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; -----------------------------------------------------------------------------
; ZX0 decoder by Einar Saukas & introspec
; "Turbo" version (126 bytes, 21% faster) - BACKWARDS VARIANT
; -----------------------------------------------------------------------------
; Parameters:
; HL: last source address (compressed data)
; DE: last destination address (decompressing)
; -----------------------------------------------------------------------------
macro dzx0_turbo_back
ld bc, 1 ; preserve default offset 1
ld (@dzx0tb_last_offset+1), bc
ld a, $80
jr @dzx0tb_literals
@dzx0tb_new_offset:
add a, a ; obtain offset MSB
call c, @dzx0tb_elias
dec b
ret z ; check end marker
dec c ; adjust for positive offset
ld b, c
ld c, (hl) ; obtain offset LSB
dec hl
srl b ; last offset bit becomes first length bit
rr c
inc bc
ld (@dzx0tb_last_offset+1), bc ; preserve new offset
ld bc, 1 ; obtain length
call c, @dzx0tb_elias_loop
inc bc
@dzx0tb_copy:
push hl ; preserve source
@dzx0tb_last_offset:
ld hl, 0 ; restore offset
add hl, de ; calculate destination - offset
lddr ; copy from offset
inc c
pop hl ; restore source
add a, a ; copy from literals or new offset?
jr c, @dzx0tb_new_offset
@dzx0tb_literals:
add a, a ; obtain length
call c, @dzx0tb_elias
lddr ; copy literals
inc c
add a, a ; copy from last offset or new offset?
jr c, @dzx0tb_new_offset
add a, a ; obtain length
call c, @dzx0tb_elias
jp @dzx0tb_copy
@dzx0tb_elias_loop:
add a, a
rl c
add a, a
ret nc
@dzx0tb_elias:
jp nz, @dzx0tb_elias_loop ; inverted interlaced Elias gamma coding
ld a, (hl) ; load another group of 8 bits
dec hl
rla
ret nc
add a, a
rl c
add a, a
ret nc
add a, a
rl c
add a, a
ret nc
add a, a
rl c
add a, a
ret nc
@dzx0tb_elias_reload:
add a, a
rl c
rl b
add a, a
ld a, (hl) ; load another group of 8 bits
dec hl
rla
ret nc
add a, a
rl c
rl b
add a, a
ret nc
add a, a
rl c
rl b
add a, a
ret nc
add a, a
rl c
rl b
add a, a
jr c, @dzx0tb_elias_reload
ret
; -----------------------------------------------------------------------------
mend
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