURE</code> is the default planning option.
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89
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90 <li><a name="index-FFTW_005fPATIENT-173"></a><code>FFTW_PATIENT</code> is like <code>FFTW_MEASURE</code>, but considers a wider
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91 range of algorithms and often produces a “more optimal” plan
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92 (especially for large transforms), but at the expense of several times
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93 longer planning time (especially for large transforms).
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94
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95 <li><a name="index-FFTW_005fEXHAUSTIVE-174"></a><code>FFTW_EXHAUSTIVE</code> is like <code>FFTW_PATIENT</code>, but considers an
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96 even wider range of algorithms, including many that we think are
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97 unlikely to be fast, to produce the most optimal plan but with a
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98 substantially increased planning time.
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99
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100 <li><a name="index-FFTW_005fWISDOM_005fONLY-175"></a><code>FFTW_WISDOM_ONLY</code> is a special planning mode in which the plan
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101 is only created if wisdom is available for the given problem, and
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102 otherwise a <code>NULL</code> plan is returned. This can be combined with
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103 other flags, e.g. ‘<samp><span class="samp">FFTW_WISDOM_ONLY | FFTW_PATIENT</span></samp>’ creates a
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104 plan only if wisdom is available that was created in
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105 <code>FFTW_PATIENT</code> or <code>FFTW_EXHAUSTIVE</code> mode. The
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106 <code>FFTW_WISDOM_ONLY</code> flag is intended for users who need to detect
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107 whether wisdom is available; for example, if wisdom is not available
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108 one may wish to allocate new arrays for planning so that user data is
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109 not overwritten.
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110
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111 </ul>
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112
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113 <h5 class="subsubheading">Algorithm-restriction flags</h5>
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114
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115 <ul>
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116 <li><a name="index-FFTW_005fDESTROY_005fINPUT-176"></a><code>FFTW_DESTROY_INPUT</code> specifies that an out-of-place transform is
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117 allowed to <em>overwrite its input</em> array with arbitrary data; this
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118 can sometimes allow more efficient algorithms to be employed.
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119 <a name="index-out_002dof_002dplace-177"></a>
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120 <li><a name="index-FFTW_005fPRESERVE_005fINPUT-178"></a><code>FFTW_PRESERVE_INPUT</code> specifies that an out-of-place transform must
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121 <em>not change its input</em> array. This is ordinarily the
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122 <em>default</em>, except for c2r and hc2r (i.e. complex-to-real)
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123 transforms for which <code>FFTW_DESTROY_INPUT</code> is the default. In the
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124 latter cases, passing <code>FFTW_PRESERVE_INPUT</code> will attempt to use
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125 algorithms that do not destroy the input, at the expense of worse
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126 performance; for multi-dimensional c2r transforms, however, no
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127 input-preserving algorithms are implemented and the planner will return
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128 <code>NULL</code> if one is requested.
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129 <a name="index-c2r-179"></a><a name="index-hc2r-180"></a>
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130 <li><a name="index-FFTW_005fUNALIGNED-181"></a><a name="index-alignment-182"></a><a name="index-fftw_005fmalloc-183"></a><a name="index-fftw_005falignment_005fof-184"></a><code>FFTW_UNALIGNED</code> specifies that the algorithm may not impose any
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131 unusual alignment requirements on the input/output arrays (i.e. no
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132 SIMD may be used). This flag is normally <em>not necessary</em>, since
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133 the planner automatically detects misaligned arrays. The only use for
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134 this flag is if you want to use the new-array execute interface to
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135 execute a given plan on a different array that may not be aligned like
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136 the original. (Using <code>fftw_malloc</code> makes this flag unnecessary
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137 even then. You can also use <code>fftw_alignment_of</code> to detect
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138 whether two arrays are equivalently aligned.)
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139
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140 </ul>
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141
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142 <h5 class="subsubheading">Limiting planning time</h5>
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143
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144 <pre class="example"> extern void fftw_set_timelimit(double seconds);
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145 </pre>
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146 <p><a name="index-fftw_005fset_005ftimelimit-185"></a>
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147 This function instructs FFTW to spend at most <code>seconds</code> seconds
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148 (approximately) in the planner. If <code>seconds ==
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149 FFTW_NO_TIMELIMIT</code> (the default value, which is negative), then
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150 planning time is unbounded. Otherwise, FFTW plans with a
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151 progressively wider range of algorithms until the the given time limit
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152 is reached or the given range of algorithms is explored, returning the
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153 best available plan.
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154 <a name="index-FFTW_005fNO_005fTIMELIMIT-186"></a>
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155
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156 <p>For example, specifying <code>FFTW_PATIENT</code> first plans in
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157 <code>FFTW_ESTIMATE</code> mode, then in <code>FFTW_MEASURE</code> mode, then
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158 finally (time permitting) in <code>FFTW_PATIENT</code>. If
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159 <code>FFTW_EXHAUSTIVE</code> is specified instead, the planner will further
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160 progress to <code>FFTW_EXHAUSTIVE</code> mode.
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161
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162 <p>Note that the <code>seconds</code> argument specifies only a rough limit; in
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163 practice, the planner may use somewhat more time if the time limit is
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164 reached when the planner is in the middle of an operation that cannot
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165 be interrupted. At the very least, the planner will complete planning
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166 in <code>FFTW_ESTIMATE</code> mode (which is thus equivalent to a time limit
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167 of 0).
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168
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169 <!-- =========> -->
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170 </body></html>
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171
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