Fast transform lengths

fft_next_len() returns the smallest length at least n that zufft transforms fastest, and fft_prev_len() the largest at most n. Padding or cropping an input to such a length with the n argument of the transforms can speed them up considerably. They are SciPy's next_fast_len() and prev_fast_len().

Usage

fft_next_len(n, real = FALSE)

fft_prev_len(n, real = FALSE)

Arguments

n

Positive whole numbers, at most 2^52.

real

TRUE for the lengths of real transforms (fft_r2c(), fft_c2r(), fft_dct(), fft_dst()).

Value

A double vector of the same length as n.

Details

Fast lengths have no prime factor above 11 for complex transforms, or above 5 for real ones (real = TRUE), as in ducc0's good_size.

See also

stats::nextn(), R's own version for stats::fft().

Other helpers: fft_freq(), fft_shift()

Examples

fft_next_len(c(1009, 999983))
#> [1]    1024 1000000
fft_next_len(1009, real = TRUE)
#> [1] 1024
fft_prev_len(1009)
#> [1] 1008