; This uses MaxTile Allocation mode, see the MaxTile documentation for more information.
; The snow effect has a lot of legacy code (from 2011) and opportunities for optimization. Try it yourself!

; 1 to enable transparency effects, 0 otherwise
!ENABLE_TRANSLUCENCY = 0

; 1 to make the snow don't update on pause, 0 otherwise.
; normally you will use '1' for regular levels and '0' for overworld.
!STOP_ON_PAUSE = 1

; Snow density or amount of snow sprites. 1 is the smallest and 128 is the maximum.
; Setting "128" will make no other sprite tiles appear since the SNES can only render
; up to 128 sprites. If you only have Mario + 1/2 enemies on your level, use 120.
!SNOW_DENSITY = 48

; 0 is max priority (appears in front of all sprites) and 3 is the lowest (appears behind all sprites)
!PRIORITY = 0

; The snowflake tile to use.
!SNOWFLAKE_TILE = $A3

; 1 for SP3/SP4, 0 for SP0/SP1
!SNOWFLAKE_SP = 1

; RAM defines. These RAM defines are only used while the snow effect is active
; (in other words, you can reuse again when you're not using snow)
!7e = $3600
!7f = $3601
!7c = $3602
!7d = $3603

!1a_copy = $3604
!1c_copy = $3606

; MaxTile shared routines
maxtile_flush_nmstl			= $0084A8
maxtile_get_sprite_slot		= $0084AC
maxtile_get_slot			= $0084B0
maxtile_finish_oam			= $0084B4

init:
	%invoke_sa1(snowfall_init)
main:
	if !ENABLE_TRANSLUCENCY == 1
		lda.b #%00110000
		sta $40
	endif

	if !STOP_ON_PAUSE == 1
		lda $73d4
		bne .return
	endif
	
	%invoke_sa1(sa1_code)
.return
	rtl
	
sa1_code:
	phb
	phk
	plb
	jsr snowfall_main
	plb
	rtl
	
snowfall_init:
	phb
	phk
	plb
	
	rep #$20
	lda $1a
	sta !1a_copy
	lda $1c
	sta !1c_copy
	sep #$20
	
	; waste time and random for best results
	ldx #$7f
-	jsr sa1rand
	dex
	bpl -

	; this loops though each snow flake and initializes it.
	ldx #$00
-	rep #$20
	txa
	asl ; x2
	asl ; x4
	asl ; x8
	sta $00
	sep #$20
	jsr snowinit
	inx
	cpx.b #!SNOW_DENSITY
	beq +
	bra -
+	plb
	rtl

snowfall_main:
	; this asks maxtile to allocate the amount of slots requested
	; which on this case is the amount of snowflakes.
	rep #$30
	ldy.w #!SNOW_DENSITY
	lda.w #!PRIORITY
	jsl maxtile_get_slot
	sep #$30
	bcs .ok
	
	; return if maxtile refused to give the requested amount
	rts
	
.ok
	; copy the pointer maxtile gave, this is important since we use
	; #$40 as data bank.
	rep #$10
	ldy $3102
	sty $02
	ldy $3100
	
	phb
	lda #$40
	pha
	plb
	
	; unroll
	
macro speedup(value)
	lda.w <value>*$08+$4030ca
	sta $0000,y
	lda.w <value>*$08+$4030cb
	sta $0001,y
	lda.w <value>*$08+$4030cd
	sta $0002,y
	lda.w <value>*$08+$4030cc
	sta $0003,y
	
	phy
	ldy $02
	lda.w <value>*$08+$4030ce
	and #$01
	sta $0000,y
	iny
	sty $02
	ply
	
	iny
	iny
	iny
	iny
endmacro

macro loop()
	%speedup(!x)
	!x #= !x+1
endmacro

	; this can be converted into a normal loop
	; the advantage is that you will be able to change
	; the amount of snow during the game play.
	
