RFFT improvement: remove the double size memory requirement #1367
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This makes the FFT engine wrappers simpler, deduplicates code (reducing potential for bugs in wrappers), at the cost of making the FallbackFFT's redundant work done again if the legacy flag is used.
Ideally rfft would be implemented more efficiently than doing a full FFT, but the FFTFallback is just a slow fill-in for juce::dsp::FFT to still work in the absence of using an external FFT engine. This makes it function like the other engines in not requiring an oversized buffer.
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On 2017, the JUCE team showed interest in fixing the double size memory requirement of
juce::FFT, this PR does this while also removing duplication and complication in the code.Fabian originally suggested making the FFTFallback's real transform more efficient and avoid producing the intermediate redundant results, but given that it is simply the fallback engine, I chose a method that makes it even less efficient (by using additional memory buffers), as I don't see making it efficient as any priority. In fact, I would consider simplifying the fallback engine even further by removing the conditional
allocas in it.