c===========================================================
c
c     vswave: Generates time-series of profiles of 
c     left-moving "wave" (f(t+x) = constant) and outputs 
c     profiles via 'vsxynt' interface.  
c
c===========================================================
      program     gpwave

      implicit    none

      integer     i4arg

      integer     maxn
      parameter ( maxn = 10 000 )

      real*8      f
      real*8      x(maxn),     y(maxn)
      integer     i,           j,          n,         nx,
     &            nt
      real*8      h,           t,          dt

      n = i4arg(1,-1)
      if( n .lt. 1  .or.  n .gt. maxn ) goto 900
      nx = n
      nt = n

      h = 1.0d0 / (nx - 1)
      x(1) = 0.0d0
      do j = 1 , nx - 1
         x(j+1) = x(j) + h
      end do

      t = 0.0d0
      dt = 1.0d0 / (nt - 1)
      do i = 1 , nt
         do j = 1 , nx 
c-----------------------------------------------------------
c           Define y(x,t)
c-----------------------------------------------------------
            y(j) = f(mod((x(j) + t),1.0d0))
         end do
c-----------------------------------------------------------
c        Output data via 'vsxynt' interface.
c-----------------------------------------------------------
         call vsxynt('wave',t,x,y,nx)
         t = t + dt 
      end do

      stop

900   continue
         write(0,*) 'usage: vswave <n>'
      stop

      end

c===========================================================
c     Gaussian function.
c===========================================================
      double precision function f(x)
         implicit      none
         real*8        x
         f = exp(-((x-0.5d0)/0.1d0)**2)
         return
      end