.export init
.feature c_comments

numL := $02 ; low byte of a 16-bit number
numH := $03 ; low byte of a 16-bit number
num2L := $04
num2H := $05
GETIN := $ffe4
CHROUT := $ffd2
CLRCHN := $e544

; one byte for each cell, 1000 bytes because 40x25 screen; is identical to
; screen memory (i.e. $a0 is alive and $20 is dead)
prev_grid := $0801

; initialisation: clear screen and set num to point to screen mem start
init:
  ; clear screen
  lda #$93
  jsr CHROUT

  ldy #$00
  lda #$00
  sta numH
  sta numL
  jsr CLRCHN
  jmp loop

; main loop
; num stores offset from grid
loop:
  jsr GETIN
  beq loop

  cmp #$1d ; right arrow
  beq if_right
  cmp #$9d ; left arrow
  beq if_left
  cmp #$91 ; up arrow
  beq if_up
  cmp #$11 ; down arrow
  beq if_down
  cmp #$2d ; reset if - pressed
  beq if_reset
  cmp #$20 ; fill cell if space pressed
  beq if_space
  cmp #$30 ; end program if 0 pressed
  bne loop
  jmp end

if_right:
  jsr inc_num
  jmp loop

if_left:
  jsr dec_num
  jmp loop

if_up:
  lda #40
  jsr s16b
  jmp loop

if_down:
  lda #40
  jsr a16b
  jmp loop

if_reset:
  lda #$93
  jsr CHROUT
  jmp loop

if_space:
  lda numL
  sta num2L
  lda numH
  clc
  adc #$04
  sta num2H

  ldy #$00
  lda (num2L),y
  cmp #$a0
  beq if_cell_filled
  jmp if_cell_empty
if_cell_filled:
  lda #$20
  jsr print_byte
  jmp loop
if_cell_empty:
  lda #$a0
  jsr print_byte
  jmp loop

; print byte in A at location in screen_mem+num; destroys Y; num2 is expected
; to be $0400 (screen_mem zero location) and is set to that at the end
print_byte:
  tay
  lda numH
  pha

  clc
  adc #$04
  sta numH

  tya
  ldy #$00
  sta (numL),y

  pla
  sta numH
  rts

; adds A to num
a16b:
  clc
  adc numL
  sta numL
  lda #$00
  adc numH
  sta numH
  rts

; adds A to num2
a16b2:
  clc
  adc num2L
  sta num2L
  lda #$00
  adc num2H
  sta num2H
  rts

; subtracts A from num (destroys X)
s16b:
  pha
  lda numL
  tsx
  inx
  sec
  sbc $0100,x
  sta numL
  lda numH
  sbc #$00
  sta numH
  pla
  rts

; might change functions to take from stack but this is easier for now as I
; only have 2 16 bit numbers to deal with anyways

; subtracts A from num2 (destroys X)
s16b2:
  pha
  lda num2L
  tsx
  inx
  sec
  sbc $0100,x
  sta num2L
  lda num2H
  sbc #$00
  sta num2H
  pla
  rts

inc_num:
  inc numL
  beq inc_H
  rts
inc_H:
  inc numH
  rts

dec_num:
  pha

  dec numL
  lda numL
  cmp #$ff
  beq dec_H
  pla
  rts

dec_H:
  dec numH
  pla
  rts

end:
  brk