Add default (undocumented) cosine windowing function

src/SampledCorrelations/CorrelationSampling.jl

@@ -83,7 +83,7 @@ function no_processing(::SampledCorrelations)
     nothing
 end
 
-function accum_sample!(sc::SampledCorrelations)
+function accum_sample!(sc::SampledCorrelations; window)
     (; data, M, observables, samplebuf, nsamples, fft!) = sc
     natoms = size(samplebuf)[5]
 
@@ -121,7 +121,17 @@ function accum_sample!(sc::SampledCorrelations)
         sample_β = @view samplebuf[β,:,:,:,j,:]
         databuf  = @view data[c,i,j,:,:,:,:]
 
-        corr = FFTW.fft(FFTW.ifft(sample_α .* conj.(sample_β),4) ./ n_contrib,4)
+        corr = FFTW.ifft(sample_α .* conj.(sample_β),4) ./ n_contrib
+
+        if window == :cosine
+          # Apply a cosine windowing to force the correlation at Δt=±(T-1) to be zero
+          # to force periodicity. In terms of the spectrum S(ω), this applys a smoothing
+          # with a characteristic lengthscale of O(1) frequency bins.
+          window_func = cos.(range(0,π,length = num_time_offsets + 1)[1:end-1]).^2
+          corr .*= reshape(window_func,1,1,1,num_time_offsets)
+        end
+
+        corr = FFTW.fft(corr,4)
 
         if isnothing(M)
             for k in eachindex(databuf)
@@ -171,7 +181,7 @@ separately prior to calling `add_sample!`. Alternatively, the initial spin
 configuration may be copied into a new `System` and this new `System` can be
 passed to `add_sample!`.
 """
-function add_sample!(sc::SampledCorrelations, sys::System) 
+function add_sample!(sc::SampledCorrelations, sys::System; window = :cosine) 
     new_sample!(sc, sys)
-    accum_sample!(sc)
+    accum_sample!(sc; window)
 end