polars-rfft 0.1.1

FFT as native Polars expressions — forward/inverse FFT, magnitude, phase, and power spectrum, powered by RustFFT.

polars-rfft

FFT as native Polars expressions. Built with Rust (RustFFT) and PyO3 for zero-copy, vectorized computation over Polars DataFrames.

Install

pip install polars-rfft

Quick start

import polars as pl
from polars_rfft import rfft

df = pl.DataFrame({"signal": [1.0, 0.0, -1.0, 0.0, 1.0, 0.0, -1.0, 0.0]})

# Forward FFT — returns struct {re, im}
df.with_columns(spectrum=rfft(pl.col("signal")).fft())

Roundtrip: FFT → IFFT

df.with_columns(
    recovered=rfft(rfft(pl.col("signal")).fft()).ifft_real()
)

Magnitude, phase, power spectrum

fft_expr = rfft(pl.col("signal")).fft()
df.with_columns(
    mag=rfft(fft_expr).magnitude(),
    phase=rfft(fft_expr).phase(),
    power=rfft(fft_expr).power_spectrum(),
)

Complex-valued input

df = pl.DataFrame({
    "z": [{"re": 1.0, "im": 0.0}, {"re": 0.0, "im": 1.0},
           {"re": -1.0, "im": 0.0}, {"re": 0.0, "im": -1.0}]
})
df.with_columns(spectrum=rfft(pl.col("z")).fft_complex())

API reference

Complex values are represented as struct columns with fields re (real) and im (imaginary).

Method	Input	Output	Description
fft()	f64	struct{re,im}	Forward FFT on real signal
fft_complex()	struct{re,im}	struct{re,im}	Forward FFT on complex signal
ifft()	struct{re,im}	struct{re,im}	Inverse FFT (normalized by 1/N)
ifft_real()	struct{re,im}	f64	Inverse FFT, real part only
magnitude()	struct{re,im}	f64
phase()	struct{re,im}	f64	atan2(im, re) in radians
power_spectrum()	struct{re,im}	f64

All methods are accessed via the rfft() wrapper:

from polars_rfft import rfft

rfft(pl.col("signal")).fft()          # forward
rfft(pl.col("spectrum")).ifft()       # inverse
rfft(pl.col("spectrum")).magnitude()  # magnitude

Direct functions

For lower overhead on smaller signals, fft_direct and ifft_direct bypass the Polars expression engine and operate directly on Series:

from polars_rfft import fft_direct, ifft_direct

re, im = fft_direct(df["signal"])           # Series → (Series, Series)
re, im = ifft_direct(re, im)               # (Series, Series) → (Series, Series)

Performance

Benchmarked end-to-end against numpy on signals of varying length. Median of 7 runs after warmup.

Expression API (rfft().fft())

Signal length	Operation	polars-rfft	numpy	Speedup
1,024	fft	0.14 ms	0.02 ms	0.2x
4,096	fft	0.24 ms	0.08 ms	0.3x
16,384	fft	0.66 ms	0.99 ms	1.5x
65,536	fft	2.2 ms	4.7 ms	2.1x
262,144	fft	10.2 ms	22.1 ms	2.2x
1,048,576	fft	33.9 ms	53.3 ms	1.9x

Direct API (fft_direct())

Bypasses Polars expression dispatch for ~0.1ms less overhead:

Signal length	Operation	fft_direct	numpy	Speedup
64	fft	0.014 ms	0.013 ms	0.9x
256	fft	0.018 ms	0.013 ms	0.7x
1,024	fft	0.040 ms	0.022 ms	0.6x
4,096	fft	0.14 ms	0.08 ms	0.5x
16,384	fft	0.64 ms	0.99 ms	1.5x
65,536	fft	2.1 ms	4.7 ms	2.3x
262,144	fft	9.9 ms	22.1 ms	2.2x
1,048,576	fft	27.9 ms	53.3 ms	1.9x

At small sizes (< 4K), numpy is faster due to its highly optimized C/Fortran backend. At 16K+ elements, RustFFT's radix algorithms dominate — up to 2.3x faster than numpy. The direct API eliminates ~0.1ms of Polars expression dispatch overhead, matching numpy at very small sizes.

Run it yourself:

pip install numpy
python benchmarks/bench_fft.py --sizes 1024 16384 262144 1048576

License

MIT