; vim:ft=asm

; X             : width
; Y             : height
; C             : current cell mask (01b or 10b)
; 0x1000 ...    : cell states

            ; Set width/height
            set X, 0x10 ; width
            set Y, 0x08 ; height

            set C, 0x1 ; 01b (curr cell state mask)

            ; Prepare cells (sets up a glider in top-left corner)
            set A, 0x01  ; x-coord
            set B, 0x00  ; y-coord
            jsr toggle

            set A, 0x02
            set B, 0x01
            jsr toggle

            set A, 0x00
            set B, 0x02
            jsr toggle

            set A, 0x01
            set B, 0x02
            jsr toggle

            set A, 0x02
            set B, 0x02
            jsr toggle

:mainloop   jsr output
            jsr tick
            set PC, mainloop


; Toggles the given cell. NOTE: only safe before starting
:toggle;(x, y)
            mul B, X
            add A, B
            add A, 0x1000

            xor [A], 0x1    ; toggle it!
            set PC, POP



; Uses the observation that the GoL rules can be summarized as `(t | n) == 3`,
; where `t` is the state of the current cell (1 or 0) and `n` is the sum of the
; states of the neighbours (i.e. the number of living neighbours).
;   In each cell memory slot, two statse are stored (in bit 0 and bit 1); the
; `C` register keeps track of a bitmask of which state is the "current" one.

; Advance a generation (in-memory)
:tick
            set I, Y        ; let I be loop counter
            mul I, X

:loop       ife I, 0        ; loop until I=0
                set PC, loopend

            add I, 0xfff    ; (Temporarily)

            set A, [1+I]    ; set A to value at cell (t)
            and A, C

            ; Set B to (sum of neighbours << (C - 1))
            set B, [I]
            and B, C

            set J, [2+I]
            and J, C
            add B, J

            sub I, X

            set J, [I]
            and J, C
            add B, J

            set J, [1+I]
            and J, C
            add B, J

            set J, [2+I]
            and J, C
            add B, J

            add I, X
            add I, X

            set J, [I]
            and J, C
            add B, J

            set J, [1+I]
            and J, C
            add B, J

            set J, [2+I]
            and J, C
            add B, J

            ; Prepare stuff
            sub C, 1
            shr B, C
            shr A, C

            ; Set B to (t | n)
            bor B, A

            ; Set the new state
            ifn B, 3
                set B, 0

            and B, 0x2   ; B to {00b, 10b} >> (c-1)
            shr B, C

            add C, 1

            sub I, X
            and [1+I], C
            xor [1+I], B

            sub I, 0x1000     ; Restore I again, also decrement by 1

            ; Next iteration
            set PC, loop

:loopend    ; clean up & return
            xor C, 0x3
            set PC, POP


; Output current state to display
:output
            set I, Y        ; let I be loop counter
            mul I, X

:loop2      ife I, 0        ; loop until I=0
                set PC, loop2end

            sub I, 1

            set A, [0x1000+I] ; set A to value at cell (t)
            and A, C

            sub C, 1
            shr A, C
            add C, 1

            mul A, 3
            add A, 0xf120

            set Z, I          ; prepare output position
            set J, I
            div Z, X
            mod J, X
            mul Z, 0x20       ; 0x20 = display width
            add J, Z
            add J, 0x8000

            set [J], A

            ; Next iteration
            set PC, loop2

:loop2end   ; clean up & return
            set PC, POP