Explanation of Lemmings file compression

[covox]

Great stuff. Unsure quite what language the code is (at first glance it looks distinctly VBish), however thankfully it's quite pseudocodey. I can't thank you enough, this is indeed a Christmas miracle.

Currently I'm writing a C port of your proceedure (tried starting from scratch using your instructions, my method invoked far too many dynamic memory hissy fits :/). Hopefully I'm aiming toward some sort of multitool to export graphics, sound and level metadata (XML, almost done) to more modern file formats/organized hierachy. If anyone's in the mood for more source code I'm happy to post what I have. Well, maybe once I've cleaned it up a bit

[Mindless]

Hopefully I'm aiming toward some sort of multitool to export graphics, sound and level metadata (XML, almost done) to more modern file formats/organized hierachy.

Could you elaborate on this a bit? I'm not sure what it is you're trying to do. XML?

[ccexplore]

Currently I'm writing a C port of your proceedure (tried starting from scratch using your instructions, my method invoked far too many dynamic memory hissy fits :/).

You can skip this step if you want, I already have working C++ code for compression and decompression.  (But I hope you will credit me for it if you do decide to use it.)  E-mail me for more info.

[covox]

Could you elaborate on this a bit? I'm not sure what it is you're trying to do. XML?

Ooh-er, could've explained myself better there At some point, I'm going to have a shot at making an open-source extendable Lemmings clone. It'll be done in C and SDL, which means (hopefully) that porting and drop-in improvements should be trivial. I'm developing this on a gcc/linux base, which should compile under Windows with mingw32. No idea if VS6/VS.NET/Microsoft Magic Mystery Compiler will touch it however ;/

The Pingus guys have done a great job, however I'm aiming toward something more lightweight and ultra-portable. Oh, and ClanLib is waaay too large to stick onto a PDA. Heaven help you if you change versions :/

To start with, I'm just trying to make the game data manageable. So all the terrain/animations will be converted to indexed PNG, and the level information converted to XML documents. XML is nice because both people and computers can read it

Each level will be stored in an XML file, and there should be some mechanism which refreshes the index (containing all the levels/different games) each time you run the program. For example, if you were to develop a level called "Taste the golden spray", and you want it positioned as Level 12 of Fun on a levelset called "My Generic Custom Game", you could add this XML to the appropriate file:

Code: [Select]

<?xml version="1.0" encoding="UTF-8"?>
<!-- Games available are Classic, ONML, Your_game_name_here -->
<LemLevel game="My Generic Custom Game" difficulty="1" level="12">
 <title>Taste the golden spray!</title>
 <settings levelset="2" release="80" save="80" speed="20" time="180"/>
 <skills climber="0" floater="0" bomber="0" blocker="0" builder="0" basher="0" miner="0" digger="0"/>

 <!-- BEGIN level metadata -->
 <Interactives>
   <interactive piecenum="4" xoffset="0" yoffset="0" flag_ground="0" flag_nooverdraw="0" flag_upsidedown="1" />
   ...
 </Interactives>
 <Terrains>
   <terrain piecenum="12" xoffset="0" yoffset="0" flag_black="1" flag_nooverdraw="0" flag_upsidedown="0" />
   ...
 </Terrains>
 <Steels>
   <steel xoffset="0" yoffset="0" xwidth="16" ywidth="16" />
   ...
 </Steels>
</LemLevel>

It's very preliminary at the moment, any suggestions are more than welcome.  There's a good chance that the schema will be extended at some point to include a broader definition of interactive objects (think the player-controlled cannons in Lemmings 2).

You can skip this step if you want, I already have working C++ code for compression and decompression.  (But I hope you will credit me for it if you do decide to use it.)  E-mail me for more info.

Thanks greatly for the offer, however I'd feel more confident if all the code is pure C. But then again, I haven't had time to compile test my attempt yet, so I may come crying back

[ccexplore]

Thanks greatly for the offer, however I'd feel more confident if all the code is pure C. But then again, I haven't had time to compile test my attempt yet, so I may come crying back

For the decompression I actually have a C version.  I don't have and can't give you a C++ version for the compression though, because it uses <list>, and C, of course, lacks any sort of standardized library for collections.

Then again, since you mentioned "open source", it may be best anyway if you write the code yourself based on Lemmingologist's posts (mine might be a bit too close to what's in the original game itself to be safely open-sourced).

[Mindless]

I don't have and can't give you a C++ version for the compression though, because it uses <list>, and C, of course, lacks any sort of standardized library for collections.

That'd be why I haven't ported the compression code to FreeBasic (I gave up on QBX because of memory issues).  I have no idea what list is...

