fftw_flags set method for fft planner algorithm selection

## Syntax

    [a,[S]]=fftw_flags(flag)

## Arguments

flag

  a string or an integer. Specifies the planner algorithm. See below.

a

  an integer. The planner code. See below.

S

  a character string. The planner name.

## Description

This function enables the selection of the algorithm used to determine the fftw planner algorithm. The planner is used to determine an efficient way to compute the fft.

Warning: the default value "FFTW_ESTIMATE" gives generally quite efficient plans. Try to change it only if the fft efficiency is really important and you have a lot of similar calls to do.

Accepted entries are :

- {FFTW_ESTIMATE} or 64. Specifies that, instead of actual measurements of different algorithms, a simple heuristic is used to pick a (probably sub-optimal) plan quickly. With this flag, the input/output arrays are not overwritten during planning. It is the default value

- FFTW_MEASURE or 0. tells FFTW to find an optimized plan by actually computing several FFTs and measuring their execution time. Depending on your machine, this can take some time (often a few seconds).

- FFTW_PATIENT or 32. It is like "FFTW_MEASURE", but considers a wider range of algorithms and often produces a “more optimal” plan (especially for large transforms), but at the expense of several times longer planning time (especially for large transforms).

- FFTW_EXHAUSTIVE or 8. It is like "FFTW_PATIENT", but considers an even wider range of algorithms, including many that we think are unlikely to be fast, to produce the most optimal plan but with a substantially increased planning time.

Remark : when using FFTW_MEASURE/FFTW_PATIENT/FFTW_EXHAUSTIVE you must call fftw twice. (first call for initialization, second and others calls for computation)

## Examples

    A=rand(1, 2^9 + 2^15);
    fftw_forget_wisdom();
    fftw_flags("FFTW_ESTIMATE");
    timer(); y=fft(A); timer()//first call determines the plan
    timer(); y=fft(A); timer() //subsequent similar calls
    fftw_flags("FFTW_MEASURE");
    fftw_forget_wisdom();
    timer(); y=fft(A); timer()//first call determines the plan, this takes about 5min
    timer(); y=fft(A); timer() //subsequent similar calls

## See also

- fft
- set_fftw_wisdom
- get_fftw_wisdom
- fftw_forget_wisdom