'*****************************************************
'* This program calculates the internal surface of a *
'* polygon defined by n points of coordinates:       *
'*         x1, y1, x2, y2,...xn, yn.                 *
'* ------------------------------------------------- *
'* Ref.: "Mathematiques 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                                   *
'*                                                   *
'*                      BASIC version by J-P Moreau. *
'*                           (www.jpmoreau.fr)       *
'*****************************************************
defint i-n

NMAX=50

DIM buffer(NMAX,NMAX)

  s=0
  npoints=13                           '13 points
  ' init buffer x(i), y(i)
  for i=1 to 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
  next
  'calculate internal surface
  for i=1 to npoints-1
    s=s+(buffer(i,1)+buffer(i+1,1))*(buffer(i,2)-buffer(i+1,2))
  next
  s=abs(s)/2
  'print surface
  cls
  print
  print " Surface is: "; s
  print
  print
END

' end of file surface.bas