FreeBasic is awesome: the power of C, yet the syntax of QB (and they're not finished yet)

[Lemmingologist]

function Binary(byte as integer) as string
     dim i, b, power as integer
     dim bin as string

     b=byte
     for i=7 downto 0
           power=pow(2,i)
           bin=bin+str(b\power)
           b=b mod power
     next

     return bin
end function

function FindRepeatedBytes(str1 as string, str2 as string, maxDistance as integer) as integer
     dim pos, curInStr, skip as integer
     dim source, find as string

     source=str1
     find=str2
     do
           //inStr is a search function which returns 0 if the string to be found does not occur,
           //and n+1 if n characters precede its first occurrence
           curInStr=inStr(source, find)
           if curInStr > 0 then
                 if curInStr mod 8 = 1 then
                       pos=skip+curInStr\8+1
                 end if
                 skip=skip+ceil(curInStr/8)
                 source=right(source, len(source)-ceil(curInStr/8)*8)
           end if
     loop until pos > 0 or curInStr=0

     return pos
end function

function Reverse(bytes as string) as string
     dim i as integer
     dim b, n as string

     b=bytes
     for i=1 to len(bytes)\8
           n=n+right(b, 8)
           b=left(b, len(b)-8)
     next

     return n
end function

function XOR(byte1 as integer, byte2 as string) as integer
     dim i, b1, power, result as integer
     dim b2 as string

     b1=byte1
     b2=byte2
     for i=7 downto 0
           power=pow(2,i)
           if str(b1\power) <> left(b2, 1) then
                 result=result+power
           end if
           b1=b1 mod power
           b2=right(b2, i)
     next

     return result
end function

dim i, j, byte, position, readPosition, count, offset, length as integer
dim newBytes, compressed, raw as string
dim in, out as binaryStream
dim file as folderItem
dim stop as boolean

file=GetOpenFolderItem("type")
if file <> nil then
     in=file.OpenAsBinaryFile(false)
     readPosition=in.length-1

     while not stop
           ProgressBar1.value=in.position/in.length*1000\1
           count=0
           do
                 stop=readPosition < 0
                 if not stop then
                       in.position=readPosition
                       byte=in.ReadByte
                       readPosition=readPosition-1
                       newBytes=Binary(byte)+newBytes
                 end if
                 offset=position+count-2
                 if not stop then
                       position=FindRepeatedBytes(raw, newBytes)
                       count=count+1
                 end if
           loop until position=0 or count > 256 or stop
           stop=readPosition < 0

           if len(newBytes) > 16 then
                 if stop then
                       if position=0 then
                             count=count-1
                             newBytes=left(newBytes, 8)
                       end if
                 else
                       readPosition=readPosition+1
                       count=count-1
                       newBytes=right(newBytes, len(newBytes)-8)
                 end if

                 if length > 0 then
                       if length <= 8 then
                             compressed=left(compressed, len(compressed)-length*8)+right(Binary(length-1), 5)+right(compressed, length*8)
                       else
                             compressed=left(compressed, len(compressed)-length*8)+"111"+Binary(length-9)+right(compressed, length*8)
                       end if
                       length=0
                 end if

                 if count=2 and offset < 256 then
                       compressed=compressed+"01"+Binary(offset)
                 elseif count=3 and offset < 512 then
                       compressed=compressed+"100"+right(Binary(offset\256), 1)+Binary(offset mod 256)
                 elseif count=4 and offset < 1024 then
                       compressed=compressed+"101"+right(Binary(offset\256), 2)+Binary(offset mod 256)
                 else
                       compressed=compressed+"110"+Binary(count-1)+right(Binary(offset\256), 4)+Binary(offset mod 256)
                 end if

                 if stop and position=0 then
                       if length < 264 then
                             compressed=compressed+Reverse(newBytes)
                             length=length+1
                       else
                             compressed=left(compressed, len(compressed)-length*8)+"111"+Binary(length-9)+right(compressed, length*8)
                             compressed=compressed+Reverse(newBytes)
                             length=1
                       end if
                 end if
           else
                 if length+count <= 264 then
                       compressed=compressed+Reverse(newBytes)
                       length=length+count
                 else
                       compressed=left(compressed, len(compressed)-264*8)+"11111111111"+right(compressed, 264*8)
                       compressed=compressed+Reverse(newBytes)
                       length=count
                 end if
           end if

           raw=left(newBytes+raw, 32768)
           newBytes=""
     wend

     if length > 0 then
           if length <= 8 then
                 compressed=left(compressed, len(compressed)-length*8)+right(Binary(length-1), 5)+right(compressed, length*8)
           else
                 compressed=left(compressed, len(compressed)-length*8)+"111"+Binary(length-9)+right(compressed, length*8)
           end if
     end if

