From ae9dd0674d002e726566a20e7756c3619244576d Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@pobox.com>
Date: Sun, 22 Jan 2023 10:42:37 -0800
Subject: [PATCH] corrupt! but it produces pixels

---
 mandel.s | 64 ++++++++++++++++++++++++--------------------------------
 1 file changed, 27 insertions(+), 37 deletions(-)

diff --git a/mandel.s b/mandel.s
index df80ed7..b4baad5 100644
--- a/mandel.s
+++ b/mandel.s
@@ -168,7 +168,7 @@ color_map:
 .endmacro
 
 .macro add32 dest, arg1, arg2
-    add 2, dest, arg2, dest
+    add 4, dest, arg2, dest
 .endmacro
 
 ; 2 + 9 * byte cycles
@@ -422,6 +422,13 @@ next:
 
 .endmacro
 
+.macro shift_round_16 arg, shift
+    .repeat shift
+        shl32 arg
+    .endrepeat
+    round16 arg
+.endmacro
+
 .proc mandelbrot
     ; input:
     ; cx: position scaled to 4.12 fixed point - -8..+7.9
@@ -445,64 +452,48 @@ initloop:
     bne initloop
 
 loop:
-    ; 1939 - 3007 cyc
-
-    ; iter++ & max-iters break = 7 cyc
-    inc iter       ; 5 cyc
-    bne keep_going ; 2 cyc
+    ; iter++ & max-iters break
+    inc iter
+    bne keep_going
     rts
 keep_going:
 
     ; 4.12: (-8 .. +7.9)
-    ; zx = zx_2  - zy_2  + cx   = 3 * 20 = 60 cyc
-    sub16 zx, zx_2, zy_2
+    ; zx = zx_2  - zy_2  + cx
+    sub16 zx, zx_2 + 2, zy_2 + 2
     add16 zx, zx, cx
 
-    ; zy = zx_zy + zx_zy + cy   = 3 * 20 = 60 cyc
-    sub16 zy, zx_zy, zx_zy
+    ; zy = zx_zy + zx_zy + cy
+    add16 zy, zx_zy + 2, zx_zy + 2
     add16 zy, zy, cy
 
     ; 8.24: (-128 .. +127.9)
-    ; zx_2 = zx * zx            = 518 - 828 cyc
+    ; zx_2 = zx * zx
     imul16 zx_2, zx, zx
 
-    ; zy_2 = zy * zy            = 518 - 828 cyc
+    ; zy_2 = zy * zy
     imul16 zy_2, zy, zy
 
-    ; zx_zy = zx * zy           = 518 - 828 cyc
+    ; zx_zy = zx * zy
     imul16 zx_zy, zx, zy
 
-    ; dist = zx_2 + zy_2        = 38 cyc
+    ; dist = zx_2 + zy_2
     add32 dist, zx_2, zy_2
 
-    ; if dist >= 4 break, else continue iterating = 7 cyc
-    lda dist + 3  ; 3 cyc
-    cmp #4        ; 2 cyc
-    bmi still_in  ; 2 cyc
+    ; if dist >= 4 break, else continue iterating
+    lda dist + 3
+    cmp #4
+    bmi still_in
     rts
 still_in:
 
-    ; shift and round zx_2 to 4.12 = (60 + 5) - (60 + 28) = 65 - 88 cyc
-    .repeat 4      ; 60 cyc
-        shl24 zx_2 ; 15 cyc
-    .endrepeat
-    round16 zx_2   ; 5-28 cycles
-
-    ; shift and round zy_2 to 4.12 = (20 + 5) - (20 + 28) = 65 - 88 cyc
-    .repeat 4      ; 60 cyc
-        shl24 zy_2 ; 15 cyc
-    .endrepeat
-    round16 zy_2   ; 5-28 cycles
-
-    ; shift and round zx_zy to 4.12 = (20 + 5) - (20 + 28) = 65 - 88 cyc
-    .repeat 4       ; 60 cyc
-        shl24 zx_zy ; 15 cyc
-    .endrepeat
-    round16 zx_zy   ; 5-28 cycles
+    shift_round_16 zx_2, 4
+    shift_round_16 zy_2, 4
+    shift_round_16 zx_zy, 4
 
     ; if may be in the lake, look for looping output with a small buffer
     ; as an optimization vs running to max iters
-    jmp loop ; 3 cycles
+    jmp loop
 
 .endproc
 
@@ -708,7 +699,6 @@ loop_sx:
     zoom_factor cx, sx, zoom, aspect_x
     zoom_factor cy, sy, zoom, aspect_y
     jsr mandelbrot
-    inc iter
     jsr pset
 
     clc