;  unlzsa2.s - 6809 decompression routine for raw LZSA2 - 197 bytes
;  compress with lzsa -f2 -r <original_file> <compressed_file>
;
;  in:  x = start of compressed data
;       y = start of decompression buffer
;  out: y = end of decompression buffer + 1
;
;  Copyright (C) 2020 Emmanuel Marty
;
;  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.

decompress_lzsa2
         ldb #$00
         stb lz2nibct      ; clear nibble available flag

lz2token ldb ,x+           ; load next token into B: XYZ|LL|MMM
         pshs b            ; save it

         lda #$00          ; clear A (high part of literals count)
         andb #$18         ; isolate LLL (embedded literals count) in B
         beq lz2nolt       ; skip if no literals
         cmpb #$18         ; LITERALS_RUN_LEN_V2?
         bne lz2declt      ; if not, we have the complete count, go unshift

         bsr lz2nibbl      ; get extra literals length nibble in B
         addb #$03         ; add LITERALS_RUN_LEN_V2
         cmpb #$12         ; LITERALS_RUN_LEN_V2 + 15 ?
         bne lz2gotlt      ; if not, we have the full literals count, go copy

         ldb ,x+           ; load extra literals count byte
         addb #$12         ; add LITERALS_RUN_LEN + 15
         bcc lz2gotlt      ; if no overflow, we got the complete count, copy

         ldb ,x+           ; load low 8 bits of little-endian literals count
         lda ,x+           ; load high 8 bits of literal count
         bra lz2gotlt      ; we now have the complete count, go copy

lz2declt lsrb              ; shift literals count into place
         lsrb
         lsrb
 
lz2gotlt tfr x,u
         tfr d,x           ; transfer 16-bit count into X
lz2cpylt lda ,u+           ; copy literal byte
         sta ,y+ 
         leax -1,x         ; decrement X and update Z flag
         bne lz2cpylt      ; loop until all literal bytes are copied
         tfr u,x

lz2nolt  puls b            ; restore token
         pshs b            ; save it again

         lslb              ; push token's X flag bit into carry
         bcs lz2replg      ; if token's X bit is set, rep or large offset

         lda #$00          ; clear A (to prepare for high 8 bits of offset)

         lslb              ; push token's Y flag bit into carry
         bcs lz2offs9      ; if token's Y bit is set, 9 bits offset

         lslb              ; push token's Z flag bit into carry
         rola              ; shift Z flag from carry into bit 0 of A
         eora #$e1         ; set bits 5-7 of offset, reverse bit 0
         sta lz2val5+1
         bsr lz2nibbl      ; get offset nibble in B
         lslb              ; nibble represents offset bits 1..4
lz2val5  orb #$aa          ; merge offset bit 0 from A
         lda #$ff          ; set bits 8-15 of offset
         bra lz2gotof

lz2offs9 lslb              ; push token's Z flag bit into carry         
         rola              ; shift Z flag from carry into bit 0 of A
         coma              ; set bits 9-15 of offset, reverse bit 8

         ldb ,x+           ; load low 8 bits of (negative, signed) offset
         bra lz2gotof

lz2nibbl ldb #$aa
         lsr lz2nibct      ; nibble ready?
         bcs lz2gotnb

         inc lz2nibct      ; flag nibble as ready for next time
         ldb ,x+           ; load two nibbles
         stb lz2nibbl+1    ; store nibble for next time (low 4 bits)

         lsrb              ; shift 4 high bits of nibble down
         lsrb
         lsrb
         lsrb

lz2gotnb andb #$0f         ; only keep low 4 bits
lz2done  rts

lz2replg lslb              ; push token's Y flag bit into carry
         bcs lz2rep16      ; if token's Y bit is set, rep or 16 bit offset

         lda #$00          ; clear A
         lslb              ; push token's Z flag bit into carry
         rola              ; shift Z flag from carry into bit 0 of A
         eora #$e1         ; set bits 13-15 of offset, reverse bit 8

         bsr lz2nibbl      ; get offset nibble in B
         lslb              ; nibble represents offset bits 9..12
         stb lz2val13+1
lz2val13 ora #$aa          ; merge offset bits 9..12 from B
         suba #$02         ; substract 512 from offset

         ldb ,x+           ; load low 8 bits of (negative, signed) offset
         bra lz2gotof

lz2rep16 bmi lz2repof      ; if token's Z flag bit is set, rep match
         
         ldd ,x++          ; load high then low 8 bits of offset

lz2gotof std lz2repof+1    ; store match offset

lz2repof ldd #$aaaa        ; load match offset
         leau d,y          ; put backreference start address in U (dst+offset)
         
         puls b            ; restore token
         
         lda #$00          ; clear A (high part of match length)
         andb #$07         ; isolate MMM (embedded match length)
         addb #$02         ; add MIN_MATCH_SIZE_V2
         cmpb #$09         ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2?
         bne lz2gotln      ; no, we have the full match length, go copy

         bsr lz2nibbl      ; get offset nibble in B
         addb #$09         ; add MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2
         cmpb #$18         ; MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2 + 15?
         bne lz2gotln      ; if not, we have the full match length, go copy

         ldb ,x+           ; load extra length byte
         addb #$18         ; add MIN_MATCH_SIZE_V2 + MATCH_RUN_LEN_V2 + 15
         bcc lz2gotln      ; if no overflow, we have the full length
         beq lz2done       ; detect EOD code

         ldb ,x+           ; load 16-bit len in D (low part in B, high in A)
         lda ,x+           ; (little endian)

lz2gotln pshs x            ; save source compressed data pointer
         tfr d,x           ; copy match length to X

lz2cpymt lda ,u+           ; copy matched byte
         sta ,y+ 
         leax -1,x         ; decrement X
         bne lz2cpymt      ; loop until all matched bytes are copied

         puls x            ; restore source compressed data pointer
         lbra lz2token     ; go decode next token

lz2nibct rmb 1             ; nibble ready flag