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; -----------------------------------------------------------------------------
; ZX0 decoder by Einar Saukas & Urusergi
; "Standard" version (68 bytes only)
; -----------------------------------------------------------------------------
; Parameters:
; HL: source address (compressed data)
; DE: destination address (decompressing)
; -----------------------------------------------------------------------------
macro dzx0_standard
ld bc, $ffff ; preserve default offset 1
push bc
inc bc
ld a, $80
@dzx0s_literals:
call @dzx0s_elias ; obtain length
ldir ; copy literals
add a, a ; copy from last offset or new offset?
jr c, @dzx0s_new_offset
call @dzx0s_elias ; obtain length
@dzx0s_copy:
ex (sp), hl ; preserve source, restore offset
push hl ; preserve offset
add hl, de ; calculate destination - offset
ldir ; copy from offset
pop hl ; restore offset
ex (sp), hl ; preserve offset, restore source
add a, a ; copy from literals or new offset?
jr nc, @dzx0s_literals
@dzx0s_new_offset:
pop bc ; discard last offset
ld c, $fe ; prepare negative offset
call @dzx0s_elias_loop ; obtain offset MSB
inc c
ret z ; check end marker
ld b, c
ld c, (hl) ; obtain offset LSB
inc hl
rr b ; last offset bit becomes first length bit
rr c
push bc ; preserve new offset
ld bc, 1 ; obtain length
call nc, @dzx0s_elias_backtrack
inc bc
jr @dzx0s_copy
@dzx0s_elias:
inc c ; interlaced Elias gamma coding
@dzx0s_elias_loop:
add a, a
jr nz, @dzx0s_elias_skip
ld a, (hl) ; load another group of 8 bits
inc hl
rla
@dzx0s_elias_skip:
ret c
@dzx0s_elias_backtrack:
add a, a
rl c
rl b
jr @dzx0s_elias_loop
mend
; -----------------------------------------------------------------------------
|
