[stella] Y-Sorting alternative for Thomas :-)

[Manuel Polik <cybergoth@xxxxxxxx>]

Hi there!

Here's how I sort the sprites in my intelligent flicker 
routine. It is adopted from Steve Judds and Pasi Ojalas 
excellent sorting article in C= Hacking #18.

It goes like this:

; Init Sortloop
            LDA #$80
            STA sprtlst+MAXSPRITE   ; Set Head node
            LDX #MAXSPRITE-1        ; Sprites to sort

; Y Sort Sprites
SortYPos
            LDY #MAXSPRITE
SortLoop:   LDA sprtlst,Y    ;Linked list of objects
            BMI Link
            STY tempVar1       ; Head index
            TAY              ;Next object
            LDA vpos,Y         ;If farther, then
            CMP vpos,X         ;move down list
            BCS SortLoop
            TYA
            LDY tempVar1        ;Insert into list
Link:       STA sprtlst,X    ;X -> rest of list
            TXA
            STA sprtlst,Y    ;beginning of list -> X
            DEX
            BPL SortYPos

Might this be even quicker than bubble-sort? :-)

Accessing data is way cool, too, this iterates once 
through the sorted(!) list:

	LDX sprtlst+MAXSPRITE
Loop
	Do Something YYYY,X
	LDA sprtlst,X
	TAX
	BPL Loop

Greetings,
	Manuel

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