title: C++ Implementation for Multiplying 2 polynomials using FFT
body: <p>I need to know if there is some good implementation available for the above.
  I found few libraries/resources , but they seemed too complex to use , as most of them were distributed within diffrent <strong>cpp</strong> files.</p>
  
  <p>Like , <a href="https://stackoverflow.com/questions/5255474/fast-fourier-transform">this post</a> , mentions quiet a few resources,but none of them fits the bill for me.</p>
  
  <p><strong>I need a compact implementation,within just 1 cpp .</strong> </p>
  
  <p>The degree for polynomial may range <strong>upto 10^6.</strong> , while the coefficients are all very small,<strong>upto 10^1.</strong></p>