     out=GetFolderItem(file.name+" Compressed").CreateBinaryFile("type")
     j=len(compressed) mod 8
     out.WriteByte j

     for i=7 downto j
           newBytes=newBytes+"0"
     next
     if left(compressed, 1) = "0" then
           if j < 4 then
                 compressed=newBytes+compressed
           elseif j=4 then
                 compressed=left(compressed, 4)+newBytes+right(compressed, len(compressed)-4)
           else
                 compressed=left(compressed, 5)+newBytes+right(compressed, len(compressed)-5)
           end if
     else
           if j=0 then
                 compressed=newBytes+compressed
           elseif j <= 4 then
                 compressed=left(compressed, j)+newBytes+right(compressed, len(compressed)-j)
           else
                 compressed=left(compressed, 7)+newBytes+right(compressed, len(compressed)-7)
           end if
     end if

     length=len(compressed)\8
     for i=1 to length
           byte=XOR(byte, left(right(compressed, i*8), 8))
     next
     byte=byte\128+(byte\64 mod 2)*2+(byte\32 mod 2)*4+(byte\16 mod 2)*8+(byte\8 mod 2)*16+(byte\4 mod 2)*32+(byte\2 mod 2)*64+(byte mod 2)*128
     out.WriteByte byte
     out.WriteLong in.length
     out.WriteLong length+10

     for i=1 to length
           byte=0
           for j=7 downto 0
                 byte=byte+val(right(compressed,1))*pow(2,j)
                 compressed=left(compressed, len(compressed)-1)
           next
           out.WriteByte byte
     next

     in.close
     out.close
end if

[Lemmingologist]

Ok, that's really the only part I cared about, since it was the part that affects tradeoffs between performance and effectiveness.

It sounds like your code searches for all possible substrings, correct? &#A0;So that in "abcdef", it will search "ab", "abc", "abcd" and "abcde"?

I'm curious what sorts of compression ratio you've gotten, and also how long it takes to compress data from the vgagr# files.

Yes, if "abcdef" is a repeated string, it will search for all of those substrings until it reaches "abcdefg", which has not occurred before in the file. It will then back up one space in the uncompressed file, remove the "g", and add code for the repeating "abcdef". I think my FindRepeatingBytes function is highly inefficient, though, as it searches through the file from beginning to end, when it is really looking for the last occurrence of the substring. This is probably why it takes a fairly long time (several minutes) to compress the VGAGR files. The compressed files are much smaller than the ones generated by LemEdit, as LemEdit only looks for repeating strings which are 16 bytes or less, and my compressor takes advantage of every bit in the "repeat number byte" as well as every bit allocated to the offset. They are still longer than the original compressed files, however. The Lemmings compressor must have been more clever about where to begin and end repeated substrings, unlike my "greedy" algorithm, which simply looks for the longest possible string starting from wherever it happens to be in the file.

[Lemmingologist]

By this do you mean the last bit in the .DAT file or the last bit in the compressed data section?

The compressed data section. I suppose I should have said "block", rather than "file".

[ccexplore]

the substring. This is probably why it takes a fairly long time (several minutes) to compress the VGAGR files.

Several minutes?!?  X_X  I guess that won't do for end-user purposes.   (Yes, I understand it's never intended as such.)

I still like to know how yours compared with DMA's (ie. the original DAT files from the game).  Are yours only larger by a few bytes, or a lot more?

As a reference, the algorithm I have is generally a few bytes larger for level DAT files, about 20% larger for VGASPEC files, and unfortunately, nearly twice as large for VGAGR files.  On the other hand, it operates with no perceptible delay (ie. under a second or less).

(Though as a practical matter, you can probably get away with not doing any real compression at all, so I guess this is more an academic exercise than anything.  :-/)

[Lemmingologist]

Several minutes?!? &#A0;X_X &#A0;I guess that won't do for end-user purposes. &#A0; (Yes, I understand it's never intended as such.)

I still like to know how yours compared with DMA's (ie. the original DAT files from the game). &#A0;Are yours only larger by a few bytes, or a lot more?

As a reference, the algorithm I have is generally a few bytes larger for level DAT files, about 20% larger for VGASPEC files, and unfortunately, nearly twice as large for VGAGR files. &#A0;On the other hand, it operates with no perceptible delay (ie. under a second or less).

(Though as a practical matter, you can probably get away with not doing any real compression at all, so I guess this is more an academic exercise than anything. &#A0;:-/)

My files are (roughly) 1% larger for VGAGR, 4% larger for levels, and 7% larger for VGASPEC. Obviously, this is not worth all the extra time my program takes. The slow speed is probably due to the fact that I'm using a high-level framework and a general-purpose search function, though. You could probably afford to increase your level of compression without sacrificing too much speed.

