pokecrystal/home/decompress.asm

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FarDecompress::
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; Decompress graphics data from a:hl to de.
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ld [wLZBank], a
ldh a, [hROMBank]
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push af
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ld a, [wLZBank]
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rst Bankswitch
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call Decompress
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pop af
rst Bankswitch
ret
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Decompress::
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; Pokemon GSC uses an lz variant (lz3) for compression.
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; This is mainly (but not necessarily) used for graphics.
; This function decompresses lz-compressed data from hl to de.
LZ_END EQU $ff ; Compressed data is terminated with $ff.
; A typical control command consists of:
LZ_CMD EQU %11100000 ; command id (bits 5-7)
LZ_LEN EQU %00011111 ; length n (bits 0-4)
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; Additional parameters are read during command execution.
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; Commands:
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LZ_LITERAL EQU 0 << 5 ; Read literal data for n bytes.
LZ_ITERATE EQU 1 << 5 ; Write the same byte for n bytes.
LZ_ALTERNATE EQU 2 << 5 ; Alternate two bytes for n bytes.
LZ_ZERO EQU 3 << 5 ; Write 0 for n bytes.
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; Another class of commands reuses data from the decompressed output.
LZ_RW EQU 2 + 5 ; bit
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; These commands take a signed offset to start copying from.
; Wraparound is simulated.
; Positive offsets (15-bit) are added to the start address.
; Negative offsets (7-bit) are subtracted from the current position.
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LZ_REPEAT EQU 4 << 5 ; Repeat n bytes from the offset.
LZ_FLIP EQU 5 << 5 ; Repeat n bitflipped bytes.
LZ_REVERSE EQU 6 << 5 ; Repeat n bytes in reverse.
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; If the value in the count needs to be larger than 5 bits,
; LZ_LONG can be used to expand the count to 10 bits.
LZ_LONG EQU 7 << 5
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; A new control command is read in bits 2-4.
; The top two bits of the length are bits 0-1.
; Another byte is read containing the bottom 8 bits.
LZ_LONG_HI EQU %00000011
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; In other words, the structure of the command becomes
; 111xxxyy yyyyyyyy
; x: the new control command
; y: the length
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; Save the output address
; for rewrite commands.
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ld a, e
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ld [wLZAddress], a
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ld a, d
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ld [wLZAddress + 1], a
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.Main:
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ld a, [hl]
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cp LZ_END
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ret z
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and LZ_CMD
cp LZ_LONG
jr nz, .short
; The count is now 10 bits.
; Read the next 3 bits.
; %00011100 -> %11100000
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ld a, [hl]
add a
add a ; << 3
add a
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; This is our new control code.
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and LZ_CMD
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push af
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ld a, [hli]
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and LZ_LONG_HI
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ld b, a
ld a, [hli]
ld c, a
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; read at least 1 byte
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inc bc
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jr .command
.short
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push af
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ld a, [hli]
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and LZ_LEN
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ld c, a
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ld b, 0
; read at least 1 byte
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inc c
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.command
; Increment loop counts.
; We bail the moment they hit 0.
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inc b
inc c
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pop af
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bit LZ_RW, a
jr nz, .rewrite
cp LZ_ITERATE
jr z, .Iter
cp LZ_ALTERNATE
jr z, .Alt
cp LZ_ZERO
jr z, .Zero
; Literal
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; Read literal data for bc bytes.
.lloop
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dec c
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jr nz, .lnext
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dec b
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jp z, .Main
.lnext
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ld a, [hli]
ld [de], a
inc de
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jr .lloop
.Iter:
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; Write the same byte for bc bytes.
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ld a, [hli]
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.iloop
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dec c
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jr nz, .inext
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dec b
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jp z, .Main
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.inext
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ld [de], a
inc de
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jr .iloop
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.Alt:
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; Alternate two bytes for bc bytes.
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dec c
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jr nz, .anext1
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dec b
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jp z, .adone1
.anext1
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ld a, [hli]
ld [de], a
inc de
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dec c
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jr nz, .anext2
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dec b
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jp z, .adone2
.anext2
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ld a, [hld]
ld [de], a
inc de
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jr .Alt
; Skip past the bytes we were alternating.
.adone1
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inc hl
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.adone2
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inc hl
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jr .Main
.Zero:
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; Write 0 for bc bytes.
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xor a
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.zloop
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dec c
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jr nz, .znext
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dec b
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jp z, .Main
.znext
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ld [de], a
inc de
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jr .zloop
.rewrite
; Repeat decompressed data from output.
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push hl
push af
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ld a, [hli]
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bit 7, a ; sign
jr z, .positive
; negative
; hl = de + -a
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and %01111111
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cpl
add e
ld l, a
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ld a, -1
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adc d
ld h, a
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jr .ok
.positive
; Positive offsets are two bytes.
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ld l, [hl]
ld h, a
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; add to starting output address
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ld a, [wLZAddress]
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add l
ld l, a
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ld a, [wLZAddress + 1]
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adc h
ld h, a
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.ok
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pop af
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cp LZ_REPEAT
jr z, .Repeat
cp LZ_FLIP
jr z, .Flip
cp LZ_REVERSE
jr z, .Reverse
; Since LZ_LONG is command 7,
; only commands 0-6 are passed in.
; This leaves room for an extra command 7.
; However, lengths longer than 768
; would be interpreted as LZ_END.
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; More practically, LZ_LONG is not recursive.
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; For now, it defaults to LZ_REPEAT.
.Repeat:
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; Copy decompressed data for bc bytes.
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dec c
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jr nz, .rnext
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dec b
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jr z, .donerw
.rnext
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ld a, [hli]
ld [de], a
inc de
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jr .Repeat
.Flip:
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; Copy bitflipped decompressed data for bc bytes.
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dec c
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jr nz, .fnext
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dec b
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jp z, .donerw
.fnext
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ld a, [hli]
push bc
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lb bc, 0, 8
.floop
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rra
rl b
dec c
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jr nz, .floop
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ld a, b
pop bc
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ld [de], a
inc de
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jr .Flip
.Reverse:
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; Copy reversed decompressed data for bc bytes.
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dec c
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jr nz, .rvnext
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dec b
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jp z, .donerw
.rvnext
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ld a, [hld]
ld [de], a
inc de
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jr .Reverse
.donerw
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pop hl
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bit 7, [hl]
jr nz, .next
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inc hl ; positive offset is two bytes
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.next
inc hl
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jp .Main