ppfft

Fourier transforms, in POST Python.

ppfft reimplements scipy.fft in POST Python — every kernel is fully-typed Python that runs under the standard CPython interpreter and compiles ahead-of-time to native code (a plain C shared library and a NumPy ufunc extension module) with the POST Python reference compiler.

Status: Planning — this repository is scaffolding, ready for an agent or contributor to claim. It is part of the PostSciPy effort to rebuild SciPy one subpackage at a time as the compiler's proving ground.

Primary compiler pressure this package generates: complex-dtype array kernels end to end; constant tables; workspaces.

Start here

1. Read the POST Python spec and the PostSciPy roadmap (package map, working rules, capability matrix).
2. Copy the layout of ppspecial, the exemplar package: ppfft/ sources, tests/, scripts/build_native.py, scripts/build_ext.py, a pixi workspace with test / build-native / build-ext tasks, a git dependency on postpython, and a ROADMAP.md tracking targets and upstream requests.
3. Start with a slice from "Compiles today" below; land it as a small PR with tests in both execution modes.

First slices

Compiles today

• Naive DFT/IDFT as (n)->(n) Complex128 gufuncs — O(n²) but proves the complex-array pipeline through compile, link, and NumPy registration
• fftfreq, fftshift/ifftshift as (n)->(n) kernels

Blocked on compiler capabilities

File these as postpython issues with minimal reproducers when you start on them — the filing is part of the work and drives the compiler roadmap.

• Radix-2/4 fast paths — need constant arrays for twiddle tables (postpython#11 follow-up) and local workspaces (spec §7)
• rfft packing and N-D transforms — after the 1-D fast path

Working rules (summary)

• Pure POST Python: no compiler-specific escape hatches; every kernel runs interpreted and compiled.
• scipy is the reference, never a runtime dependency. Tests may use it optionally; prefer deterministic hardcoded reference values.
• Compiler gaps go upstream as postpython issues with reproducers, not silent workarounds.
• Verify against a postpython checkout on main.
• Document accuracy targets and reference sources per function.

The full rules and the definition of done live in the PostSciPy roadmap.