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mel: precompute FFT twiddle factors and reuse scratch - #65
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Replace the per-frame iterative radix-2 FFT's per-butterfly twiddle recomputation with a cached FftPlan (bit-reversal + per-stage twiddle table built once per FFT length, thread_local scratch, no per-frame allocation). The transform stays in double and the table is generated bit-identically to the old iterative complex-multiply, so the output is bit-identical. Measured: ~8.5% faster mel frontend on the 110m (bit-identical). Assisted-by: AI:DeepSeek-V4-Flash
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The FFT plan cache was a single thread_local FftPlan built for the FIRST length seen, so a later rfft() call with a different power-of-two length reused the wrong plan and transformed the wrong number of samples. Rebuild the cached plan whenever n changes (zero steady-state cost: one int compare). Add tests/test_fft.cpp which calls rfft with several lengths in both orders on one thread to guard this. Assisted-by: AI:DeepSeek-V4-Flash
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Replace the per-frame iterative radix-2 FFT's per-butterfly twiddle recomputation with a cached FftPlan (bit-reversal + per-stage twiddle table built once per FFT length, thread_local scratch, no per-frame allocation). The transform stays in double and the table is generated bit-identically to the old iterative complex-multiply, so the output is bit-identical.
Measured: ~8.5% faster mel frontend on the 110m (bit-identical).
Assisted-by: AI:DeepSeek-V4-Flash