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<title>Words of Wisdom-Saving Plans - FFTW 3.1.2</title>
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(version 3.1.2, 23 June 2006).
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Copyright (C) 2003 Matteo Frigo.
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<h3 class="section">3.3 Words of Wisdom—Saving Plans</h3>
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<p><a name="index-wisdom-121"></a><a name="index-saving-plans-to-disk-122"></a>
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FFTW implements a method for saving plans to disk and restoring them.
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In fact, what FFTW does is more general than just saving and loading
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plans. The mechanism is called <dfn>wisdom</dfn>. Here, we describe
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this feature at a high level. See <a href="FFTW-Reference.html#FFTW-Reference">FFTW Reference</a>, for a less casual
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but more complete discussion of how to use wisdom in FFTW.
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<p>Plans created with the <code>FFTW_MEASURE</code>, <code>FFTW_PATIENT</code>, or
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<code>FFTW_EXHAUSTIVE</code> options produce near-optimal FFT performance,
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but may require a long time to compute because FFTW must measure the
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runtime of many possible plans and select the best one. This setup is
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designed for the situations where so many transforms of the same size
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must be computed that the start-up time is irrelevant. For short
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initialization times, but slower transforms, we have provided
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<code>FFTW_ESTIMATE</code>. The <code>wisdom</code> mechanism is a way to get the
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best of both worlds: you compute a good plan once, save it to
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disk, and later reload it as many times as necessary. The wisdom
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mechanism can actually save and reload many plans at once, not just
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<a name="index-FFTW_005fMEASURE-123"></a><a name="index-FFTW_005fPATIENT-124"></a><a name="index-FFTW_005fEXHAUSTIVE-125"></a><a name="index-FFTW_005fESTIMATE-126"></a>
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Whenever you create a plan, the FFTW planner accumulates wisdom, which
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is information sufficient to reconstruct the plan. After planning,
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you can save this information to disk by means of the function:
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<pre class="example"> void fftw_export_wisdom_to_file(FILE *output_file);
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<p><a name="index-fftw_005fexport_005fwisdom_005fto_005ffile-127"></a>
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The next time you run the program, you can restore the wisdom with
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<code>fftw_import_wisdom_from_file</code> (which returns non-zero on success),
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and then recreate the plan using the same flags as before.
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<pre class="example"> int fftw_import_wisdom_from_file(FILE *input_file);
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<p><a name="index-fftw_005fimport_005fwisdom_005ffrom_005ffile-128"></a>
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Wisdom is automatically used for any size to which it is applicable, as
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long as the planner flags are not more “patient” than those with which
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the wisdom was created. For example, wisdom created with
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<code>FFTW_MEASURE</code> can be used if you later plan with
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<code>FFTW_ESTIMATE</code> or <code>FFTW_MEASURE</code>, but not with
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<code>FFTW_PATIENT</code>.
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<p>The <code>wisdom</code> is cumulative, and is stored in a global, private
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data structure managed internally by FFTW. The storage space required
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is minimal, proportional to the logarithm of the sizes the wisdom was
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generated from. If memory usage is a concern, however, the wisdom can
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be forgotten and its associated memory freed by calling:
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<pre class="example"> void fftw_forget_wisdom(void);
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<p><a name="index-fftw_005fforget_005fwisdom-129"></a>
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Wisdom can be exported to a file, a string, or any other medium.
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For details, see <a href="Wisdom.html#Wisdom">Wisdom</a>.
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