From 3d6abe38d72347d6b254070edb50cf56f236f5d7 Mon Sep 17 00:00:00 2001
From: "B. Watson" <urchlay@slackware.uk>
Date: Tue, 8 Nov 2022 03:07:04 -0500
Subject: Add/fix comments, profiling for initscreen and render. Runtime ~3:05
 now.

---
 drunkwalk.s | 41 ++++++++++++++++++++++++++---------------
 1 file changed, 26 insertions(+), 15 deletions(-)

(limited to 'drunkwalk.s')

diff --git a/drunkwalk.s b/drunkwalk.s
index 02ab67d..8cea4cf 100644
--- a/drunkwalk.s
+++ b/drunkwalk.s
@@ -1,11 +1,20 @@
 ;;; Subroutine: drunkwalk
-;;; Walk the point around randomly until it either is
-;;; adjacent to a set pixel or goes out of bounds.
-;;; Return with Z=0 if out of bounds, Z=1 if it hit a pixel.
+;;; 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.
-; 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.
+
+; 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:
@@ -24,29 +33,29 @@ oob:
 
 move_pixel:
  bit RANDOM     ; 4 ; use top 2 bits (probably more random, definitely faster)
- bmi lr         ; 2/3
- bvc down       ; 2/3
+ bmi lr         ; 3/4
+ bvc down       ; 3/4
 up:
  dex            ; 2 ; N=1 V=1 up
 selfmod_ymin = * + 1
  cpx #0         ; 2
- beq oob        ; 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        ; 2
+ beq oob        ; 3
  stx cursor_y
  jmp check_lrd
 lr:
- bvc right      ; 2/3
+ bvc right      ; 3/4
 left:
  dey            ; 3 ; N=0 V=1 left
 selfmod_xmin = * + 1
  cpy #0         ; 2
- beq oob        ; 2
+ beq oob        ; 3
  sty cursor_x
  ; check left neighbor (we just vacated the right one)
  dey
@@ -58,7 +67,7 @@ right:
  iny            ; 3 ; N=0 V=0 right
 selfmod_xmax = * + 1
  cpy #0         ; 2
- beq oob        ; 2
+ beq oob        ; 3
  sty cursor_x
  ; check right neighbor (we just vacated the left one)
  iny
@@ -83,9 +92,10 @@ check_ud:
  sta pixptr2+1
  lda (pixptr2),y
  bne stick
- jmp move_pixel
+ 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
@@ -108,9 +118,10 @@ check_lru:
  sta pixptr2+1
  lda (pixptr2),y
  bne stick
- jmp move_pixel
+ 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
@@ -133,7 +144,7 @@ check_lrd:
  sta pixptr2+1
  lda (pixptr2),y
  bne stick
- jmp move_pixel
+ jmp move_pixel ; pixel didn't stick, move it again.
 
 stick:
  ; pixel stuck next to an existing pixel, draw it and return.
-- 
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