pocketfft -- https://github.com/mreineck/pocketfft

Vendored from branch `cpp` at commit c90e55b3d529f8efa40ed01a20de22405f45fc65
(2026-06-30). Only `pocketfft_hdronly.h` is vendored, byte-for-byte unmodified;
sha256 3e9a05318d8e3b1446bda1c4617e6a103cdd23599ae0a776a92a6e8800e92fdc.

The license text below is reproduced verbatim from the head of that header.

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This file is part of pocketfft.

Copyright (C) 2010-2024 Max-Planck-Society
Copyright (C) 2019-2020 Peter Bell

For the odd-sized DCT-IV transforms:
  Copyright (C) 2003, 2007-14 Matteo Frigo
  Copyright (C) 2003, 2007-14 Massachusetts Institute of Technology

For the prev_good_size search:
  Copyright (C) 2024 Tan Ping Liang, Peter Bell

For the safeguards against integer overflow in good_size search:
  Copyright (C) 2024 Cris Luengo

Authors: Martin Reinecke, Peter Bell

All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright notice, this
  list of conditions and the following disclaimer in the documentation and/or
  other materials provided with the distribution.
* Neither the name of the copyright holder nor the names of its contributors may
  be used to endorse or promote products derived from this software without
  specific prior written permission.

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