reduce two bytes by replacing jae and xor with adc (#63)

README.md

@@ -81,9 +81,9 @@ It is so small I could fit it into a single QR:
 
 <img src="/demo/qr.png" width="250"/>
 
-It's 60 bytes.
+It's 58 bytes.
 
-How little is 60 bytes? Well, this line of text weighs more than 70 bytes.
+How little is 58 bytes? Well, this line of text weighs more than 70 bytes.
 
 And so does this arbitrary sequence of emojis: 👩🏼‍❤️‍💋‍👨🏼🧔🏽‍♀️👩🏼‍❤️‍💋‍👨🏼
 
@@ -94,19 +94,17 @@ An **empty** C program generated with `gcc -Os -w -xc - <<< "main;"` on linux-x8
   <br/>
 
 ```
-c53000b80000cd108b3f
-8d22e54021c30837bbd0
-0778f5e4606bc00ad414
-d5449801c739df73dc30
-1d79d8ad893a7bdc880f
-83eb5079f95b202779d6
+c53000b80000cd108b3f8d22e5402
+1c3300fbbd0077af5e4606bc00ad4
+14d5449801c739dfad10257bd9893
+a74de880f83eb5079f95b88277bd8
 ```
 </details>
 
 ### Comparison
 
 ||My version|MattKC's version|ibara's version|
 |-|-|-|-|
-|Bytes|60|~1400|2024|
+|Bytes|58|~1400|2024|
 |QR|<img src="/demo/qr.png" width="250"/>|<img src="https://mattkc.com/etc/snakeqr/code.png" width="250"/>|<img src="https://raw.githubusercontent.com/ibara/snakeqr/master/snakeqr.png" width="250"/>|
 |Link|https://github.com/donno2048/snake|https://mattkc.com/etc/snakeqr/|https://github.com/ibara/snakeqr|

docs/update.py

@@ -4,8 +4,8 @@
 
 hexdata = open(0).read().replace("\n", "")
 
-length = len(hexdata) // 2
+length = len(hexdata)
 
 div = next(filter(lambda x: not length % x, range(int(length ** .5), length)))
 
-open("README.md", "w").write(open(Path(__file__).parent.resolve() / 'template.md').read().format(size = length, hex = "\n".join(findall('..' * div, hexdata)), platform = get_platform(), empty_size = Path("a.out").stat().st_size))
+open("README.md", "w").write(open(Path(__file__).parent.resolve() / 'template.md').read().format(size = length // 2, hex = "\n".join(findall('.' * div, hexdata)), platform = get_platform(), empty_size = Path("a.out").stat().st_size))

snake.asm

@@ -1,6 +1,6 @@
 ; register usage during main loop
 ; DS: 0xB800, segment of screen buffer
-; BX: 0x7D0, screen size (40x25x2 bytes), used in food generation, edge checks and for snake character, also used for screen accesses but constantly reinitialized
+; BX: 0x7D0, screen size (40x25x2 bytes), used in food generation, edge checks, also used for screen accesses but constantly reinitialized
 ; DI: position of the snake head
 ; SI: pointer to memory location where the current position of the snake head is stored (actual pointer is BP+SI because it defaults to SS)
 lds si, [bx+si]         ; SI=0x100 and BX=0x0 at program start in most DOS versions, this initializes DS and SI (machine code at 0x100 is c5 30 00 b8)
@@ -13,27 +13,26 @@ start:                  ; reset game
 .food:                  ; create new food item
     in ax, 0x40         ;   read 16 bit timer counter into AX for randomization
     and bx, ax          ;     mask with BX to make divisible by 4 and less than or equal to screen size
-    or [bx], dh         ;   place food item and check if position was empty by applying OR with DH (assumed to be 1)
+    xor [bx], cl        ;   place food item and check if position was empty by applying XOR with CL (assumed to be 0xFF)
 .input:                 ; handle keyboard input
     mov bx, 0x7D0       ; initialize BX
-    js .food            ;     if position was occupied by snake or wall in food generation => try again, if we came from main loop SF=0
+    jp .food            ;     if position was occupied by snake or wall in food generation => try again, if we came from main loop PF=0
     in al, 0x60         ;   read scancode from keyboard controller - bit 7 is set in case key was released
     imul ax, BYTE 0xA   ;   we want to map scancodes for arrow up (0x48/0xC8), left (0x4B/0xCB), right (0x4D/0xCD), down (0x50/0xD0) to movement offsets
     aam 0x14            ;     IMUL (AH is irrelevant here), AAM and AAD with some magic constants maps up => -80, left => -2, right => 2, down => 80
     aad 0x44            ;     using arithmetic instructions is more compact than checks and conditional jumps
     cbw                 ;     but causes weird snake movements though with other keys
     add di, ax          ; add offset to head position
     cmp di, bx          ; check if head crossed vertical edge by comparing against screen size in BX
-    jae start           ;   if DI<0 or DI>=BX => game over
-    xor [di], bl        ; XOR head position with snake character
-    jns start           ;   if it already had snake or wall in it, SF=0 from XOR => game over
     lodsw               ; load 0x2007 into AX from off-screen screen buffer and advance head pointer
+    adc [di], ah        ; ADC head position with 0x20 to set snake character
+    jnp start           ;   if it already had snake or wall in it or if it crossed a vertical edge, PF=0 from ADC => game over
     mov [bp+si], di     ; store head position, use BP+SI to default to SS
-    jnp .food           ; if food was consumed, PF=0 from XOR => generate new food
+    jz .food            ; if food was consumed, ZF=1 from ADC => generate new food
 .wall:                  ; draw an invisible wall on the left side
     mov [bx], cl        ;   store wall character
-    sub bx, 0x50        ;   go one line backwards
+    sub bx, BYTE 0x50   ;   go one line backwards
     jns .wall           ; jump to draw the next wall
     pop bx              ; no food was consumed so pop tail position into BX
-    and [bx], ah        ; clear old tail position on screen, AND also clears SF
-    jns .input          ; loop to keyboard input
+    mov [bx], ah        ; clear old tail position on screen
+    jnp .input          ; loop to keyboard input, PF=0 from SUB