!
!       project "gravity field"
!
!-----------------------------------------------------------------------
module realfield
  implicit none

!-----------------------------------------------------------------------
!      definition of structures
!
type massbody
  real                  :: posx, posy
  real                  :: mass          = 1.0
  integer               :: serial        = 666
end type

!-----------------------------------------------------------------------
contains
!-----------------------------------------------------------------------

function compute_gravity(fx, fy, body)
  real, intent(in)              :: fx, fy
  type(massbody), intent(in)    :: body
  real                          :: compute_gravity
  real                          :: rx, ry, dist

  rx = fx - body%posx
  ry = fy - body%posy
  dist = sqrt( (rx*rx) + (ry*ry) )
  if (dist .LT. 4.50) then
    write (0, *) "dist too small ", dist
    compute_gravity = 0e0
  endif
  compute_gravity = body%mass / (dist ** 2)

end function

!-----------------------------------------------------------------------
!-
!   Compute the gravity field in a pre-allocated array relative
!   to the massbody 'moon'. Nobody know where the magic number
!   come from, sorry.
!-
subroutine compute_a_field(field, moon)
  real, dimension(:,:), intent(out)  :: field
  type(massbody), intent(in)         :: moon

  integer                            :: ix, iy
  real                               :: fx, fy
  real                               :: grav

  ! print *, "pic size  ", ubound(field, 1), "W", ubound(field, 2), "H"
  ! print *, "mass body ", moon

  do ix=1, ubound(field, 1)
    fx = real(ix)
    do iy=1, ubound(field, 2)
      fy = real(iy)
      grav = compute_gravity(fx, fy, moon)
      field(ix,iy) = grav
    enddo
  enddo

end subroutine
!-----------------------------------------------------------------------

!-----------------------------------------------------------------------

end module