program linear_motion_wall_room

 1001 write(*,*)'x?'
      read(*,*)x
      write(*,*)'y?'
      read(*,*)y
      write(*,*)'Vx?'
      read(*,*)vx
      write(*,*)'Vy?'
      read(*,*)vy
      xold=x
      yold=y

      w_left=5.0
      w_right=7.5
      w_top=7.0
      w_bottom=3.0

      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,-1)

      call draw_wall(w_left,w_right,w_top,w_bottom)

      room=r+0.02
      call extra_room(room,1.0,x_min,x_max,y_min,y_max)
      call extra_room(room,-1.0,w_left,w_right,w_bottom,w_top)

      dt=0.00001
 1003 do i=1,10000000
        x = x + vx*dt
        y = y + vy*dt
c       call pbc(x,y,x_min,x_max,y_min,y_max)
        call box(x,y,vx,vy,x_min,x_max,y_min,y_max)
        call wall(x,y,xold,yold,vx,vy,w_left,w_right
     &,w_top,w_bottom)
        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,2=continue)'
      read(*,*)j
      if(j==0)stop
      if(j==1)goto 1001
      if(j==2)goto 1003
      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

      subroutine wall(x,y,xo,yo,vx,vy,wl,wr,wt,wb)
      if((y<wt).and.(y>wb))then
        if((xo<wl).and.(x>wl))vx=(-1)*vx
        if((xo>wr).and.(x<wr))vx=(-1)*vx
      end if
      if((x<wr).and.(x>wl))then
        if((yo<wb).and.(y>wb))vy=(-1)*vy
        if((yo>wt).and.(y<wt))vy=(-1)*vy
      end if
      end

      subroutine draw_wall(wl,wr,wt,wb)
      real wx(5),wy(5)
      wx(1)=wl
      wy(1)=wt
      wx(2)=wr
      wy(2)=wt
      wx(3)=wr
      wy(3)=wb
      wx(4)=wl
      wy(4)=wb
      wx(5)=wx(1)
      wy(5)=wy(1)
c     call pgsfs(0)
c     call pgpoly(4,wx,wy)
      call pgline(5,wx,wy)
      end

      subroutine extra_room(room,coeff,x_min,x_max,y_min,y_max)
      x_min = x_min + coeff*room
      x_max = x_max - coeff*room
      y_min = y_min + coeff*room
      y_max = y_max - coeff*room
      end