	!x = 0
	rep !SNOW_DENSITY : %loop()
	
	plb
	sep #$10
	
	lda $9d
	bne +
	
	; this is used for detecting changes in the scrolling
	; and apply the changes to the snow flakes.
	rep #$20
	lda $1a
	sec
	sbc !1a_copy
	sta $0e
	lda $1c
	sec
	sbc !1c_copy
	sta $0c
	lda $1a
	sta !1a_copy
	lda $1c
	sta !1c_copy
	sep #$20
	
	; loop though each snow flake
	ldx #$00
-	rep #$20
	txa
	asl ; x2
	asl ; x4
	asl ; x8
	sta $00
	sep #$20
	jsr snowcode
	inx
	cpx.b #!SNOW_DENSITY
	beq +
	bra -
+	
	rts

; A snow.
; $00-$01 = RAM ptr.

!xSpeed = $4030c8,x
!ySpeed = $4030c9,x
!xPos = $4030ca,x
!yPos = $4030cb,x
!yxppccct = $4030cc,x
!snowtile = $4030cd,x
!free = $4030ce,x
!backX = $4030cf,x

macro snowinit_randomstart()
	phx
	sep #$10
	jsr sa1rand
	rep #$10
	plx
endmacro

snowinit:							;\ snow init
	phx								;|
	php								;| preserve x & p
	sei 							;|
	rep #$10						;| disable irq & x/y = 16-bit
	ldx $00							;/ load ram pointer
	
	lda #$01						;\ setup division
	sta $2250						;/
	
	lda #$00
	sta !free
	
	%snowinit_randomstart()
	
	lda !7c
	clc
	adc #$08
	eor !7d
	and #$07
	sta !xSpeed
	
	%snowinit_randomstart()
	
	lda !7c
	and #$01
	clc
	adc #!SNOWFLAKE_TILE
	sta !snowtile
	
	%snowinit_randomstart()
	
	lda !7c
	and #$01
	beq .noTransparency
	
	lda.b #%00111110|!SNOWFLAKE_SP
	sta !yxppccct
	
	bra .skip
.noTransparency
	
	lda.b #%00110000|!SNOWFLAKE_SP
	sta !yxppccct
	
.skip
	
	%snowinit_randomstart()
	
	rep #$20
	lda !7c
	and #$7fff
	sta $2251
	lda #$5555
	sta $2253
	sep #$20
	lda $2306
	clc
	adc #$01
	sta !ySpeed
	
	%snowinit_randomstart()
	
	lda !7c
	sta !xPos
	sta !backX
	
	%snowinit_randomstart()
	
	lda !7c
	sta !yPos
	
	plp
	plx
	rts
	
	
;=========================;
; Snow Frame-a-Frame Code ;
;=========================;
	
snowcode:
	phx
	php
	rep #$10
	ldx $00
	
	lda !backX
	clc
	adc !xSpeed
	sta !backX
	
	rep #$20
	and #$00ff
	asl a
	tay
	sep #$20
	
	lda.w CosTable+1,y	
	adc !xPos
	sta !xPos

	lda.w CosTable+1,y
	bmi +
	lda #$00
+	adc !free
	sta !free
	
	lda !xPos
	sec
	sbc $0e
	sta !xPos
	xba
	lda !free
	sbc $0f
	sta !free
	
	rep #$20
	xba
	cmp #$fff0
	sep #$20
	bpl +
	lda #$ff
	sta !xPos
	inc
	sta !free
+
	lda !free
	cmp #$01
	bne +
	lda #$ff
	sta !free
	lda #$f9
	sta !xPos
+
	
	lda !yPos
	clc
	adc !ySpeed
	sec
	sbc $0c
	sta !yPos
	
	cmp #$df
	bcs .Reset
	
.nvm
	plp
	plx
	rts
	
.Reset
	cmp #$f0
	bcs .nvm
	
	jsr snowinit
	
	lda #$f9
	sta !yPos
	
	plp
	plx
	rts

;=====================;
; Cossine Table       ;
;=====================;

