Note: *UNTESTED AS IS*
;In: HL: source
; DE: destination
depack: ;inc hl ;skip original file length
; inc hl ;which is stored in 4 bytes
; inc hl
; inc hl
ld a,128
exx
ld de,1
exx
depack_loop:
call getbits
jr c,output_compressed ;if set, we got lz77 compression
ldi ;copy byte from compressed data to destination [literal byte]
jr depack_loop
;handle compressed data
output_compressed:
ld c,[hl] ;get lowest 7 bits of offset, plus offset extension bit
inc hl ;to next byte in compressed data
output_match:
ld b,0
bit 7,c
jr z,output_match1 ;no need to get extra bits if carry not set
call getbits
call rlbgetbits
call rlbgetbits
call rlbgetbits
jr c,output_match1 ;since extension mark already makes bit 7 set
res 7,c ;only clear it if the bit should be cleared
output_match1:
inc bc
;return a gamma-encoded value
;length returned in HL
exx ;to second register set!
ld h,d
ld l,e ;initial length to 1
ld b,e ;bitcount to 1
;determine number of bits used to encode value
get_gamma_value_size:
exx
call getbits
exx
jr nc,get_gamma_value_size_end ;if bit not set, bitlength of remaining is known
inc b ;increase bitcount
jr get_gamma_value_size ;repeat...
get_gamma_value_bits:
exx
call getbits
exx
adc hl,hl ;insert new bit in HL
get_gamma_value_size_end:
djnz get_gamma_value_bits ;repeat if more bits to go
get_gamma_value_end:
inc hl ;length was stored as length-2 so correct this
exx ;back to normal register set
ret c
;HL' = length
push hl ;address compressed data on stack
exx
push hl ;match length on stack
exx
ld h,d
ld l,e ;destination address in HL...
sbc hl,bc ;calculate source address
pop bc ;match length from stack
ldir ;transfer data
pop hl ;address compressed data back from stack
jr depack_loop
rlbgetbits:
rl b
getbits:
add a,a
ret nz
ld a,[hl]
inc hl
rla
ret
