!*****************************************************
!* This program calculates the internal surface of a *
!* polygon defined by n points of coordinates:       *
!*         x1, y1, x2, y2,...xn, yn.                 *
!* ------------------------------------------------- *
!* Ref.: "Mathématiques en Turbo-Pascal By M. Ducamp *
!* and A. Reverchon (vol 2), Eyrolles, Paris, 1988"  *
!* [BIBLI 05].                                       *
!* ------------------------------------------------- *
!* SAMPLE RUN:                                       *
!*                                                   *
!* npoints=13                                        *
!* x1=3    y1=3                                      *
!* x2=12   y2=3                                      *
!* x3=12   y3=6                                      *
!* x4=11   y4=6                                      *
!* x5=11   y5=7                                      *
!* x6=15   y6=7                                      *
!* x7=15   y7=9                                      *
!* x8=8    y8=9                                      *
!* x9=8    y9=6                                      *
!* x10=4   y10=6                                     *
!* x11=4   y11=9                                     *
!* x12=3   y12=9                                     *
!* x13=3   y13=3                                     *
!*                                                   *
!* Surface is:    47.00000                           *
!*                                                   *
!*                        F90 version by J-P Moreau. *
!*                            (www.jpmoreau.fr)      *
!*****************************************************
PROGRAM Surface

parameter(NMAX=50)

real*4 buffer(NMAX,NMAX)

  s=0
  npoints=13                           !13 points
  ! init buffer x(i), y(i)
  do i=1, npoints
    buffer(1,1)= 3; buffer(1,2)=3
    buffer(2,1)=12; buffer(2,2)=3
    buffer(3,1)=12; buffer(3,2)=6
    buffer(4,1)=11; buffer(4,2)=6
    buffer(5,1)=11; buffer(5,2)=7
    buffer(6,1)=15; buffer(6,2)=7
    buffer(7,1)=15; buffer(7,2)=9
    buffer(8,1)= 8; buffer(8,2)=9
    buffer(9,1)= 8; buffer(9,2)=6
    buffer(10,1)=4; buffer(10,2)=6
    buffer(11,1)=4; buffer(11,2)=9
    buffer(12,1)=3; buffer(12,2)=9
    buffer(13,1)=3; buffer(13,2)=3
  end do
  !calculate internal surface
  do i=1, npoints-1
    s=s+(buffer(i,1)+buffer(i+1,1))*(buffer(i,2)-buffer(i+1,2))
  end do
  s=abs(s)/2
  !print surface
  print *,''
  print *,' Surface is: ', s
  print *,''
  print *,''
END

! end of file surface.f90