program tst
c
      parameter(ntms=25,imx=73,dlon=5.)
      dimension ut(ntms),gcmlon(imx),ix(imx)
      data censlt/12./
c
      do i=1,ntms
        ut(i) = float(i-1)
      enddo
      do i=1,imx
        gcmlon(i) = -180.+(i-1)*5.
      enddo
      do it=1,ntms
        slt0 = censlt-12.
        if (slt0.lt.0.) slt0 = slt0-12.
        ix0 = ixslt(slt0,ut(it),xlon,gcmlon,imx,dlon)
        write(6,"('censlt=',f5.2,' ut=',f5.2,' slt0=',f5.2,
     +    ' ix0=',i3,' xlon=',f7.2)") censlt,ut(it),slt0,ix0,xlon
        iflag = 0
        do i=1,imx
          ix(i) = mod(ix0+i-1,imx)
          if (ix(i).eq.0) ix(i) = imx
          if (i.gt.1.and.ix(i-1).gt.ix(i)) iflag = 1
          if (iflag.gt.0) ix(i) = ix(i)+1
        enddo
        write(6,"('ix=',/(18i4))") ix
      enddo
      stop
      end
c
      function ixslt(slt,ut,xlon,gcmlon,imx,dlon)
      dimension gcmlon(imx)
c
c Find nearest tgcm longitude index corresponding to 
c given solar local time (slt) and ut:
c (also return the longitude at slt in xlon)
c
      xlon = (slt-ut) * 15.
      if (xlon.ge.180.) xlon = xlon-360.
      if (xlon.lt.-180.) xlon = xlon+360.
      ilon = ixfind(gcmlon,imx,xlon,dlon)
      if (ilon.le.0) then
        write(6,"(' ')")
        write(6,"('>>> ixslt: slt=',f8.3,' ut=',f8.3,' ilon=',i3,
     +    ' apparent error from ixfind: xlon=',f12.3)")
     +    slt,ut,ilon,xlon
        stop 'ilon'
      endif
      ixslt = ilon
      return
      end
c
c------------------------------------------------------------------
c Begin file /home/sting/foster/lib/ixfind.f
c------------------------------------------------------------------
c
      function ixfind(z,nz,val,del)
c
c Find nearest index to val in z(nz), with increment del
c If not found, return -1
c
      dimension z(nz)
c
      do n=1,nz
        if (val.ge.z(n)-0.5*del.and.val.le.z(n)+0.5*del) then
          ixfind = n
          return
        endif
      enddo
      ixfind = -1
      return
      end