processor 6502
	include "vcs.h"
	include "macro.h"
	include "xmacro.h"

; For lots of games, we'd like to display more than two sprites.
; There are lots of different ways to tackle this on the VCS,
; but we're going to try for a generalized approach that lets
; use have N different sprites at any X-Y coordinate, each with
; its own bitmap and color table. This is tricky because we can
; only do so much on each scanline.

; Our approach is to separate the problem into three phases.
; In the sort phase, we sort all sprites by Y coordinate.
; We do one sort pass per frame, so it may take several frames
; for the sort to stabilize.
; In the positioning phase, we look at the sprites in Y-sorted
; order, looking several lines ahead to see if a sprite is
; coming up. We then allocate it to one of the two player
; objects in hardware and set its position using the SetHorizPos
; method. We can set one or both of the player objects this way.

; In the display phase, we display the objects which we previously
; assigned and positioned. First we figure out how many scanlines are
; required. If only one object is scheduled, we just use its height.
; If two objects are scheduled, we go until the bottommost line has
; been displayed. We then loop through, fetching pixels and colors
; for one or both objects (up to four lookup tables) and setting
; registers at the appropriate time. We don't have time to do much
; else, so we don't look for any new objects to schedule until
; we're done with this loop.

; This scheme can only display up to two objects on a given
; scanline, so if the system tries to schedule a third, it will 
; be ignored. Also, the positioning routine takes a few scanlines
; to complete, so if the top of a sprite is too close to the
; bottom of another sprite, the latter may not be displayed.
;
; To mitigate this, we increment a priority counter when a
; sprite entry is missed. In the sort phase, we move those sprites
; ahead of lower priority sprites in the sort order. This makes
; overlapping sprites flicker instead of randomly disappear.

	seg.u Variables
        org $80

Scanline byte	; current scanline
CurIndex byte	; current sprite # to try to schedule

PData0	word	; pointer (lo/hi) to player 0 bitmap data
PData1	word	; pointer to player 1 bitmap data
PColr0	word	; pointer to player 0 color data
PColr1	word	; pointer to player 1 color data
SIndx0	byte	; next y-position to draw player 0
		; or during draw, index into sprite
                ; zero means not assigned
SIndx1	byte	; ... for player 1
SSize0	byte	; sprite size for player 0
SSize1	byte	; sprite size for player 1

NSprites	equ 8       ; max # of sprites
XPos0		ds NSprites ; x coord for each sprite
YPos0		ds NSprites ; y coord for each sprite
Sorted0		ds NSprites ; sorted list of sprite indices
Priority0	ds NSprites ; sprite priority list, if missed

MinYDist	equ 7	; min. Y distance to consider sprite

; Fetchs the approximate scanline (could be off by +/- 1)
; into A. Takes 11 or 14 cycles.
	MAC GET_APPROX_SCANLINE
        ldy INTIM
        lda Timer2Scanline,y
        bne .Ok
        lda Timer2Scanline-1,y
.Ok
        ENDM

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

	seg Code
        org $f000

; Initialize and set initial X and Y offsets of objects.
Start
	CLEAN_START
	ldx #0
        lda #10
        ldy #40
InitLoop
        sty XPos0,x
        sta YPos0,x
        clc
        adc #19
        iny
        iny
        iny
        iny
        iny
        iny
        inx
        cpx #NSprites
        bne InitLoop
; Initialize initial sort order
        ldx #0
InitLoop2
	txa
	sta Sorted0,x
        inx
        cpx #NSprites
        bne InitLoop2

; Next frame loop
NextFrame
; VSYNC and VBLANK periods
	VERTICAL_SYNC
	TIMER_SETUP 24
; Do joystick movement
        jsr MoveJoystick
; Do one iteration of bubble sort on sprite indices
	ldx #NSprites-2
SortLoop
	jsr SwapSprites
        dex
        bpl SortLoop	; loop until <= 0
; Reset scanline counter and sprite objects
	ldx #0
        stx CurIndex
        stx SIndx0
        stx SIndx1
        stx SSize0
        stx SSize1
        TIMER_WAIT
; end of VBLANK

; Scanline loop
        TIMER_SETUP 216	; timer <- #$ff
        lda #$90
        sta COLUBK
NextFindSprite
; Try to schedule sprites to both players
	jsr FindAnotherSprite
	jsr FindAnotherSprite
; Apply fine offsets
	sta WSYNC	; start next scanline
	sta HMOVE	; apply the previous fine position(s)
; See if time to draw
	jsr DrawSprites
; Repeat until all scanlines drawn
	sta HMCLR	; reset the old horizontal position(s)
	lda INTIM
        cmp #$14	; scanline 198
        bcs NextFindSprite
        lda #201	; + 9 lines, end exactly
        jsr WaitForScanline
; end of Scanline loop

