ENH: Vendor pocketfft as the ITKPocketFFT third-party module

[hjmjohnson]

Move the header-only pocketfft from the FFT module include directory into a
proper Modules/ThirdParty/PocketFFT module, vendored from the
InsightSoftwareConsortium/pocketfft
fork via the standard UpdateFromUpstream.sh mechanism.

This gives pocketfft parity with every other ITK third-party inclusion —
even though it is a single header file — so that future upstream pocketfft
updates are maintained the same consistent way as eigen, DCMTK, VNL, etc.
(fork with a welcome branch, for/itk-* overlay, UpdateFromUpstream.sh).

Upstream pocketfft has also already merged the fix for the compiler warnings
reported on the CDash build
https://open.cdash.org/builds/11378398/build
(mreineck/pocketfft#32), so the vendored copy is warning-clean with no
ITK-local patch required.

What changed

• New Modules/ThirdParty/PocketFFT module (Eigen3 header-only model):
  itk-module.cmake, CMakeLists.txt with an ITK_USE_SYSTEM_POCKETFFT
  switch, and UpdateFromUpstream.sh pinned to
  for/itk-pocketfft-cpp-c90e55b.
• itk_pocketfft.h is the single entry point: it is the only header that
  includes pocketfft_hdronly.h. ITK-specific configuration lives here (not in
  the vendored copy), applied before the include:
  • POCKETFFT_NAMESPACE = itk::detail::pocketfft — confines pocketfft symbols
    to an ITK-private namespace so they cannot collide with a ::pocketfft
    from another library (Make pocketfft namespace customizable using POCKETFFT_NAMESPACE definition mreineck/pocketfft#30).
  • POCKETFFT_CACHE_SIZE = 16.
• The vendored pocketfft_hdronly.h keeps upstream defaults, so the fork stays
  a faithful mirror of upstream cpp (which includes Use nullptr instead of 0 in arr default constructor mreineck/pocketfft#32).
• ITKFFT now DEPENDS ITKPocketFFT; the FFT filters and the deprecated
  vnl_fft_1d shim include itk_pocketfft.h and use the fully-qualified
  itk::detail::pocketfft:: API.

Validation

• ITKFFT builds; all 20 PocketFFT tests pass.
• Install places itk_pocketfft.h and itkpocketfft/{pocketfft_hdronly.h, LICENSE.md} under a single include root; an external consumer including only
  itk_pocketfft.h compiles and links, with POCKETFFT_CACHE_SIZE == 16.
• Symbol check: zero global ::pocketfft symbols; all in
  itk::detail::pocketfft.
• UpdateFromUpstream.sh re-import reproduces the vendored header byte-for-byte
  with correct two-parent merge topology.
• pre-commit run --all-files passes.

[blowekamp]

Looks correct so far

[greptile-apps]

Greptile Summary

This PR extracts pocketfft_hdronly.h from the ITKFFT module's include directory into a proper Modules/ThirdParty/PocketFFT third-party module, bringing it into parity with Eigen3 and other vendored dependencies. ITK-specific configuration (namespace isolation to itk::detail::pocketfft and cache size) is moved into a generated itk_pocketfft.h wrapper rather than embedded in the vendored copy.

• New ITKPocketFFT module follows the Eigen3 header-only pattern: CMakeLists.txt with ITK_USE_SYSTEM_POCKETFFT switch, UpdateFromUpstream.sh pinned to for/itk-pocketfft-cpp-c90e55b, and a configure_file-generated entry-point header.
• All six PocketFFT filter headers and vnl_fft_1d.h updated to #include \"itk_pocketfft.h\" and use the fully-qualified itk::detail::pocketfft:: namespace throughout.

Confidence Score: 4/5

The module restructuring is mechanically correct and follows the established Eigen3 pattern closely. The vendored copy and build wiring look sound; the two items worth a second look before merge are the incomplete LICENSE.md and the undocumented system-pocketfft namespace fragility.

The core refactor — moving pocketfft into ThirdParty, generating the entry-point header, updating all callers — is clean and consistent with existing ITK conventions. The LICENSE.md shipped with the vendored copy omits several copyright holders named in the header it licenses, which is a BSD-3-Clause attribution gap. The system-pocketfft path also silently degrades (compile failure) when another library has already pulled in pocketfft under its default namespace; a short comment would make this failure mode diagnosable.

