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Module « scipy.fft »

Fonction prev_fast_len - module scipy.fft

Signature de la fonction prev_fast_len

def prev_fast_len(target, real=False) 

Description

help(scipy.fft.prev_fast_len)

Find the previous fast size of input data to ``fft``.
Useful for discarding a minimal number of samples before FFT.

SciPy's FFT algorithms gain their speed by a recursive divide and conquer
strategy. This relies on efficient functions for small prime factors of the
input length. Thus, the transforms are fastest when using composites of the
prime factors handled by the fft implementation. If there are efficient
functions for all radices <= `n`, then the result will be a number `x`
<= ``target`` with only prime factors <= `n`. (Also known as `n`-smooth
numbers)

Parameters
----------
target : int
    Maximum length to search until. Must be a positive integer.
real : bool, optional
    True if the FFT involves real input or output (e.g., `rfft` or `hfft`
    but not `fft`). Defaults to False.

Returns
-------
out : int
    The largest fast length less than or equal to ``target``.

Notes
-----
The result of this function may change in future as performance
considerations change, for example, if new prime factors are added.

Calling `fft` or `ifft` with real input data performs an ``'R2C'``
transform internally.

In the current implementation, prev_fast_len assumes radices of
2,3,5,7,11 for complex FFT and 2,3,5 for real FFT.

Examples
--------
On a particular machine, an FFT of prime length takes 16.2 ms:

>>> from scipy import fft
>>> import numpy as np
>>> rng = np.random.default_rng()
>>> max_len = 93059  # prime length is worst case for speed
>>> a = rng.standard_normal(max_len)
>>> b = fft.fft(a)

Performing FFT on the maximum fast length less than max_len
reduces the computation time to 1.5 ms, a speedup of 10.5 times:

>>> fft.prev_fast_len(max_len, real=True)
92160
>>> c = fft.fft(a[:92160]) # discard last 899 samples

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