NoMoreScanlines
; Clear all colors to black before overscan
	ldx #0
	stx COLUBK
	stx COLUP0
	stx COLUP1
	stx COLUPF
; 30-2 lines of overscan
	TIMER_SETUP 28
        TIMER_WAIT
; Go to next frame
	jmp NextFrame

; We were too late to display a sprite.
; Put it earlier in the sort order and try next frame.
; X = sort index
.MissedSprite subroutine
; Have we already looked at all the sprites?
; Increment priority for this sort entry
	inc Priority0,x
; Go to next sort index, until we get to the end
        inx
        stx CurIndex
.OutOfSprites
        rts
; Try to assign the next sprite in the sort order into
; one of the two player slots.
; If sprite found, uses at least 3 scanlines for SetHorizPos.
FindAnotherSprite ; subroutine entry point
; Get the approximate scanline
	GET_APPROX_SCANLINE
        clc
        adc #MinYDist
        sta Scanline
; Calculate the distance to next sprite 
	ldx CurIndex
	cpx #NSprites
        bcs .OutOfSprites
        ldy Sorted0,x	; get sprite index # in Y-sorted order
        lda YPos0,y	; get Y position of sprite
        cmp Scanline	; SpriteY - Scanline
; Don't schedule the sprite if it's too soon or its scanline
; has already passed -- mark it missed
	bmi .MissedSprite ; passed it? (or > 127 lines away)
; A sprite is starting soon, now we need to schedule it
; to either one of the player objects
        lda XPos0,y
; Is player 1 available?
        ldx SIndx1
	bne .Plyr1NotReady
; Due to timing issues, we have artifacts if player 1 is
; too close to the left edge of the screen. So we'd prefer to
; put those sprites in the player 0 slot.
        cmp #34		; X < 34
        bcc .Plyr1NotReady
; First let's set its horizontal offset
        ldx #1
        jsr SetHorizPos	; set horizontal position (does WSYNC)
; Assign the sprite's Y position to player 1 
	lda YPos0,y
        sta SIndx1
; Get index into SpriteDataMap (index * 4)
	ldx MultBy4,y
; Copy addresses of pixel/color maps to player 1
        lda SpriteDataMap,x
        sta PData1
        lda SpriteDataMap+1,x
        sta PData1+1
        lda SpriteDataMap+2,x
        sta PColr1
        lda SpriteDataMap+3,x
        sta PColr1+1
; Get the sprite height as the first byte of the color map
        ldy #0
        lda (PColr1),y
        sta SSize1
        jmp .SetupDone
.Plyr1NotReady
        ldx SIndx0
        bne .NoNearSprite	; both players in use
; Player 0 is available
; This is essentially the same as the player 1 routine
        ldx #0
        jsr SetHorizPos
	lda YPos0,y
	sta SIndx0
	ldx MultBy4,y
        lda SpriteDataMap,x
        sta PData0
        lda SpriteDataMap+1,x
        sta PData0+1
        lda SpriteDataMap+2,x
        sta PColr0
        lda SpriteDataMap+3,x
        sta PColr0+1
        ldy #0
        lda (PColr0),y
        sta SSize0
.SetupDone
	inc CurIndex	; go to next sprite in sort order
.NoNearSprite
	rts

