Meta [
	Title:   "Mortal Coil Atari 8-bit demo"
	Version: 13-Feb-2022
	Author:  ["Freddy Offenga" "Kaj de Vos"]
	Rights:  "Copyright (c) 2017,2018 Freddy Offenga, 2020-2022 Kaj de Vos"
	License: {
		PD/CC0
		http://creativecommons.org/publicdomain/zero/1.0/
	}
	Purpose: "Kind of rotating starfield using one missile and DMA off!"
	Notes: {
		Ported by Kaj de Vos from the original in assembly by Freddy Offenga:
		https://github.com/FreddyOffenga/mortal-coil

		The original won first place in the 128 bytes compo at Outline 2017.
		https://outlinedemoparty.nl
	}
]

; Define fixed memory addresses

Unsafe!!! constant reference volatile byte! [
	; Operating system software addresses

	RTCLOK3= ~14
	SDMCTL=  ~022F

	; Hardware addresses

	; GTIA

	HPOSM0=  ~D004
	COLPM0=  ~D012
	GRACTL=  ~D01D

	; POKEY

	AUDC1=   ~D201
	RANDOM=  ~D20A

	; ANTIC

	WSYNC=   ~D40A
	VCOUNT
]

; Declare byte arrays
Let [wave shifts colours] binary! ~100

Byte! [byte index reverse]

GRACTL: %11
SDMCTL: 0  ; Switch off display DMA

; Give the compiler the opportunity to reuse 0

; Fill SHIFTS and COLOURS with random values.
; Use indexing instead of series references for arrays upto length 256 on 6502.
; Shift the indexing to 0-based to make the index fit in a byte.
; We don't need that here in pre-computation for speed, but the generated code will be smaller.

For index [0  (count shifts) - 1] [
	poke next shifts  index RANDOM
	poke next colours  index RANDOM
]

; Pre-compute a base sine WAVE

; Quarter sine wave
Let [sine step] natural! 0  ; Set to zero for Atari 400/800

; Give the compiler the opportunity to reuse 0
Index: 0

; Have this loop count down to 1, to generate an efficient end test for 0
For reverse [~40 1] [
	Increment index

	; Don't use intermediate variables with CC65 if possible.
	; But don't use complex expressions as value to store into indexed series with CC65.
	Byte: second-of sine: sine + step  ; Quarter sine wave

	Poke skip wave ~C0  index byte
	Poke skip wave ~80  reverse byte  ; Mirror the sine wave
	Byte: byte x-or %0111'1111  ; Mirror the sine wave again
	Poke skip wave ~40  index byte
	Poke wave reverse byte

	Step: step + 8
]

; Animate the coil

Forever [
	Until is-zero VCOUNT []  ; Wait for vertical sync

	Index: RTCLOK3  ; Scroll the coil

	Repeat 254  overflow [  ; Scanlines (minus 1); limit for NTSC
		; Don't use complex expressions as indexes with CC65
		Byte: (pick next shifts  index)  ; Scatter the sine wave into a column
			+ RTCLOK3  ; Rotate the coil

		AUDC1:  ; Sound control
			HPOSM0:  ; Missile position
				WSYNC:  ; Wait a scanline
					(pick next wave  index)
					+ (pick next wave  byte)  ; Wave the wave
					>> 1  ; Shrink the coil
					+ ~40  ; Centre the coil

		COLPM0: pick next colours  index  ; Colour the missile

		Increment index  ; Scatter the sine wave into a coil
	]
]