Version analysis of FLIP

       Name: FLIP
       Type: Subroutine
   Category: Stardust
    Summary: Reflect the stardust particles in the screen diagonal and redraw
             the stardust field


 Swap the x- and y-coordinates of all the stardust particles and draw the new
 set of particles. Called by LOOK1 when we switch views.

 This is a quick way of making the stardust field in the new view feel
 different without having to generate a whole new field. If you look carefully
 at the stardust field when you switch views, you can just about see that the
 new field is a reflection of the previous field in the screen diagonal, i.e.
 in the line from bottom left to top right. This is the line where x = y when
 the origin is in the middle of the screen, and positive x and y are right and
 up, which is the coordinate system we use for stardust).


.FLIP

Cassette
\LDA MJ                 \ These instructions are commented out in the original
\BNE FLIP-1             \ source. They would have the effect of not swapping the
                        \ stardust if we had mis-jumped into witchspace

Master
 LDA #DUST              \ Switch to stripe 3-2-3-2, which is cyan/red in the
 STA COL                \ space view

Cassette, Flight, Docked, 6502SP, Master
Electron
 LDY NOSTM              \ Set Y to the current number of stardust particles, so
                        \ we can use it as a counter through all the stardust
 LDY #NOST              \ Set Y to the number of stardust particles, so we can
                        \ use it as a counter through all the stardust

.FLL1

 LDX SY,Y               \ Copy the Y-th particle's y-coordinate from SY+Y into X

 LDA SX,Y               \ Copy the Y-th particle's x-coordinate from SX+Y into
 STA Y1                 \ both Y1 and the particle's y-coordinate
 STA SY,Y

 TXA                    \ Copy the Y-th particle's original y-coordinate into
 STA X1                 \ both X1 and the particle's x-coordinate, so the x- and
 STA SX,Y               \ y-coordinates are now swapped and (X1, Y1) contains
                        \ the particle's new coordinates

 LDA SZ,Y               \ Fetch the Y-th particle's distance from SZ+Y into ZZ
 STA ZZ

 JSR PIXEL2             \ Draw a stardust particle at (X1,Y1) with distance ZZ

 DEY                    \ Decrement the counter to point to the next particle of
                        \ stardust

 BNE FLL1               \ Loop back to FLL1 until we have moved all the stardust
                        \ particles

 RTS                    \ Return from the subroutine