; Draw any scheduled sprites.
DrawSprites subroutine
; Wait for next precise scanline
	lda #0		     ; 0 = wait for next
        jsr WaitForScanline
        lda Timer2Scanline,y ; lookup scanline #
        sta Scanline         ; save it
; Calculate # of lines to draw for each sprite
; Sprite Y - current scanline + sprite height
	lda SIndx0
        beq .Empty0	; sprite 0 is inactive?
	sec
	sbc Scanline
        clc
        adc SSize0
        sta SIndx0	; SIndx0 += SSize0 - Scanline
.Empty0
	lda SIndx1
        beq .Empty1	; sprite 1 is inactive?
	sec
	sbc Scanline
        clc
        adc SSize1
        sta SIndx1	; SIndx1 += SSize1 - Scanline
.Empty1
; Find out the maximum # of lines to draw
; by taking the maximum of the two sprite heights
	cmp SIndx0
        bpl .Cmp1	; sindx0 < sindx1?
        lda SIndx0
.Cmp1
	tax		; X = # of lines left to draw
        beq .NoSprites	; X = 0? we're done
        sta WSYNC	; next scanline
.DrawNextScanline
; Make sure player 0 index is within bounds
	ldy SIndx0
        cpy SSize0
        bcs .Inactive0	; index >= size? (or index < 0)
; Lookup pixels for player 0
        lda (PData0),y
; Do WSYNC and then quickly store pixels for player 0
        sta WSYNC
	sta GRP0
; Lookup/store colors for player 0
        lda (PColr0),y
        sta COLUP0
.DrawSprite1
; Make sure player 1 index is within bounds
	ldy SIndx1
        cpy SSize1
        bcs .Inactive1	; index >= size? (or index < 0)
; Lookup/store pixels and colors for player 1
; Note that we are already 30-40 pixels into the scanline
; by this point...
        lda (PData1),y
        sta GRP1
        lda (PColr1),y
        sta COLUP1
.Inactive1
; Decrement the two sprite indices
        dey
        sty SIndx1
        dec SIndx0
; Repeat until we've drawn all the scanlines for this job
	dex
        bne .DrawNextScanline
; Free up both player objects by zeroing them out
	stx SIndx0
        stx SIndx1
        stx SSize0
        stx SSize1
	sta WSYNC
	stx GRP0
	stx GRP1
; No sprites were drawn; just exit
.NoSprites
	rts
.Inactive0
; Alternate player 0 path when it is inactive
	sta WSYNC
	lda #0
	sta GRP0
        sta COLUP0
        beq .DrawSprite1 ; always taken due to lda #0

; Perform one sort iteration
; X register contains sort index (0 to NSprites-1)
SwapSprites subroutine
; First compare Priority[i] and Priority[i+1]
	lda Priority0,x
        cmp Priority0+1,x
        bcs .CompareYPos
; If Priority[i] < Priority[i+1], do the swap
; anyway after resetting priorities
        lda #0
	sta Priority0,x
        sta Priority0+1,x	; reset
        ldy Sorted0+1,x
        bcc .DoSwap	; swap due to priority
.CompareYPos
; Compare Y[i] and Y[i+1]
        ldy Sorted0,x
        lda YPos0,y
        ldy Sorted0+1,x
        cmp YPos0,y
        bcc .NoSwap	; Y[i] < Y[i+1]? don't swap
.DoSwap
; Swap Sorted[i] and Sorted[i+1]
	lda Sorted0,x	; A <- Sorted[i]
        sty Sorted0,x	; Y -> Sorted[i]
        sta Sorted0+1,x ; A -> Sorted[i+1]
.NoSwap
	rts

; Read joystick movement and apply to object 0
MoveJoystick subroutine
; Move vertically
	ldx YPos0
	lda #%00010000	;Up?
	bit SWCHA
	bne .SkipMoveUp
        cpx #8
        bcc .SkipMoveUp
        dex
.SkipMoveUp
	lda #%00100000	;Down?
	bit SWCHA 
	bne .SkipMoveDown
        cpx #170
        bcs .SkipMoveDown
        inx
.SkipMoveDown
	stx YPos0
; Move horizontally
        ldx XPos0
	lda #%01000000	;Left?
	bit SWCHA
	bne .SkipMoveLeft
        cpx #5
        bcc .SkipMoveLeft
        dex
.SkipMoveLeft
	lda #%10000000	;Right?
	bit SWCHA 
	bne .SkipMoveRight
        cpx #140
        bcs .SkipMoveRight
        inx
.SkipMoveRight
	stx XPos0
	rts


; SetHorizPos - Sets the horizontal position of an object.
; The X register contains the index of the desired object:
;  X=0: player 0
;  X=1: player 1
;  X=2: missile 0
;  X=3: missile 1
;  X=4: ball
; NOTE: This version of the routine does a NEWLINE after executing
; because when the beam is at the far right side of the screen
; there is little time to do so before wrapping to the next line.
; It does NOT do a HMOVE and HCLR.
SetHorizPos subroutine
	sta WSYNC	; start a new line
	sec		; set carry flag
.DivideLoop
	sbc #15		; subtract 15
	bcs .DivideLoop	; branch until negative
	eor #7		; calculate fine offset
	asl
	asl
	asl
	asl
	sta HMP0,x	; set fine offset
	sta RESP0,x	; fix coarse position
	rts		; return to caller

;    Pass: A = desired scanline
; Returns: Y = timer value - 1
        align $10
WaitForScanline subroutine
        ldy INTIM       ; Fetch timer value
.Wait
        cpy INTIM
        beq .Wait       ; Wait for it to change
        sta WSYNC       ; Sync with scan line
        cmp Timer2Scanline,y ; lookup scanline
        bcs WaitForScanline  ; repeat until >=
        rts