Modules/ThirdParty/PocketFFT/src/itkpocketfft/LICENSE.md (incomplete copyright attribution) and Modules/ThirdParty/PocketFFT/src/itk_pocketfft.h.in (undocumented namespace-collision risk for ITK_USE_SYSTEM_POCKETFFT)

Important Files Changed

Filename	Overview
Modules/ThirdParty/PocketFFT/CMakeLists.txt	New top-level module CMakeLists following the Eigen3 pattern; correctly sets up vendored vs system paths, runs configure_file, and exposes ITKPocketFFT_INCLUDE_DIRS. Missing install of vendored files is handled in src/CMakeLists.txt via itk_module_impl() → add_subdirectory(src).
Modules/ThirdParty/PocketFFT/src/itk_pocketfft.h.in	Generated entry-point header that sets POCKETFFT_NAMESPACE and POCKETFFT_CACHE_SIZE before the upstream include. Uses #ifndef guards which create a fragility for the system-pocketfft path when another library has already included pocketfft_hdronly.h with its default namespace.
Modules/ThirdParty/PocketFFT/src/itkpocketfft/LICENSE.md	LICENSE.md covers only the 2010-2018 Max-Planck-Society copyright but pocketfft_hdronly.h names additional copyright holders (Peter Bell 2019-2020, Matteo Frigo/MIT 2003-2014, Tan Ping Liang/Peter Bell 2024, Cris Luengo 2024). The license file is incomplete relative to the code being distributed.
Modules/ThirdParty/PocketFFT/itk-module.cmake	Minimal module declaration with no dependencies; follows standard ITK third-party module pattern. DEPENDS followed immediately by DESCRIPTION is correct CMake argument-parsing idiom for zero dependencies.
Modules/ThirdParty/PocketFFT/UpdateFromUpstream.sh	Standard ITK update-third-party script pointing to the ISC fork; exact_tree_match=false is appropriate since only two files are imported from upstream. Tag pinned to for/itk-pocketfft-cpp-c90e55b.
Modules/Filtering/FFT/include/itkPocketFFTCommon.h	Updated to use itk_pocketfft.h and itk::detail::pocketfft:: namespace throughout; logic is unchanged.
Modules/Filtering/FFT/itk-module.cmake	Adds ITKPocketFFT to ITKFFT's DEPENDS list, correctly modeling the new module dependency.
Modules/ThirdParty/VNL/src/vxl/core/vnl/algo/vnl_fft_1d.h	Updated deprecated shim to include itk_pocketfft.h and use itk::detail::pocketfft:: namespace. Comment saying 'Requires ITKFFT module' is still accurate since ITKFFT transitively supplies ITKPocketFFT include dirs.
Modules/ThirdParty/PocketFFT/src/itkpocketfft/pocketfft_hdronly.h	Vendored upstream pocketfft header; POCKETFFT_CACHE_SIZE default reverted to 0 (ITK's value is now set in itk_pocketfft.h.in), and arr() default-initializes pointer to nullptr instead of 0. Content otherwise mirrors upstream.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    ITKFFT["ITKFFT module\n(itk-module.cmake)"]
    ITKPocketFFT["ITKPocketFFT module\n(ThirdParty)"]
    itk_pocketfft_h["itk_pocketfft.h\n(generated via configure_file)\n- #define POCKETFFT_NAMESPACE itk::detail::pocketfft\n- #define POCKETFFT_CACHE_SIZE 16"]
    pocketfft_hdronly["itkpocketfft/pocketfft_hdronly.h\n(vendored upstream)"]
    system_pocketfft["<pocketfft_hdronly.h>\n(system, if ITK_USE_SYSTEM_POCKETFFT=ON)"]
    filters["PocketFFT filter headers\n(itkPocketFFTCommon.h, etc.)"]
    vnl["vnl_fft_1d.h\n(deprecated shim)"]

    ITKFFT -->|DEPENDS| ITKPocketFFT
    ITKPocketFFT --> itk_pocketfft_h
    itk_pocketfft_h -->|vendored default| pocketfft_hdronly
    itk_pocketfft_h -->|ITK_USE_SYSTEM_POCKETFFT=ON| system_pocketfft
    filters -->|#include| itk_pocketfft_h
    vnl -->|#include| itk_pocketfft_h

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flowchart TD
    ITKFFT["ITKFFT module\n(itk-module.cmake)"]
    ITKPocketFFT["ITKPocketFFT module\n(ThirdParty)"]
    itk_pocketfft_h["itk_pocketfft.h\n(generated via configure_file)\n- #define POCKETFFT_NAMESPACE itk::detail::pocketfft\n- #define POCKETFFT_CACHE_SIZE 16"]
    pocketfft_hdronly["itkpocketfft/pocketfft_hdronly.h\n(vendored upstream)"]
    system_pocketfft["<pocketfft_hdronly.h>\n(system, if ITK_USE_SYSTEM_POCKETFFT=ON)"]
    filters["PocketFFT filter headers\n(itkPocketFFTCommon.h, etc.)"]
    vnl["vnl_fft_1d.h\n(deprecated shim)"]

    ITKFFT -->|DEPENDS| ITKPocketFFT
    ITKPocketFFT --> itk_pocketfft_h
    itk_pocketfft_h -->|vendored default| pocketfft_hdronly
    itk_pocketfft_h -->|ITK_USE_SYSTEM_POCKETFFT=ON| system_pocketfft
    filters -->|#include| itk_pocketfft_h
    vnl -->|#include| itk_pocketfft_h

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_{Reviews (1): Last reviewed commit: "ENH: Vendor pocketfft as the ITKPocketFF..." | Re-trigger Greptile}

[dzenanz]

Mostly looks good.