# fft-test

**Mercurial revision 5786:5c8b45563631 | 2026-08-04**

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## Description

`fft-test` is a utility developed as part of the **T-UGO tools (2006-2011)** under the **Unstructured Ocean Grid initiative**. It tests the **equivalence between FFT implementations** in the **FFTPACK** and **GSL (GNU Scientific Library)** libraries.

This tool is primarily intended for **validation and comparison** purposes, ensuring consistency between the two FFT libraries.

> **Note:** This program does not accept command-line arguments. It runs predefined tests internally.

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## Output

The program will output the results of the FFT transformations for both libraries, allowing you to compare their equivalence. Example output:

```
vvv --- n=5 j=4 --- vvv
FFTPACK results
0
0
0
0
1

GSL FFT results
1 
0.309017 
0.951057 
-0.809017 
0.587785 
^^^ --- n=5 j=4 --- ^^^
```

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## Notes

1. **Dependencies:**
  - **FFTPACK**: A widely used library for computing Fast Fourier Transforms (FFTs).
  - **GSL (GNU Scientific Library)**: A numerical library for C and C++ programmers, providing FFT routines for real data.
2. **Test Cases:**
  - The program tests FFT transformations for arrays of sizes `n=4` and `n=5`.
  - For each size, it tests impulse responses (e.g., `value = 1` if `i == j`, else `0`).
3. **Output Interpretation:**
  - The output displays the FFT results for both libraries side by side.
  - Users can visually compare the results to ensure equivalence.
4. **Purpose:**
  - This tool is **not intended for production use** but rather for **debugging and validation** of FFT implementations.

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## See Also

- [FFTPACK](https://www.netlib.org/fftpack/): A library for computing Fast Fourier Transforms.
- [GNU Scientific Library (GSL)](https://www.gnu.org/software/gsl/): A numerical library for C and C++ programmers.
- [T-UGO Tools](https://www.t-ugo.org/): Unstructured Ocean Grid initiative tools.