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bundles / scipy latest / scipy / fftpack / _helper / next_fast_len

function

scipy.fftpack._helper:next_fast_len

source: /scipy/fftpack/_helper.py :54

Signature

def   next_fast_len ( target )

Summary

Find the next fast size of input data to fft, for zero-padding, etc.

Extended Summary

SciPy's FFTPACK has efficient functions for radix {2, 3, 4, 5}, so this returns the next composite of the prime factors 2, 3, and 5 which is greater than or equal to target. (These are also known as 5-smooth numbers, regular numbers, or Hamming numbers.)

Parameters

target : int

Length to start searching from. Must be a positive integer.

Returns

out : int

The first 5-smooth number greater than or equal to target.

Notes

Examples

On a particular machine, an FFT of prime length takes 133 ms: 
from scipy import fftpack
import numpy as np
rng = np.random.default_rng()
min_len = 10007  # prime length is worst case for speed
a = rng.standard_normal(min_len)
b = fftpack.fft(a)
✓
Zero-padding to the next 5-smooth length reduces computation time to 211 us, a speedup of 630 times: 
fftpack.next_fast_len(min_len)
b = fftpack.fft(a, 10125)
✓
Rounding up to the next power of 2 is not optimal, taking 367 us to compute, 1.7 times as long as the 5-smooth size: 
b = fftpack.fft(a, 16384)
✓

Aliases

  • scipy.fftpack.next_fast_len

Referenced by

This package