; Bitmap data "standing" position
Frame0
	.byte #0
	.byte #%01101100;$F6
	.byte #%00101000;$86
	.byte #%00101000;$86
	.byte #%00111000;$86
	.byte #%10111010;$C2
	.byte #%10111010;$C2
	.byte #%01111100;$C2
	.byte #%00111000;$C2
	.byte #%00111000;$16
	.byte #%01000100;$16
	.byte #%01111100;$16
	.byte #%01111100;$18
	.byte #%01010100;$18
	.byte #%01111100;$18
	.byte #%11111110;$F2
	.byte #%00111000;$F4

; Bitmap data "throwing" position
Frame1
	.byte #0
	.byte #%01101100;$F6
	.byte #%01000100;$86
	.byte #%00101000;$86
	.byte #%00111000;$86
	.byte #%10111010;$C2
	.byte #%10111101;$C2
	.byte #%01111101;$C2
	.byte #%00111001;$C2
	.byte #%00111000;$16
	.byte #%01101100;$16
	.byte #%01111100;$16
	.byte #%01111100;$18
	.byte #%01010100;$18
	.byte #%01111100;$18
	.byte #%11111110;$F2
	.byte #%00111000;$F4

; Color data for each line of sprite
ColorFrame0
	.byte #17	; height
	.byte #$F6;
	.byte #$86;
	.byte #$86;
	.byte #$86;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$C2;
	.byte #$16;
	.byte #$16;
	.byte #$16;
	.byte #$18;
	.byte #$18;
	.byte #$18;
	.byte #$F2;
	.byte #$F4;

; Enemy cat-head graphics data
EnemyFrame0
	.byte #0
        .byte #%00111100;$AE
        .byte #%01000010;$AE
        .byte #%11100111;$AE
        .byte #%11111111;$AC
        .byte #%10011001;$8E
        .byte #%01111110;$8E
        .byte #%11000011;$98
        .byte #%10000001;$98

; Enemy cat-head color data
EnemyColorFrame0
	.byte #9	; height
        .byte #$AE;
        .byte #$AC;
        .byte #$A8;
        .byte #$AC;
        .byte #$8E;
        .byte #$8E;
        .byte #$98;
        .byte #$94;

; Mapping of sprite objects (0-7) to sprite data
SpriteDataMap
	.word Frame0,ColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word EnemyFrame0,EnemyColorFrame0
	.word Frame1,ColorFrame0

; Multiplication by 4 table
; faster than tya/asl/asl/tay
MultBy4
	.byte #$00,#$04,#$08,#$0c
	.byte #$10,#$14,#$18,#$1c
	.byte #$20,#$24,#$28,#$2c
	.byte #$30,#$34,#$38,#$3c

; Timer -> Scanline table
        align $100
Timer2Scanline
	.byte 215,  0,214,213,212,211,210,  0,209,208,207,206,205,204,  0,203 
	.byte 202,201,200,199,  0,198,197,196,195,194,  0,193,192,191,190,189 
	.byte 188,  0,187,186,185,184,183,  0,182,181,180,179,178,  0,177,176 
	.byte 175,174,173,172,  0,171,170,169,168,167,  0,166,165,164,163,162 
	.byte   0,161,160,159,158,157,156,  0,155,154,153,152,151,  0,150,149 
	.byte 148,147,146,  0,145,144,143,142,141,140,  0,139,138,137,136,135 
	.byte   0,134,133,132,131,130,  0,129,128,127,126,125,124,  0,123,122 
	.byte 121,120,119,  0,118,117,116,115,114,  0,113,112,111,110,109,108 
	.byte   0,107,106,105,104,103,  0,102,101,100, 99, 98,  0, 97, 96, 95 
	.byte  94, 93, 92,  0, 91, 90, 89, 88, 87,  0, 86, 85, 84, 83, 82,  0 
	.byte  81, 80, 79, 78, 77, 76,  0, 75, 74, 73, 72, 71,  0, 70, 69, 68 
	.byte  67, 66,  0, 65, 64, 63, 62, 61, 60,  0, 59, 58, 57, 56, 55,  0 
	.byte  54, 53, 52, 51, 50,  0, 49, 48, 47, 46, 45, 44,  0, 43, 42, 41 
	.byte  40, 39,  0, 38, 37, 36, 35, 34,  0, 33, 32, 31, 30, 29, 28,  0 
	.byte  27, 26, 25, 24, 23,  0, 22, 21, 20, 19, 18,  0, 17, 16, 15, 14 
	.byte  13, 12,  0, 11, 10,  9,  8,  7,  0,  6,  5,  4,  3,  2,  0,  1

; Epilogue
	org $fffc
	.word Start
	.word Start