CosTable:
dw $0100, $0100, $0100, $00FF, $00FF, $00FE, $00FD, $00FC, $00FB, $00FA, $00F8, $00F7
dw $00F5, $00F3, $00F1, $00EF, $00ED, $00EA, $00E7, $00E5, $00E2, $00DF, $00DC, $00D8
dw $00D5, $00D1, $00CE, $00CA, $00C6, $00C2, $00BE, $00B9, $00B5, $00B1, $00AC, $00A7
dw $00A2, $009D, $0098, $0093, $008E, $0089, $0084, $007E, $0079, $0073, $006D, $0068
dw $0062, $005C, $0056, $0050, $004A, $0044, $003E, $0038, $0032, $002C, $0026, $001F
dw $0019, $0013, $000D, $0006, $0000, $FFFA, $FFF3, $FFED, $FFE7, $FFE1, $FFDA, $FFD4
dw $FFCE, $FFC8, $FFC2, $FFBC, $FFB6, $FFB0, $FFAA, $FFA4, $FF9E, $FF98, $FF93, $FF8D
dw $FF87, $FF82, $FF7C, $FF77, $FF72, $FF6D, $FF68, $FF63, $FF5E, $FF59, $FF54, $FF4F
dw $FF4B, $FF47, $FF42, $FF3E, $FF3A, $FF36, $FF32, $FF2F, $FF2B, $FF28, $FF24, $FF21
dw $FF1E, $FF1B, $FF19, $FF16, $FF13, $FF11, $FF0F, $FF0D, $FF0B, $FF09, $FF08, $FF06
dw $FF05, $FF04, $FF03, $FF02, $FF01, $FF01, $FF00, $FF00, $FF00, $FF00, $FF00, $FF01
dw $FF01, $FF02, $FF03, $FF04, $FF05, $FF06, $FF08, $FF09, $FF0B, $FF0D, $FF0F, $FF11
dw $FF13, $FF16, $FF19, $FF1B, $FF1E, $FF21, $FF24, $FF28, $FF2B, $FF2F, $FF32, $FF36
dw $FF3A, $FF3E, $FF42, $FF47, $FF4B, $FF4F, $FF54, $FF59, $FF5E, $FF63, $FF68, $FF6D
dw $FF72, $FF77, $FF7C, $FF82, $FF87, $FF8D, $FF93, $FF98, $FF9E, $FFA4, $FFAA, $FFB0
dw $FFB6, $FFBC, $FFC2, $FFC8, $FFCE, $FFD4, $FFDA, $FFE1, $FFE7, $FFED, $FFF3, $FFFA
dw $0000, $0006, $000D, $0013, $0019, $001F, $0026, $002C, $0032, $0038, $003E, $0044
dw $004A, $0050, $0056, $005C, $0062, $0068, $006D, $0073, $0079, $007E, $0084, $0089
dw $008E, $0093, $0098, $009D, $00A2, $00A7, $00AC, $00B1, $00B5, $00B9, $00BE, $00C2
dw $00C6, $00CA, $00CE, $00D1, $00D5, $00D8, $00DC, $00DF, $00E2, $00E5, $00E7, $00EA
dw $00ED, $00EF, $00F1, $00F3, $00F5, $00F7, $00F8, $00FA, $00FB, $00FC, $00FD, $00FE
dw $00FF, $00FF, $0100, $0100

;=====================;
; SA-1 Random Number  ;
;                     ;
; $7c-$7d - result    ;
; $7e-$7f - ram used  ;
;=====================;
	
sa1rand:
	phx
	ldx #$01
	jsr .rand
	dex
	jsr .rand
	plx
	rts
	
.rand
	lda !7e
	asl
	asl
	sec
	adc !7e
	sta !7e
	asl !7f
	lda #$20
	bit !7f
	bcc .label1
	beq .label2
	bne .label3
.label1
	bne .label2
.label3
	inc !7f
.label2
	lda !7f
	eor !7e
	sta !7c,x
	rts