lzsa/asm/6809/unlzsa1b.s

;  unlzsa1b.s - 6809 backward decompression routine for raw LZSA1 - 116 bytes
;  compress with lzsa -r -b <original_file> <compressed_file>
;
;  in:  x = last byte of compressed data
;       y = last byte of decompression buffer
;  out: y = first byte of decompressed data
;
;  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_lzsa1
         leax 1,x
         leay 1,y

lz1token ldb ,-x           ; load next token into B: O|LLL|MMMM
         pshs b            ; save it

         clra              ; clear A (high part of literals count)
         andb #$70         ; isolate LLL (embedded literals count) in B
         beq lz1nolt       ; skip if no literals
         cmpb #$70         ; LITERALS_RUN_LEN?
         bne lz1declt      ; if not, we have the complete count, go unshift

         ldb ,-x           ; load extra literals count byte
         addb #$07         ; add LITERALS_RUN_LEN
         bcc lz1gotlt      ; if no overflow, we got the complete count, copy
         bne lz1midlt

         ldd ,--x          ; load 16 bit count in D (low part in B, high in A)
         bra lz1gotlt      ; we now have the complete count, go copy

lz1midlt tfr b,a           ; copy high part of literals count into A
         ldb ,-x           ; load low 8 bits of literals count
         bra lz1gotlt      ; we now have the complete count, go copy

lz1declt lsrb              ; shift literals count into place
         lsrb
         lsrb
         lsrb
 
lz1gotlt tfr x,u
         tfr d,x           ; transfer 16-bit count into X
lz1cpylt lda ,-u           ; copy literal byte
         sta ,-y 
         leax -1,x         ; decrement X and update Z flag
         bne lz1cpylt      ; loop until all literal bytes are copied
         tfr u,x

lz1nolt  ldb ,s            ; get token again, don't pop it from the stack
         
         tstb              ; test O bit (small or large offset)
         bmi lz1bigof

         ldb ,-x           ; O clear: load 8 bit (negative, signed) offset
         lda #$ff          ; set high 8 bits
         bra lz1gotof

lz1bigof ldd ,--x          ; O set: load long 16 bit (negative, signed) offset
lz1gotof nega              ; reverse sign of offset in D
         negb
         sbca #0
         leau d,y          ; put backreference start address in U (dst+offset)
         
         puls b            ; restore token
         
         clra              ; clear A (high part of match length)
         andb #$0F         ; isolate MMMM (embedded match length)
         addb #$03         ; add MIN_MATCH_SIZE
         cmpb #$12         ; MATCH_RUN_LEN?
         bne lz1gotln      ; no, we have the full match length, go copy

         ldb ,-x           ; grab extra match length byte
         addb #$12         ; add MIN_MATCH_SIZE + MATCH_RUN_LEN
         bcc lz1gotln      ; if no overflow, we have the full length
         bne lz1midln

         ldd ,--x          ; load 16-bit len in D (low part in B, high in A)
         bne lz1gotln      ; check if we hit EOD (16-bit length = 0)
      
         rts               ; done, bail

lz1midln tfr b,a           ; copy high part of len into A
         ldb ,-x           ; grab low 8 bits of len in B

lz1gotln pshs x            ; save source compressed data pointer
         tfr d,x           ; copy match length to X

lz1cpymt lda ,-u           ; copy matched byte
         sta ,-y 
         leax -1,x         ; decrement X
         bne lz1cpymt      ; loop until all matched bytes are copied

         puls x            ; restore source compressed data pointer
         bra lz1token      ; go decode next token