N-body bench

[bearophile]

Since my post someone has added a Fortran version based on the 
algorithm used in the C++11 code. It's a little faster than the 
C++11 code and it's much nicer looking:
http://benchmarksgame.alioth.debian.org/u32/program.php?test=nbody&lang=ifc&id=5


pure subroutine advance(tstep, x, v, mass)
   real*8, intent(in) :: tstep
   real*8, dimension(4,nb), intent(inout) :: x, v
   real*8, dimension(nb), intent(in) :: mass
   real*8 :: r(4,N),mag(N)

   real*8 :: distance, d2
   integer :: i, j, m
   m = 1
   do i = 1, nb
      do j = i + 1, nb
         r(1,m) = x(1,i) - x(1,j)
         r(2,m) = x(2,i) - x(2,j)
         r(3,m) = x(3,i) - x(3,j)
         m = m + 1
      end do
   end do

   do m = 1, N
      d2 = r(1,m)**2 + r(2,m)**2 + r(3,m)**2
      distance = 1/sqrt(real(d2))
      distance = distance * (1.5d0 - 0.5d0 * d2 * distance * 
distance)
      !distance = distance * (1.5d0 - 0.5d0 * d2 * distance * 
distance)
      mag(m) = tstep * distance**3
   end do

   m = 1
   do i = 1, nb
      do j = i + 1, nb
         v(1,i) = v(1,i) - r(1,m) * mass(j) * mag(m)
         v(2,i) = v(2,i) - r(2,m) * mass(j) * mag(m)
         v(3,i) = v(3,i) - r(3,m) * mass(j) * mag(m)

         v(1,j) = v(1,j) + r(1,m) * mass(i) * mag(m)
         v(2,j) = v(2,j) + r(2,m) * mass(i) * mag(m)
         v(3,j) = v(3,j) + r(3,m) * mass(i) * mag(m)

         m = m + 1
      end do
   end do

   do i = 1, nb
      x(1,i) = x(1,i) + tstep * v(1,i)
      x(2,i) = x(2,i) + tstep * v(2,i)
      x(3,i) = x(3,i) + tstep * v(3,i)
   end do
   end subroutine advance


Bye,
bearophile