N step fft in D language

[Kadir Erdem Demir]

On Sunday, 15 September 2013 at 15:39:14 UTC, John Colvin wrote:
> On Sunday, 15 September 2013 at 15:15:28 UTC, Kadir Erdem Demir 
> wrote:
>> I am using fft function from std.numeric
>>
>> Complex!double[] resultfft = fft(timeDomainAmplitudeVal);
>>
>> The parameter timeDomainAmplitudeVal is audio amplitude data. 
>> Sample rate 44100 hz and there is 131072(2^16) samples
>>
>> I am seeing that resultfft has the same size as 
>> timeDomainAmplitudeVal(131072) which does not fits my 
>> project(also makes no sense).
>
> That's what the FFT does. See here: 
> http://stackoverflow.com/questions/4364823/how-to-get-frequency-from-fft-result

I believe I am well aware of the things which are explained in 
the link. There is a sentence in link which says  : "The first 
bin in the FFT is DC (0 Hz), the second bin is Fs / N, where Fs 
is the sample rate and N is the size of the FFT."

My question how can I determine the "N" which is the size of FFT ?
In fftw library one can define N like :
fftw_create_plan(N, FFTW_FORWARD, FFTW_ESTIMATE);
In D do we have a way to do that ?