snake6502/src/lzgmini.asm

	SEG zeropageSegment
srcPointer WORD
dstPointer WORD
inEnd WORD
offset WORD
length BYTE
symbol BYTE
marker1 BYTE
marker2 BYTE
marker3 BYTE
marker4 BYTE
copy WORD

	SEG loaderSegment
inflate SUBROUTINE
    clc
    ldy #10
    lda (srcPointer),y
    adc srcPointer
	sta	inEnd
	dey
	lda	(srcPointer),y
	adc	srcPointer + 1
	sta	inEnd + 1
	clc
	lda	inEnd
	adc	#16
	sta	inEnd
	lda	inEnd + 1
	adc	#0
	sta	inEnd + 1

	; Get the marker symbols
	ldy	#16
	lda	(srcPointer),y
	sta	marker1
	iny
	lda	(srcPointer),y
	sta	marker2
	iny
	lda	(srcPointer),y
	sta	marker3
	iny
	lda	(srcPointer),y
	sta	marker4

	; Skip header + marker symbols (16 + 4 bytes)
	clc
	lda	srcPointer
	adc	#20
	sta	srcPointer
	lda	srcPointer + 1
	adc	#0
	sta	srcPointer + 1

	; Main decompression loop
	ldy	#0				; Make sure that Y is zero
.mainloop:
	lda	srcPointer		; done?
	cmp	inEnd
	bne	.notdone
	lda	srcPointer + 1
	cmp	inEnd + 1
	bne	.notdone
	rts
.notdone:
	lda	(srcPointer),y				; A = symbol
	sta	symbol
    sta $d020
	inc	srcPointer
	bne	.noinc1
	inc	srcPointer + 1
.noinc1:
	cmp	marker1			; Marker1?
	beq	.domarker1
	cmp	marker2			; Marker2?
	beq	.domarker2
	cmp	marker3			; Marker3?
	beq	.domarker3
	cmp	marker4			; Marker4?
	beq	.domarker4
.literal:
	lda	symbol
	sta	(dstPointer),y			; Plain copy
	inc	dstPointer
	bne	.mainloop
	inc	dstPointer + 1
	bne	.mainloop

.domarker1:
	jmp	.domarker1b

	; marker4 - "Near copy (incl. RLE)"
.domarker4:
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc3
	inc	srcPointer + 1
.noinc3:
	cmp	#0
	beq	.literal				; Single occurance of the marker symbol (rare)
	tax
	lsr
	lsr
	lsr
	lsr
	lsr
	sta	offset
	inc	offset
	lda	#0
	sta	offset + 1			; offset = (b >> 5) + 1
	txa
	and	#$1f
	tax
	lda	.LZG_LENGTH_DECODE_LUT,x
	sta	length				; length = .LZG_LENGTH_DECODE_LUT[b & 0x1f]
	jmp	.docopy

	; marker3 - "Short copy"
.domarker3:
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc4
	inc	srcPointer + 1
.noinc4:
	cmp	#0
	beq	.literal				; Single occurance of the marker symbol (rare)
	tax
	lsr
	lsr
	lsr
	lsr
	lsr
	lsr
	clc
	adc	#3
	sta	length				; length = (b >> 6) + 3
	txa
	and	#$3f
	adc	#8
	sta	offset
	lda	#0
	sta	offset + 1			; offset = (b & 0x3f) + 8
	beq	.docopy

	; marker2 - "Medium copy"
.domarker2:
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc5
	inc	srcPointer + 1
.noinc5:
	cmp	#0
	beq	.literal				; Single occurance of the marker symbol (rare)
	tax
	lsr
	lsr
	lsr
	lsr
	lsr
	sta	offset + 1
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc6
	inc	srcPointer + 1
.noinc6:
	clc
	adc	#8
	sta	offset
	bcc	.noinc7
	inc	offset + 1			; offset = (((b & 0xe0) << 3) | b2) + 8
.noinc7:
	txa
	and	#$1f
	tax
	lda	.LZG_LENGTH_DECODE_LUT,x
	sta	length				; length = .LZG_LENGTH_DECODE_LUT[b & 0x1f]
	bne	.docopy

.literal2:
	jmp	.literal

	; marker1 - "Distant copy"
.domarker1b:
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc8
	inc	srcPointer + 1
.noinc8:
	cmp	#0
	beq	.literal2			; Single occurance of the marker symbol (rare)
	and	#$1f
	tax
	lda	.LZG_LENGTH_DECODE_LUT,x
	sta	length				; length = .LZG_LENGTH_DECODE_LUT[b & 0x1f]
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc9
	inc	srcPointer + 1
.noinc9:
	sta	offset + 1
	lda	(srcPointer),y
	inc	srcPointer
	bne	.noinc10
	inc	srcPointer + 1
.noinc10:
	clc
	adc	#$08
	sta	offset
	lda	offset + 1
	adc	#$08
	sta	offset + 1			; offset = ((b2 << 8) | (*src++)) + 2056

	; Copy corresponding data from history window
.docopy:
	sec
	lda	dstPointer
	sbc	offset
	sta	copy
	lda	dstPointer + 1
	sbc	offset + 1
	sta	copy + 1
.loop1:
    lda	(copy),y
	sta	(dstPointer),y
	iny
	cpy	length
	bne	.loop1
	ldy	#0				; Make sure that Y is zero

	clc
	lda	dstPointer
	adc	length
	sta	dstPointer
	bcc	.noinc11
	inc	dstPointer + 1
.noinc11:
	jmp	.mainloop

; Lookup Table for decoding the copy length parameter
.LZG_LENGTH_DECODE_LUT
	BYTE    2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,35,48,72,128