;  decompress_small.S - space-efficient decompressor implementation for 8088
;
;  Copyright (C) 2019 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.

   segment .text
   bits 16

;  ---------------------------------------------------------------------------
;  Decompress raw LZSA1 block
;  inputs:
;  * ds:si: raw LZSA1 block
;  * es:di: output buffer
;  output:
;  * ax:    decompressed size
;  ---------------------------------------------------------------------------

lzsa1_decompress:
   push di                 ; remember decompression offset
   cld                     ; make string operations (lods, movs, stos..) move forward

   xor cx,cx

.decode_token:
   mov ax,cx               ; clear ah - cx is zero from above or from after rep movsb in .copy_match
   lodsb                   ; read token byte: O|LLL|MMMM
   mov dx,ax               ; keep token in dl
   
   and al,070H             ; isolate literals length in token (LLL)
   mov cl,4
   shr al,cl               ; shift literals length into place

   cmp al,07H              ; LITERALS_RUN_LEN?
   jne .got_literals       ; no, we have the full literals count from the token, go copy

   lodsb                   ; grab extra length byte
   add al,07H              ; add LITERALS_RUN_LEN
   jnc .got_literals       ; if no overflow, we have the full literals count, go copy
   jne .mid_literals

   lodsw                   ; grab 16-bit extra length
   jmp short .got_literals

.mid_literals:
   lodsb                   ; grab single extra length byte
   inc ah                  ; add 256

.got_literals:
   xchg cx,ax
   rep movsb               ; copy cx literals from ds:si to es:di

   test dl,dl              ; check match offset size in token (O bit)
   js .get_long_offset

   xchg ax,cx              ; clear ah - cx is zero from the rep movsb above
   lodsb
   dec ah
   jmp short .get_match_length

.get_long_offset:
   lodsw                   ; Get 2-byte match offset

.get_match_length:
   xchg ax,dx              ; dx: match offset  ax: original token
   and al,0FH              ; isolate match length in token (MMMM)
   add al,3                ; add MIN_MATCH_SIZE

   cmp al,012H             ; MATCH_RUN_LEN?
   jne .got_matchlen       ; no, we have the full match length from the token, go copy

   lodsb                   ; grab extra length byte
   add al,012H             ; add MIN_MATCH_SIZE + MATCH_RUN_LEN
   jnc .got_matchlen       ; if no overflow, we have the entire length
   jne .mid_matchlen       

   lodsw                   ; grab 16-bit length
   test ax,ax              ; bail if we hit EOD
   je short .done_decompressing 
   jmp short .got_matchlen

.mid_matchlen:
   lodsb                   ; grab single extra length byte
   inc ah                  ; add 256

.got_matchlen:
   xchg cx,ax              ; copy match length into cx
   push ds                 ; save ds:si (current pointer to compressed data)
   xchg si,ax          
   push es
   pop ds
   mov si,di               ; ds:si now points at back reference in output data
   add si,dx
   rep movsb               ; copy match
   xchg si,ax              ; restore ds:si
   pop ds
   jmp short .decode_token ; go decode another token

.done_decompressing:
   pop ax                  ; retrieve the original decompression offset
   xchg ax,di              ; compute decompressed size
   sub ax,di
   ret                     ; done