seg_intersect.F

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!||====================================================================
!||    seg_intersect       ../engine/source/materials/fail/failwave/seg_intersect.F
!||--- called by ------------------------------------------------------
!||    set_failwave_nod3   ../engine/source/materials/fail/failwave/set_failwave_nod3.F
!||    set_failwave_nod4   ../engine/source/materials/fail/failwave/set_failwave_nod4.F
!||====================================================================
       INTEGER FUNCTION seg_intersect(X1    ,Y1    ,X2    ,Y2    ,X3    ,Y3    ,
     .                                X4    ,Y4    ,XINT  ,YINT  ,IDEBUG       )
C-----------------------------------------------
C   I m p l i c i t   T y p e s
C-----------------------------------------------
#include      "implicit_f.inc"
C-----------------------------------------------
C   D u m m y   A r g u m e n t s
C-----------------------------------------------
      my_real  :: x1,y1,x2,y2,x3,y3,x4,y4,xint,yint
C-----------------------------------------------
C   L o c a l   V a r i a b l e s
C-----------------------------------------------
      INTEGER  :: ok,idebug
      my_real  :: ax,bx,ay,by,cx,cy,dm,alpha,beta
c----------------------------------------------------------
c     Calculated intersection of two 2D segments: N1-N2 and N3-N4
c     intersection finds => ok = 1, otherwise ok = 0
c     alpha, beta = ratio of the intersection point of each segment
C=======================================================================
      ok = 0
      ax = x2 - x1
      ay = y2 - y1
      bx = x4 - x3
      by = y4 - y3
c
      dm = ay*bx - ax*by
      IF (dm /= zero) THEN  ! the lines are not parallel
        cx = x3 - x1
        cy = y3 - y1
c
         alpha = (bx * cy - by * cx) / dm
         beta  = (ax * cy - ay * cx) / dm
c
         IF (alpha >= zero .and. alpha <= one .and.
     .       beta  >= zero .and. beta  <= one) THEN
           ok = 1
           xint = x1 + alpha * ax
           yint = y1 + alpha * ay
         ENDIF
c         IF (IDEBUG == 1) print*,'ALPHA,BETA=',ALPHA,BETA
       ENDIF
       seg_intersect = ok
c-----------
      RETURN
      END