[Lemmingologist]

dim byte, bytePos as integer
dim in, out as binaryStream

function ReadBits(count as integer) as integer
     dim i, sum as integer

     for i=count-1 downto 0
           sum=sum+pow(2,i)*(byte mod 2)
           byte=byte\2
           if bytePos < 7 then
                 bytePos=bytePos+1
           else
                 bytePos=0
                 byte=in.ReadByte
                 in.position=in.position-2
           end if
     next

     return sum
end function

dim i, bits, start, blockSize, compressedBlockSize, fileSize, itemCount, repeat, offset, length as integer
dim stop, endOfFile as boolean
dim file as folderItem

file=GetOpenFolderItem("type") //displays "Open File" dialog box
fileSize=0
itemCount=0
if file <> nil then
     while not endOfFile
           in=file.OpenAsBinaryFile(false)
           out=GetFolderItem("temp").CreateBinaryFile("type") //creates a binary file named temp
           in.position=fileSize
           start=in.ReadByte
           bits=in.ReadByte
           blockSize=in.ReadLong
           compressedBlockSize=in.ReadLong
           in.position=fileSize+compressedBlockSize-1
           byte=in.ReadByte
           bytePos=0
           in.position=in.position-2

           if start > 0 then
                 if ReadBits(1)=0 then
                       if start < 4 then
                             bits=ReadBits(9-start)
                             length=ReadBits(3)
                       elseif start=4 then
                             length=ReadBits(3)*2
                             bits=ReadBits(4)
                             length=length+ReadBits(1)
                       else
                             length=ReadBits(4)
                             bits=ReadBits(8-start)
                       end if
                 else
                       if start < 4 then
                             bits=ReadBits(10-start)
                             length=ReadBits(8)+8
                       elseif start=4 then
                             bits=ReadBits(2)
                             length=ReadBits(1)*128
                             bits=ReadBits(4)
                             length=length+ReadBits(7)+8
                       else
                             bits=ReadBits(2)
                             length=ReadBits(4)*16
                             bits=ReadBits(8-start)
                             length=length+ReadBits(4)+8
                       end if
                 end if
           else
                 bits=ReadBits(8)
                 if ReadBits(1)=0 then
                       length=ReadBits(4)
                 else
                       bits=ReadBits(2)
                       length=ReadBits(8)+8
                 end if
           end if

           while in.position >= fileSize+5
                 for i=0 to length
                       bits=ReadBits(8)
                       out.WriteByte bits
                 next
                 stop=(in.position < fileSize+5)

                 while not stop
                       if ReadBits(1) = 0 then //0
                             if ReadBits(1) = 0 then //00
                                   length=ReadBits(3)
                                   stop=true
                             else //01
                                   repeat=1
                                   offset=ReadBits(8)
                             end if
                       else //1
                             if ReadBits(1) = 0 then //10
                                   if ReadBits(1) = 0 then //100
                                         repeat=2
                                         offset=ReadBits(9)
                                   else //101
                                         repeat=3
                                         offset=ReadBits(10)
                                   end if
                             else //11
                                   if ReadBits(1) = 0 then //110
                                         repeat=ReadBits(8)
                                         offset=ReadBits(12)
                                   else //111
                                         length=ReadBits(8)+8
                                         stop=true
                                   end if
                             end if
                       end if

                       if not stop then
                             for i=0 to repeat
                                   out.position=out.position-offset-1
                                   bits=out.ReadByte
                                   out.position=out.position+offset
                                   out.WriteByte bits
                             next
                       end if
                 wend
           wend

           in.close
           out.close
           in=GetFolderItem("temp").OpenAsBinaryFile(false)
           out=GetFolderItem(file.name+" Item "+str(itemCount)).CreateBinaryFile("type")
           in.position=in.length-1
           for i=1 to in.length-blockSize
                 bits=in.ReadByte
                 in.position=in.position-2
           next
           for i=1 to blockSize
                 out.WriteByte in.ReadByte
                 in.position=in.position-2
           next
           in.close
           out.close
           GetFolderItem("temp").delete

           fileSize=fileSize+compressedBlockSize
           if fileSize >= file.length then
                 endOfFile=true
           end if
           itemCount=itemCount+1
     wend
end if

[Lemmingologist]

I have updated my compression and decompression code (above on this page). The changes include:
_ Checksum calculation (thanks to ccexplore)
_ Support for 12-bit offsets with less than 5 repeating bytes
_ Fixed a bug in compressor which sometimes caused the first byte of the file to be encoded incorrectly.
_ Increased compression speed by optimizing searches for repeated strings