c
c
c
c
c =================================================
subroutine cellave(xlow,ylow,dx,dy,wl)
c =================================================
implicit double precision (a-h,o-z)
external fss
logical fl(5),alll,allr
dimension x(10),y(10),xx(5),yy(5)
common/fsscorn/ xc0,yc0,xc1,yc1
c
c # compute wl, fraction of cell that lies in left state.
c # For initial data with two states ql and qr separated by a
c # discontinuity. The curve along which the discontinuity lies is
c # specified by the function fdisc, which should return a value that
c # is negative on the side where ql lies and positive on the qr side.
c
c # xlow,ylow is the coordinate of the lower left corner of the cell.
c # dx, dy are grid spacing in x and y.
c
xx(1) = xlow
xx(2) = xlow
xx(3) = xlow+dx
xx(4) = xlow+dx
xx(5) = xx(1)
yy(1) = ylow
yy(2) = ylow+dy
yy(3) = ylow+dy
yy(4) = ylow
yy(5) = yy(1)
alll = .true.
allr = .true.
c
do 20 i=1,4
fl(i) = fdisc(xx(i),yy(i)) .lt. 0.d0
alll = alll .and. fl(i)
allr = allr .and. (.not. fl(i))
20 continue
fl(5) = fl(1)
c
if (alll) then
wl = 1.d0
return
endif
if (allr) then
wl = 0.d0
return
endif
c
iv = 0
do 40 i=1,4
if (fl(i)) then
iv = iv+1
x(iv) = xx(i)
y(iv) = yy(i)
endif
if (fl(i).neqv.fl(i+1)) then
iv = iv+1
xc0 = xx(i)
yc0 = yy(i)
xc1 = xx(i+1)
yc1 = yy(i+1)
ss = zeroin(0.d0, 1.d0, fss, 1d-8)
c write(27,*) 'xc,yc,ss:',xc0,yc0,xc1,yc1,ss
x(iv) = xx(i) + ss*(xx(i+1)-xx(i))
y(iv) = yy(i) + ss*(yy(i+1)-yy(i))
endif
40 continue
c
c # compute area:
c
if (iv.eq.0) then
wl = 0.d0
return
endif
c
x(iv+1) = x(1)
y(iv+1) = y(1)
area = 0.d0
do 50 i=1,iv
area = area + .5d0*(y(i)+y(i+1))*(x(i+1)-x(i))
c write(27,*) ' x,y:',x(i),y(i)
50 continue
c
wl = area / (dx*dy)
c write(27,*) 'area,wl:',area,wl
c
return
end