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(fftw.info)Discussion on Specific Plans

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Discussion on Specific Plans
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   We recommend the use of the specific planners, even in cases where
you will be transforming arrays different from those passed to the
specific planners, as they confer the following advantages:

   * The resulting plans will be optimized for your specific arrays and
     strides.  This may or may not make a significant difference, but it
     certainly doesn't hurt.  (The ordinary planner does its planning
     based upon a stride-one temporary array that it allocates.)

   * Less intermediate storage is required during the planning process.
     (The ordinary planner uses O(`N') temporary storage, where `N' is
     the maximum dimension, while it is creating the plan.)

   * For multi-dimensional transforms, new parameters become accessible
     for optimization by the planner.  (Since multi-dimensional arrays
     can be very large, we don't dare to allocate one in the ordinary
     planner for experimentation.  This prevents us from doing certain
     optimizations that can yield dramatic improvements in some cases.)

   On the other hand, note that *the specific planner destroys the
contents of the `in' and `out' arrays*.