# n-body Classic Fortran #4 program

## source code

```! The Computer Language Benchmarks Game
! https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
!
!   Translated from Mark C. Lewis nbody.java by Simon Geard
!   Revised by Mike Garrahan
!   minor changes for speed improvement by Gilbert Brietzke
!
! ifort -O2 -xHost -o nbody nbody.f90

program nbody
implicit none
integer, parameter :: dp=kind(1.d0)
real(dp),parameter :: TSTEP=0.01d0, PI=3.141592653589793d0
real(dp),parameter :: SOLAR_MASS=4*PI*PI,DAYS_PER_YEAR=365.24d0
integer ,parameter :: NB=5,NPAIR=NB*(NB-1)/2
real(dp) :: x(3,NB), v(3,NB), mass(NB), e
integer :: nstep,i
character(len=8) :: argv
call init(x,v,mass)
e = energy(x,v,mass)
write (*,'(f12.9)') e
do i = 1, nstep
end do
e = energy(x,v,mass)
write (*,'(f12.9)') e
contains
real(dp), intent(inout) :: x(3,NB),v(3,NB)
real(dp), intent(in) :: mass(NB)
real(dp) :: r(3,NPAIR),rmag(3),mag(NPAIR)
integer :: i,j,k
k = 1
do i = 1, NB - 1
do j = i + 1, NB
r(:,k) = x(:,i) - x(:,j)
k = k + 1
end do
end do
mag = TSTEP/norm2(r,dim=1)**3
k = 1
do i = 1, NB - 1
do j = i + 1, NB
v(:,i) = v(:,i) - mass(j)*mag(k)*r(:,k)
v(:,j) = v(:,j) + mass(i)*mag(k)*r(:,k)
k = k + 1
end do
end do
x = x + TSTEP*v
function energy(x,v,mass)
real(dp), intent(in) :: x(3,NB),v(3,NB),mass(NB)
real(dp) :: energy,pe
integer :: i,j
energy = 0.5d0*dot_product(mass,sum(v**2,dim=1))
do i = 1, NB - 1
do j = i + 1, NB
pe = pe - mass(i)*mass(j)/norm2(x(:,i) - x(:,j))
end do
end do
energy = energy + pe
end function energy
subroutine init(x,v,mass)
real(dp),intent(out)::x(3,NB),v(3,NB),mass(NB)
real(dp),dimension(3,NB),parameter :: xi=reshape([0.d0,0.d0,0.d0,&
&4.84143144246472090d+00,-1.16032004402742839d+00,-1.03622044471123109d-01,&
&8.34336671824457987d+00, 4.12479856412430479d+00,-4.03523417114321381d-01,&
&1.28943695621391310d+01,-1.51111514016986312d+01,-2.23307578892655734d-01,&
&1.53796971148509165d+01,-2.59193146099879641d+01, 1.79258772950371181d-01],&
&[3,NB])
real(dp),dimension(3,NB),parameter:: vi=reshape([0.d0,0.d0,0.d0,&
&1.66007664274403694d-03,7.69901118419740425d-03,-6.90460016972063023d-05,&
&-2.76742510726862411d-03,4.99852801234917238d-03, 2.30417297573763929d-05,&
&2.96460137564761618d-03,2.37847173959480950d-03,-2.96589568540237556d-05,&
&2.68067772490389322d-03,1.62824170038242295d-03,-9.51592254519715870d-05],&
&[3,NB])
real(dp),dimension(NB),parameter :: massi=[1.0d0,9.54791938424326609d-04,&
&2.85885980666130812d-04, 4.36624404335156298d-05, 5.15138902046611451d-05]

x = xi
v = vi*DAYS_PER_YEAR
mass = massi * SOLAR_MASS
v(:,1) = v(:,1) - matmul(v, mass)/mass(1)
end subroutine init
end program nbody
```

## notes, command-line, and program output

```NOTES:
Fortran Compiler Classic
2021.12.0 20240222

Mon, 03 Jun 2024 19:52:13 GMT

MAKE:
mv nbody.ifc-4.ifc nbody.ifc-4.f90
~/intel/oneapi/compiler/latest/bin/ifort -O3 -march=ivybridge -ipo -static nbody.ifc-4.f90 -o nbody.ifc-4.ifc_run
ifort: remark #10448: Intel(R) Fortran Compiler Classic (ifort) is now deprecated and will be discontinued late 2024. Intel recommends that customers transition now to using the LLVM-based Intel(R) Fortran Compiler (ifx) for continued Windows* and Linux* support, new language support, new language features, and optimizations. Use '-diag-disable=10448' to disable this message.
ipo: warning #11021: unresolved __ehdr_start
Referenced in libc.a(dl-support.o)
ld: /home/dunham/intel/oneapi/compiler/2024.1/lib/libifcoremt.a(for_close_proc.o): in function `for__close_proc':
for_close_proc.c:(.text+0x1df): warning: Using 'dlopen' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
rm nbody.ifc-4.f90

2.86s to complete and log all make actions

COMMAND LINE:
./nbody.ifc-4.ifc_run 50000000

PROGRAM OUTPUT:
-0.169075164
-0.169059907
```