; kernel - this is where the action happens. assemble with nasm

BITS 16
org 0x500

	call cursor_hide
	call rand_seed
	call rand_position
	call disp_hero
loop:
 	call key_read
 	call key_handle
	jmp loop

char_emit:			; al=char
	mov ah, 0x02		; fn: set cursor position
	xor bh, bh		; page number
	mov dx, [pos_x]		; dh=row, dl=col
	int 0x10
	mov ah, 0x09		; fn: write char
	mov bl, [color]
	mov cx, 1		; reps
	int 0x10
	ret

cursor_hide:
	mov ah, 0x01
	mov ch, 0x3F
	int 0x10
	ret

disp_clear:
	mov al, 0x20
	call char_emit
	ret

disp_hero:
	mov al, 0x40
	call char_emit
	ret

key_handle:			; input al=char
 	mov bp, actions
	add al, al		; byte to word size
 	xor ah, ah
	add bp, ax		; apply the key offset
 	cmp word [bp], 0	; action for this key?
 	jz .nope
	call [bp]
.nope:
	ret

key_read:			; out: al=char
	xor ah, ah
	int 0x16
	ret

move_ss:
	cmp byte [pos_y], 23
	jae .nope
	call disp_clear
	inc byte [pos_y]
	call disp_hero
.nope:
	ret

move_nn:
	cmp byte [pos_y], 0
	jz .nope
	call disp_clear
	dec byte [pos_y]
	call disp_hero
.nope:
	ret

move_ww:
	cmp byte [pos_x], 0
	jz .nope
	call disp_clear
	dec byte [pos_x]
	call disp_hero
.nope:
	ret

move_ee:
	cmp byte [pos_x], 79
	jae .nope
 	call disp_clear
	inc byte [pos_x]
	call disp_hero
.nope:
	ret

; NOTE these "glide" on an edge. avoiding that would be more complicated
move_sw:
	call move_ss
	call move_ww
	ret

move_se:
	call move_ss
	call move_ee
	ret

move_nw:
	call move_nn
	call move_ww
	ret

move_ne:
	call move_nn
	call move_ee
	ret

rand_seed:
	xor ax, ax
	int 0x1a		; fn: system time
	mov [seed], dx
	ret

; the "RNG" from rogue 3.6.3
rand_value:			; in=bx, out=dx (or dl)
	mov ax, [seed]
	mov cx, 11109
	mul cx
	add ax, 13849
	mov [seed], ax
	ror ax, 1
	xor dx, dx
	div bx
	ret

rand_position:
	mov bx, 80
	call rand_value
	mov [pos_x], dl
	mov bx, 24
	call rand_value
 	mov [pos_y], dl
	ret

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; VARIABLES

color: db 0xf			; white

pos_x: db 40			; position - must be grouped with y for DX
pos_y: db 12

seed: dw 0

; traditional rogue(6) keys. remap them if you want something else
actions:
	times 98 dw 0
	dw move_sw		; b
	times 5 dw 0
	dw move_ww		; h
	dw 0
	dw move_ss		; j
	dw move_nn		; k
	dw move_ee		; l
	dw 0
	dw move_se		; n
	times 6 dw 0
	dw move_ne		; u
	times 3 dw 0
	dw move_nw		; y