program linear_motion

 1001 write(*,*)'x?'
      read(*,*)x
      write(*,*)'y?'
      read(*,*)y
      write(*,*)'Vx?'
      read(*,*)vx
      write(*,*)'Vy?'
      read(*,*)vy
      
      x_min=0.0
      x_max=10.0
      y_min=0.0
      y_max=10.0
      r=0.1
      if(pgopen('/xwin')<=0)stop
      call pgenv(x_min,x_max,y_min,y_max,1,0)

      dt=0.00001
      do i=1,1000000
        x = x + vx*dt
        y = y + vy*dt
        call pbc(x,y,x_min,x_max,y_min,y_max)
c       call box(x,y,vx,vy,x_min,x_max,y_min,y_max)
        call pgbbuf
         call pgsci(0)
         call pgcirc(xold,yold,r)
         call pgsci(1)
         call pgcirc(x,y,r)
        call pgebuf
        xold=x
        yold=y
      end do

 1002 write(*,*)'Again?(1=yes,0=no)'
      read(*,*)j
      if(j==0)stop
      if(j==1)goto 1001
      goto 1002
      end

      subroutine box(x,y,vx,vy,x_min,x_max,y_min,y_max)
c     if(x>x_max)vx=(-1)*vx
c     if(x<x_min)vx=(-1)*vx
      if((x<x_min).or.(x>x_max))vx=(-1)*vx
c     if(y<y_min)vy=(-1)*vy
c     if(y>y_max)vy=(-1)*vy
      if((y<y_min).or.(y>y_max))vy=(-1)*vy
      end

      subroutine pbc(x,y,x_min,x_max,y_min,y_max)
      if(x<x_min) x = x + (x_max-x_min)
      if(x>x_max) x = x - (x_max-x_min)
      if(y<y_min) y = y + (y_max-y_min)
      if(y>y_max) y = y - (y_max-y_min)
      end