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; unaplib_68000.s - aPLib decompressor for 68000 - 156 bytes
;
; in: a0 = start of compressed data
; a1 = start of decompression buffer
; out: d0 = decompressed size
;
; Copyright (C) 2020 Emmanuel Marty
; With parts of the code inspired by Franck "hitchhikr" Charlet
;
; This software is provided 'as-is', without any express or implied
; warranty. In no event will the authors be held liable for any damages
; arising from the use of this software.
;
; Permission is granted to anyone to use this software for any purpose,
; including commercial applications, and to alter it and redistribute it
; freely, subject to the following restrictions:
;
; 1. The origin of this software must not be misrepresented; you must not
; claim that you wrote the original software. If you use this software
; in a product, an acknowledgment in the product documentation would be
; appreciated but is not required.
; 2. Altered source versions must be plainly marked as such, and must not be
; misrepresented as being the original software.
; 3. This notice may not be removed or altered from any source distribution.
apl_decompress:
movem.l a2-a6/d2-d3,-(sp)
moveq #-128,d1 ; initialize empty bit queue
; plus bit to roll into carry
lea 32000.w,a2 ; load 32000 offset constant
lea 1280.w,a3 ; load 1280 offset constant
lea 128.w,a4 ; load 128 offset constant
move.l a1,a5 ; save destination pointer
.literal: move.b (a0)+,(a1)+ ; copy literal byte
.after_lit: moveq #3,d2 ; set LWM flag
.next_token: bsr.s .get_bit ; read 'literal or match' bit
bcc.s .literal ; if 0: literal
bsr.s .get_bit ; read '8+n bits or other type' bit
bcs.s .other_match ; if 11x: other type of match
bsr.s .get_gamma2 ; 10: read gamma2-coded high offset bits
sub.l d2,d0 ; high offset bits == 2 when LWM == 3 ?
bcc.s .no_repmatch ; if not, not a rep-match
bsr.s .get_gamma2 ; read repmatch length
bra.s .got_len ; go copy large match
.no_repmatch: lsl.l #8,d0 ; shift high offset bits into place
move.b (a0)+,d0 ; read low offset byte
move.l d0,d3 ; copy offset into d3
bsr.s .get_gamma2 ; read match length
cmp.l a2,d3 ; offset >= 32000 ?
bge.s .inc_by_2 ; if so, increase match len by 2
cmp.l a3,d3 ; offset >= 1280 ?
bge.s .inc_by_1 ; if so, increase match len by 1
cmp.l a4,d3 ; offset < 128 ?
bge.s .got_len ; if so, increase match len by 2
.inc_by_2: addq.l #1,d0 ; increase match len by 1
.inc_by_1: addq.l #1,d0 ; increase match len by 1
.got_len: move.l a1,a6 ; calculate backreference address
sub.l d3,a6 ; (dest - match offset)
subq.l #1,d0 ; dbf will loop until d0 is -1, not 0
.copy_match: move.b (a6)+,(a1)+ ; copy matched byte
dbf d0,.copy_match ; loop for all matched bytes
moveq #2,d2 ; clear LWM flag
bra.s .next_token ; go decode next token
.other_match: bsr.s .get_bit ; read '7+1 match or short literal' bit
bcs.s .short_match ; if 111: 4 bit offset for 1-byte copy
moveq #1,d0 ; 110: prepare match length
moveq #0,d3 ; clear high bits of offset
move.b (a0)+,d3 ; read low bits of offset + length bit
lsr.b #1,d3 ; shift offset into place, len into carry
beq.s .done ; check for EOD
addx.b d0,d0 ; len = (1 << 1) + carry bit, ie. 2 or 3
bra.s .got_len ; go copy match
.short_match: moveq #0,d0 ; clear short offset before reading 4 bits
bsr.s .get_dibits ; read a data bit into d0, one into carry
addx.b d0,d0 ; shift second bit into d0
bsr.s .get_dibits ; read a data bit into d0, one into carry
addx.b d0,d0 ; shift second bit into d0
tst.b d0 ; check offset value
beq.s .write_zero ; if offset is zero, write a 0
move.l a1,a6 ; calculate backreference address
sub.l d0,a6 ; (dest - short offset)
move.b (a6),d0 ; read matched byte
.write_zero: move.b d0,(a1)+ ; write matched byte or 0
bra.s .after_lit ; set LWM flag and go decode next token
.done: move.l a1,d0 ; pointer to last decompressed byte + 1
sub.l a6,d0 ; minus start of decompression buffer = size
movem.l (sp)+,a2-a6/d2-d3
rts
.get_gamma2: moveq #1,d0 ; init to 1 so it gets shifted to 2 below
.gamma2_loop: bsr.s .get_dibits ; read data bit, shift into d0
; and read continuation bit
bcs.s .gamma2_loop ; loop until a 0 continuation bit is read
rts
.get_dibits: bsr.s .get_bit ; read bit
addx.l d0,d0 ; shift into d0
; fall through
.get_bit: add.b d1,d1 ; shift bit queue, high bit into carry
bne.s .got_bit ; queue not empty, bits remain
move.b (a0)+,d1 ; read 8 new bits
addx.b d1,d1 ; shift bit queue, high bit into carry
; and shift 1 from carry into bit queue
.got_bit: rts
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