ref: ae50b1148ae48aa083e6d26dd27a662fffb950ed
dir: /home/decompress.asm/
FarDecompress:: ; b40 ; Decompress graphics data from a:hl to de. ld [wLZBank], a ld a, [hROMBank] push af ld a, [wLZBank] rst Bankswitch call Decompress pop af rst Bankswitch ret ; b50 Decompress:: ; b50 ; Pokemon Crystal uses an lz variant for compression. ; 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) ; Additional parameters are read during command execution. ; Commands: 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. ; Another class of commands reuses data from the decompressed output. LZ_RW EQU 2 + 5 ; bit ; 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. 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. ; 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 ; 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 ; In other words, the structure of the command becomes ; 111xxxyy yyyyyyyy ; x: the new control command ; y: the length ; For more information, refer to the code below and in extras/gfx.py. ; Save the output address ; for rewrite commands. ld a, e ld [wLZAddress], a ld a, d ld [wLZAddress + 1], a .Main: ld a, [hl] cp LZ_END ret z and LZ_CMD cp LZ_LONG jr nz, .short .long ; The count is now 10 bits. ; Read the next 3 bits. ; %00011100 -> %11100000 ld a, [hl] add a add a ; << 3 add a ; This is our new control code. and LZ_CMD push af ld a, [hli] and LZ_LONG_HI ld b, a ld a, [hli] ld c, a ; read at least 1 byte inc bc jr .command .short push af ld a, [hli] and LZ_LEN ld c, a ld b, 0 ; read at least 1 byte inc c .command ; Increment loop counts. ; We bail the moment they hit 0. inc b inc c pop af 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: ; Read literal data for bc bytes. .lloop dec c jr nz, .lnext dec b jp z, .Main .lnext ld a, [hli] ld [de], a inc de jr .lloop .Iter: ; Write the same byte for bc bytes. ld a, [hli] .iloop dec c jr nz, .inext dec b jp z, .Main .inext ld [de], a inc de jr .iloop .Alt: ; Alternate two bytes for bc bytes. dec c jr nz, .anext1 dec b jp z, .adone1 .anext1 ld a, [hli] ld [de], a inc de dec c jr nz, .anext2 dec b jp z, .adone2 .anext2 ld a, [hld] ld [de], a inc de jr .Alt ; Skip past the bytes we were alternating. .adone1 inc hl .adone2 inc hl jr .Main .Zero: ; Write 0 for bc bytes. xor a .zloop dec c jr nz, .znext dec b jp z, .Main .znext ld [de], a inc de jr .zloop .rewrite ; Repeat decompressed data from output. push hl push af ld a, [hli] bit 7, a ; sign jr z, .positive .negative ; hl = de - a ; Since we can't subtract a from de, ; Make it negative and add de. and %01111111 cpl add e ld l, a ld a, -1 adc d ld h, a jr .ok .positive ; Positive offsets are two bytes. ld l, [hl] ld h, a ; add to starting output address ld a, [wLZAddress] add l ld l, a ld a, [wLZAddress + 1] adc h ld h, a .ok pop af 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. ; More practically, LZ_LONG is not recursive. ; For now, it defaults to LZ_REPEAT. .Repeat: ; Copy decompressed data for bc bytes. dec c jr nz, .rnext dec b jr z, .donerw .rnext ld a, [hli] ld [de], a inc de jr .Repeat .Flip: ; Copy bitflipped decompressed data for bc bytes. dec c jr nz, .fnext dec b jp z, .donerw .fnext ld a, [hli] push bc lb bc, 0, 8 .floop rra rl b dec c jr nz, .floop ld a, b pop bc ld [de], a inc de jr .Flip .Reverse: ; Copy reversed decompressed data for bc bytes. dec c jr nz, .rvnext dec b jp z, .donerw .rvnext ld a, [hld] ld [de], a inc de jr .Reverse .donerw pop hl bit 7, [hl] jr nz, .next inc hl ; positive offset is two bytes .next inc hl jp .Main ; c2f