;;; Subroutine: drunkwalk
;;; Spawn a particle, walk it randomly until it's adjacent to a set
;;; pixel, then draw it. If it goes out of bounds, start over (spawn
;;; another).
;;; This is the innermost loop, so it should be as optimized as
;;; possible.

; Here be self-modifying code. All the 'cpx #0' and 'cpy #0' get
; their operands modified by set_limits.

; BEWARE!
; Y and X are backwards: Y holds the X coordinate, and X holds the
; Y coordinate. Has to be, because there's no (zpind),x addressing
; mode.

; TODO: Fix the cycle counts. I did them from my years-old memory, and
; I got at least one wrong, after looking it up again.

drunkwalk:
oob:
 ldy RANDOM ; spawn a new particle
 lda (spawn_x),y
 sta cursor_x
 lda (spawn_y),y
 sta cursor_y
 tax
 ldy cursor_x

 lda lineaddrs_l,x    ; 5
 sta pixptr           ; 3
 lda lineaddrs_h,x    ; 5
 sta pixptr+1         ; 3

move_pixel:
 bit RANDOM     ; 4 ; use top 2 bits (probably more random, definitely faster)
 bmi lr         ; 3/4
 bvc down       ; 3/4
up:
 dex            ; 2 ; N=1 V=1 up
selfmod_ymin = * + 1
 cpx #0         ; 2
 beq oob        ; 3
 stx cursor_y
 jmp check_lru
down:
 inx            ; 2 ; N=1 V=0 down
selfmod_ymax = * + 1
 cpx #0         ; 2
 beq oob        ; 3
 stx cursor_y
 jmp check_lrd
lr:
 bvc right      ; 3/4
left:
 dey            ; 3 ; N=0 V=1 left
selfmod_xmin = * + 1
 cpy #0         ; 2
 beq oob        ; 3
 sty cursor_x
 ; check left neighbor (we just vacated the right one)
 dey
 lda (pixptr),y
 bne stick
 iny
 jmp check_ud    ; 3 ; still have to check Y (up/down) neighbors.
right:
 iny            ; 3 ; N=0 V=0 right
selfmod_xmax = * + 1
 cpy #0         ; 2
 beq oob        ; 3
 sty cursor_x
 ; check right neighbor (we just vacated the left one)
 iny
 lda (pixptr),y
 bne stick
 dey
 ; fall through to check_ud

check_ud:
 ; check up neighbor
 lda lineaddrs_l-1,x
 sta pixptr2
 lda lineaddrs_h-1,x
 sta pixptr2+1
 lda (pixptr2),y
 bne stick

 ; check down neighbor
 lda lineaddrs_l+1,x
 sta pixptr2
 lda lineaddrs_h+1,x
 sta pixptr2+1
 lda (pixptr2),y
 bne stick
 jmp move_pixel ; pixel didn't stick, move it again.

check_lru:
 ; pixel's Y coord changed, must update pointer.
 lda lineaddrs_l,x    ; 5
 sta pixptr           ; 3
 lda lineaddrs_h,x    ; 5
 sta pixptr+1         ; 3

 ; check left neighbor
 dey
 lda (pixptr),y
 bne stick
 iny
 ; check right neighbor
 iny
 lda (pixptr),y
 bne stick
 dey
 ; check up neighbor
 lda lineaddrs_l-1,x
 sta pixptr2
 lda lineaddrs_h-1,x
 sta pixptr2+1
 lda (pixptr2),y
 bne stick
 jmp move_pixel ; pixel didn't stick, move it again.

check_lrd:
 ; pixel's Y coord changed, must update pointer.
 lda lineaddrs_l,x    ; 5
 sta pixptr           ; 3
 lda lineaddrs_h,x    ; 5
 sta pixptr+1         ; 3

 ; check left neighbor
 dey
 lda (pixptr),y
 bne stick
 iny
 ; check right neighbor
 iny
 lda (pixptr),y
 bne stick
 dey
 ; check down neighbor
 lda lineaddrs_l+1,x
 sta pixptr2
 lda lineaddrs_h+1,x
 sta pixptr2+1
 lda (pixptr2),y
 bne stick
 jmp move_pixel ; pixel didn't stick, move it again.

stick:
 ; pixel stuck next to an existing pixel, draw it and return.
 ldx cursor_y
 lda lineaddrs_l,x
 sta pixptr
 lda lineaddrs_h,x
 sta pixptr+1
 ldy cursor_x
 lda #1
 sta (pixptr),y
 rts