[Haskell-cafe] C-like Haskell

28 Jan 2009

      I'm trying to do a calculation for Gauss' circle problem, which counts the
integer lattice points with distance to the origin <= r.  It's sequence
A000328 on the AT&T integer sequence database.  I can't figure out a way to
do it quickly in Haskell for r around 10^9.

Here's my attempt, which takes about 75s for r=10^8.

circ2 r = (1+4*r) + 4 * (circ2' (rs+1) r 1 0)
          where
            rs = r^2
--            circ2' :: Int64 -> Int64 -> Int64 -> Int64 -> Int64                        
            circ2' rad x y sum
                | xrs) {
                        rad-=2*x;
                        rad++;
                        x--;
                }
                sum+=1+2*(x-y);
                rad+=2*y+1;
                y++;

        }
        sum*=4;
        sum++;

        return sum;
}

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[Haskell-cafe] C-like Haskell

drblanco