ref: e13f03ae4447b741a7af9b4c28b7c36f38311c0d
dir: /engine/decompress.asm/
FarDecompress: ; b40 ; Decompress graphics data at a:hl to de ; put a away for a sec ld [$c2c4], a ; save bank ld a, [hROMBank] push af ; bankswitch ld a, [$c2c4] rst Bankswitch ; what we came here for call Decompress ; restore bank pop af rst Bankswitch ret ; b50 Decompress: ; b50 ; Pokemon Crystal uses an lz variant for compression. ; This is mainly used for graphics, but the intro's ; tilemaps also use this compression. ; This function decompresses lz-compressed data at hl to de. ; Basic rundown: ; A typical control command consists of: ; -the command (bits 5-7) ; -the count (bits 0-4) ; -and any additional params ; $ff is used as a terminator. ; Commands: ; 0: literal ; literal data for some number of bytes ; 1: iterate ; one byte repeated for some number of bytes ; 2: alternate ; two bytes alternated for some number of bytes ; 3: zero (whitespace) ; 0x00 repeated for some number of bytes ; Repeater control commands have a signed parameter used to determine the start point. ; Wraparound is simulated: ; Positive values are added to the start address of the decompressed data ; and negative values are subtracted from the current position. ; 4: repeat ; repeat some number of bytes from decompressed data ; 5: flipped ; repeat some number of flipped bytes from decompressed data ; ex: $ad = %10101101 -> %10110101 = $b5 ; 6: reverse ; repeat some number of bytes in reverse from decompressed data ; If the value in the count needs to be larger than 5 bits, ; control code 7 can be used to expand the count to 10 bits. ; A new control command is read in bits 2-4. ; The new 10-bit count is split: ; bits 0-1 contain the top 2 bits ; another byte is added containing the latter 8 ; So, the structure of the control command becomes: ; 111xxxyy yyyyyyyy ; | | | | ; | | our new count ; | the control command for this count ; 7 (this command) ; For more information, refer to the code below and in extras/gfx.py . ; save starting output address ld a, e ld [$c2c2], a ld a, d ld [$c2c3], a .loop ; get next byte ld a, [hl] ; done? cp $ff ; end ret z ; get control code and %11100000 ; 10-bit param? cp $e0 ; LZ_HI jr nz, .normal ; 10-bit param: ; get next 3 bits (%00011100) ld a, [hl] add a add a ; << 3 add a ; this is our new control code and %11100000 push af ; get param hi ld a, [hli] and %00000011 ld b, a ; get param lo ld a, [hli] ld c, a ; read at least 1 byte inc bc jr .readers .normal ; push control code push af ; get param ld a, [hli] and %00011111 ld c, a ld b, $0 ; read at least 1 byte inc c .readers ; let's get started ; inc loop counts since we bail as soon as they hit 0 inc b inc c ; get control code pop af ; command type bit 7, a ; 80, a0, c0 jr nz, .repeatertype ; literals cp $20 ; LZ_ITER jr z, .iter cp $40 ; LZ_ALT jr z, .alt cp $60 ; LZ_ZERO jr z, .zero ; else $00 ; 00 ; LZ_LIT ; literal data for bc bytes .loop1 ; done? dec c jr nz, .next1 dec b jp z, .loop .next1 ld a, [hli] ld [de], a inc de jr .loop1 ; 20 ; LZ_ITER ; write byte for bc bytes .iter ld a, [hli] .iterloop dec c jr nz, .iternext dec b jp z, .loop .iternext ld [de], a inc de jr .iterloop ; 40 ; LZ_ALT ; alternate two bytes for bc bytes ; next pair .alt ; done? dec c jr nz, .alt0 dec b jp z, .altclose0 ; alternate for bc .alt0 ld a, [hli] ld [de], a inc de dec c jr nz, .alt1 ; done? dec b jp z, .altclose1 .alt1 ld a, [hld] ld [de], a inc de jr .alt ; skip past the bytes we were alternating .altclose0 inc hl .altclose1 inc hl jr .loop ; 60 ; LZ_ZERO ; write 00 for bc bytes .zero xor a .zeroloop dec c jr nz, .zeronext dec b jp z, .loop .zeronext ld [de], a inc de jr .zeroloop ; repeats ; 80, a0, c0 ; repeat decompressed data from output .repeatertype push hl push af ; get next byte ld a, [hli] ; absolute? bit 7, a jr z, .absolute ; relative ; a = -a and %01111111 ; forget the bit we just looked at cpl ; add de (current output address) add e ld l, a ld a, $ff ; -1 adc d ld h, a jr .repeaters .absolute ; get next byte (lo) ld l, [hl] ; last byte (hi) ld h, a ; add starting output address ld a, [$c2c2] add l ld l, a ld a, [$c2c3] adc h ld h, a .repeaters pop af cp $80 ; LZ_REPEAT jr z, .repeat cp $a0 ; LZ_FLIP jr z, .flip cp $c0 ; LZ_REVERSE jr z, .reverse ; e0 -> 80 ; 80 ; LZ_REPEAT ; repeat some decompressed data .repeat ; done? dec c jr nz, .repeatnext dec b jr z, .cleanup .repeatnext ld a, [hli] ld [de], a inc de jr .repeat ; a0 ; LZ_FLIP ; repeat some decompressed data w/ flipped bit order .flip dec c jr nz, .flipnext dec b jp z, .cleanup .flipnext ld a, [hli] push bc ld bc, $0008 .fliploop rra rl b dec c jr nz, .fliploop ld a, b pop bc ld [de], a inc de jr .flip ; c0 ; LZ_REVERSE ; repeat some decompressed data in reverse .reverse dec c jr nz, .reversenext dec b jp z, .cleanup .reversenext ld a, [hld] ld [de], a inc de jr .reverse .cleanup ; get type of repeat we just used pop hl ; was it relative or absolute? bit 7, [hl] jr nz, .next ; skip two bytes for absolute inc hl ; skip one byte for relative .next inc hl jp .loop ; c2f