proc screensaver {w} {
   destroy $w ;# just to make sure
   toplevel $w -bg black
   wm overrideredirect $w 1
   wm geometry $w [winfo screenwidth $w]x[winfo screenheight $w]+0+0
   focus $w ;# so it gets keypresses
   bind $w <Key>    [list destroy $w]
   bind $w <Button> [list destroy $w]
   bind $w <Motion> [list destroy $w]
   return $w
 }

 exec xset s activate

 # Handle command line options.
 switch -glob -- [lindex $argv 0] {
        "" { # Do nothing special. Start the screen saver }
        /s { # Same as above }
        /p { 
                # No preview mode
                exit
        }
        /c* { 
                # No configuration options. Show a message box.
                wm withdraw .
                tk_messageBox -title "TclScreenSaver Configuration" -message "There are no configuration options available for this screen saver."
                exit
        }
 }

 proc InitScreenSaver {} {
        global CursorStartX
        global CursorStartY

        # Make the main window solid black.
        . configure -background black

        # Make the main window fill the entire screen.
        wm geometry . [winfo screenwidth .]x[winfo screenheight .]+0+0

        # Take away the window decoration.
        wm overrideredirect . 1

        # Make the screensaver ontop of all other windows.  Catch any errors in case we´re not running under Windows.
        catch {wm attributes . -topmost 1}

        # Make sure the window has the current focus.
        raise .

        # Save the starting position of the mouse pointer.
        set CursorStartX [winfo pointerx .]
        set CursorStartY [winfo pointery .]

        # Hide the mouse pointer in the lower-right corner of the screen.
        event generate . <Motion> -warp true -x [winfo screenwidth .] -y [winfo screenheight .]
        update

        # Exit the application when mouse or keyboard activity is detected.
        bind . <KeyPress>         ExitScreenSaver
        bind . <ButtonPress>         ExitScreenSaver
        bind . <Motion>                 ExitScreenSaver

        return
 }

 proc ExitScreenSaver {} {
        global CursorStartX
        global CursorStartY

        # Erase previous binding on mouse motion so that we don´t get an infinite loop with the next line.
        bind . <Motion> {}

        # Move the mouse pointer back to the starting position.
        event generate . <Motion> -warp true -x $CursorStartX -y $CursorStartY

        # Close the application.  The 'after idle' is needed or the mouse pointer won't be re-positioned.
        after idle exit
 }


 proc CreateAnimation {widget} {
        global PositionX PositionY
        global IncrementX IncrementY
        global IncrementThreshold

        # This is the maximum number of pixels the widget will move in any direction at a time.
        set IncrementThreshold        4

        # Set the amount the widget will initially move in the X and Y directions. Make sure both are not zero or the widget will stand still..
        set IncrementX($widget) 0
        set IncrementY($widget) 0
        while { $IncrementX($widget) == 0  &&  $IncrementY($widget) == 0 } {
                set IncrementX($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
                set IncrementY($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
        }

        # The initital starting position coordinates of the widget, which will be a random spot on the screen.
        set PositionX($widget) [expr { int(rand()*[winfo screenwidth .]) }]
        set PositionY($widget) [expr { int(rand()*[winfo screenheight .]) }]

        RedisplayAnimation $widget

        return
 }


 proc RedisplayAnimation {widget} {
        global PositionX PositionY
        global IncrementX IncrementY
        global IncrementThreshold

        if { ![winfo exists $widget] } {
                return
        }

        # Increase/decrease the X and Y coordinates of the widget, according to the current increment values.
        incr PositionX($widget) $IncrementX($widget)
        incr PositionY($widget) $IncrementY($widget)

        if {$PositionX($widget) <= 0} {
                # The widget we are animating has hit the left edge of the screen.  

                # Make sure it isn´t past the edge.
                set PositionX($widget) 0

                # The new X increment value must be positive, but the new Y increment value can be positive, negative, or zero.
                set IncrementX($widget) [expr { int(rand()*$IncrementThreshold)+1 }]
                set IncrementY($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
        }

        if {$PositionX($widget) >= ([winfo screenwidth .] - [winfo width $widget]) } {
                # The widget has hit the right edge of the screen.

                set PositionX($widget) [expr { [winfo screenwidth .] - [winfo width $widget] }]

                # X must be negative, but Y can be any.
                set IncrementX($widget) [expr { int(rand()*-$IncrementThreshold)-1 }]
                set IncrementY($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
        }

        if {$PositionY($widget) <= 0} {
                # The widget has hit the top edge of the screen.

                set PositionY($widget) 0

                # Y must be positive, but X can be any.
                set IncrementX($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
                set IncrementY($widget) [expr { int(rand()*$IncrementThreshold)+1 }]
        }

        if {$PositionY($widget) >= ([winfo screenheight .] - [winfo height $widget]) } {
                # The widget has hit the bottom edge of the screen.

                set PositionY($widget) [expr { [winfo screenheight .] - [winfo height $widget] }]

                # Y must be negative, but X can be any.
                set IncrementX($widget) [expr { int(rand()*($IncrementThreshold*2)-$IncrementThreshold) }]
                set IncrementY($widget) [expr { int(rand()*-$IncrementThreshold)-1 }]
        }


        # Adjust the position of the widget.
        place $widget -x $PositionX($widget) -y $PositionY($widget)

        # Call this procedure again in the near future.
        after 10 [list RedisplayAnimation $widget]

        return
 }

 proc UpdateTime {label} {
        $label configure -text [clock format [clock seconds] -format %c]
        after 1000 [list UpdateTime $label]
        return
 }


 InitScreenSaver
 CreateAnimation [label .title -text "Tcl Screen Saver" -font {-weight bold} -bg black -fg white -padx 0 -pady 0]
 CreateAnimation [label .author -text "by Paul Walton" -font {-weight bold} -bg black -fg white -padx 0 -pady 0]
 CreateAnimation [label .time -text "" -font {-weight bold} -bg black -fg green -padx 0 -pady 0]
 UpdateTime .time

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