bcd.asm

; divide hMoney by hDivideBCDDivisor
; return output in hDivideBCDQuotient (same as hDivideBCDDivisor)
; used only to halve player money upon losing a fight
DivideBCDPredef::
DivideBCDPredef2::
DivideBCDPredef3:: ; only used function
DivideBCDPredef4::
	call GetPredefRegisters

DivideBCD::
	xor a
	ldh [hDivideBCDBuffer], a
	ldh [hDivideBCDBuffer+1], a
	ldh [hDivideBCDBuffer+2], a
	ld d, $1
.mulBy10Loop
; multiply the divisor by 10 until the leading digit is nonzero
; to set up the standard long division algorithm
	ldh a, [hDivideBCDDivisor]
	and $f0
	jr nz, .next
	inc d
	ldh a, [hDivideBCDDivisor]
	swap a
	and $f0
	ld b, a
	ldh a, [hDivideBCDDivisor+1]
	swap a
	ldh [hDivideBCDDivisor+1], a
	and $f
	or b
	ldh [hDivideBCDDivisor], a
	ldh a, [hDivideBCDDivisor+1]
	and $f0
	ld b, a
	ldh a, [hDivideBCDDivisor+2]
	swap a
	ldh [hDivideBCDDivisor+2], a
	and $f
	or b
	ldh [hDivideBCDDivisor+1], a
	ldh a, [hDivideBCDDivisor+2]
	and $f0
	ldh [hDivideBCDDivisor+2], a
	jr .mulBy10Loop

.next
	push de
	push de
	call DivideBCD_getNextDigit
	pop de
	ld a, b
	swap a
	and $f0
	ldh [hDivideBCDBuffer], a
	dec d
	jr z, .next2
	push de
	call DivideBCD_divDivisorBy10
	call DivideBCD_getNextDigit
	pop de
	ldh a, [hDivideBCDBuffer]
	or b
	ldh [hDivideBCDBuffer], a
	dec d
	jr z, .next2
	push de
	call DivideBCD_divDivisorBy10
	call DivideBCD_getNextDigit
	pop de
	ld a, b
	swap a
	and $f0
	ldh [hDivideBCDBuffer+1], a
	dec d
	jr z, .next2
	push de
	call DivideBCD_divDivisorBy10
	call DivideBCD_getNextDigit
	pop de
	ldh a, [hDivideBCDBuffer+1]
	or b
	ldh [hDivideBCDBuffer+1], a
	dec d
	jr z, .next2
	push de
	call DivideBCD_divDivisorBy10
	call DivideBCD_getNextDigit
	pop de
	ld a, b
	swap a
	and $f0
	ldh [hDivideBCDBuffer+2], a
	dec d
	jr z, .next2
	push de
	call DivideBCD_divDivisorBy10
	call DivideBCD_getNextDigit
	pop de
	ldh a, [hDivideBCDBuffer+2]
	or b
	ldh [hDivideBCDBuffer+2], a
.next2
	ldh a, [hDivideBCDBuffer]
	ldh [hDivideBCDQuotient], a ; the same memory location as hDivideBCDDivisor
	ldh a, [hDivideBCDBuffer+1]
	ldh [hDivideBCDQuotient+1], a
	ldh a, [hDivideBCDBuffer+2]
	ldh [hDivideBCDQuotient+2], a
	pop de
	ld a, $6
	sub d
	and a
	ret z
.divResultBy10loop
	push af
	call DivideBCD_divDivisorBy10
	pop af
	dec a
	jr nz, .divResultBy10loop
	ret

DivideBCD_divDivisorBy10:
	ldh a, [hDivideBCDDivisor+2]
	swap a
	and $f
	ld b, a
	ldh a, [hDivideBCDDivisor+1]
	swap a
	ldh [hDivideBCDDivisor+1], a
	and $f0
	or b
	ldh [hDivideBCDDivisor+2], a
	ldh a, [hDivideBCDDivisor+1]
	and $f
	ld b, a
	ldh a, [hDivideBCDDivisor]
	swap a
	ldh [hDivideBCDDivisor], a
	and $f0
	or b
	ldh [hDivideBCDDivisor+1], a
	ldh a, [hDivideBCDDivisor]
	and $f
	ldh [hDivideBCDDivisor], a
	ret

DivideBCD_getNextDigit:
	ld bc, $3
.loop
	ld de, hMoney ; the dividend
	ld hl, hDivideBCDDivisor
	push bc
	call StringCmp
	pop bc
	ret c
	inc b
	ld de, hMoney + 2 ; since SubBCD works starting from the least significant digit
	ld hl, hDivideBCDDivisor + 2
	push bc
	call SubBCD
	pop bc
	jr .loop


AddBCDPredef::
	call GetPredefRegisters

AddBCD::
	and a
	ld b, c
.add
	ld a, [de]
	adc [hl]
	daa
	ld [de], a
	dec de
	dec hl
	dec c
	jr nz, .add
	jr nc, .done
	ld a, $99
	inc de
.fill
	ld [de], a
	inc de
	dec b
	jr nz, .fill
.done
	ret


SubBCDPredef::
	call GetPredefRegisters

SubBCD::
	and a
	ld b, c
.sub
	ld a, [de]
	sbc [hl]
	daa
	ld [de], a
	dec de
	dec hl
	dec c
	jr nz, .sub
	jr nc, .done
	ld a, $00
	inc de
.fill
	ld [de], a
	inc de
	dec b
	jr nz, .fill
	scf
.done
	ret