The levels, graphics, and sound files for PC Lemmings are all compressed using the .DAT compression format. A .DAT file contains one or more data items (for example, levels), and each item in the file is stored as a "block." The compressed data are read in bitwise, starting at the end of a block and working backwards. A few pieces of information, however, are stored in a header at the beginning of each block, and must be read in before jumping to the end.


Header

Byte 0: instructions for handling the tail end of the block. There are eight different possibilities, represented by the values 00 through 07. The instructions depend on the structure of the end of the file as well, which can be determined by looking at the last bit.
If the last bit in the file is 0, then the instructions are determined as follows:
00 - skip last 10 bits, read next 3 bits
01 - skip last 9 bits, read next 3 bits
02 - skip last 8 bits, read next 3 bits
03 - skip last 7 bits, read next 3 bits
04 - skip last 2 bits, read next 2 bits, skip next 4 bits, read next 1 bit
05 - skip last 2 bits, read next 3 bits, skip next 3 bits
06 - skip last 2 bits, read next 3 bits, skip next 2 bits
07 - skip last 2 bits, read next 3 bits, skip next 1 bit
If the last bit in the file is 1, the instructions are slightly different:
00 - skip last 11 bits, read next 8 bits
01 - skip last 10 bits, read next 8 bits
02 - skip last 9 bits, read next 8 bits
03 - skip last 8 bits, read next 8 bits
04 - skip last 3 bits, read next 4 bits, skip next 4 bits, read next 4 bits
05 - skip last 3 bits, read next 4 bits, skip next 3 bits, read next 4 bits
06 - skip last 3 bits, read next 4 bits, skip next 2 bits, read next 4 bits
07 - skip last 3 bits, read next 4 bits, skip next 1 bit, read next 4 bits

Byte 1: checksum. Don't know how this is calculated, but it doesn't appear to make any difference.
Bytes 2-5: four-byte big-endian integer indicating the size, in bytes, of the uncompressed block.
Bytes 6-9: four-byte big-endian integer indicating the size, in bytes, of the compressed block.


To be continued in reply, as there appears to be a limit on message length.

Data

Once the header has been read, skip to the end of the block and follow the instructions indicated by byte 0 of the header. Remember, the data are being read backwards, which means that "next" really means the bits that come before in the compressed file. Also, integers which are not in the header should be read in reverse bit order (for example, 110 = 3, and 01000000 = 2). The bits that were read, rather than skipped, form an integer specifying the first "length." If 8 bits were read instead of 3 (that is, the last bit of the file is 1), add 8 to this length. Now, do the following until all of the data have been read (reading backwards in the file from the current position, one bit at a time, and making sure not to treat the header as data):

(1) Read in a number of bytes equal to length + 1, and write them directly to the decompressed file, reversing the order of the bits in each byte. Remember that these bytes may start in the middle of a file byte - do not skip any bits. Proceed to (2).

(2) The next few bits indicate the nature of the bits that come after them. (Since bits are being read backwards one at a time, the actual order of these bits in the file will be reversed).
00 - read next 3 bits as an integer. This number is the next length. Repeat (1).
01 - there will be 2 repeated bytes. Read next 8 bits as an offset. Proceed to (3).
100 - there will be 3 repeated bytes. Read next 9 bits as an offset. Proceed to (3).
101 - there will be 4 repeated bytes. Read next 10 bits as an offset. Proceed to (3).
110 - read next 8 bits as an integer, "n". There will be n + 1 repeated bytes. Read the next 12 bits as an offset. Proceed to (3).
111 - read next 8 bits as an integer, "n". The next length is equal to n + 8. Repeat (1).

(3) Copy the indicated number of repeated bytes from the decompressed file, starting from the position equal to the number of bytes already decompressed, minus the offset, minus 1. Paste these bytes at the end of the decompressed file.
For example, if 50 bytes have been decompressed so far, and the offset is 20, copy bytes starting from position 50 - 20 - 1 = 29. If there are, for example, 5 repeated bytes, then copy 5 bytes starting from byte number 29, and paste them at the end of the file. Repeat (2).

After all of the data in the block have been read and the decompressed file has been written, the order of the bytes in the decompressed file will then need to be reversed, as the original file was read and compressed in reverse order. This must be done bytewise - read the last byte of the decompressed file, paste it in as the first byte of the new file; read the second to last byte and paste it in as the new second byte, and so on.


And that is all there is to the Lemmings compression format! I have included program code for compression and decompression as replies to this post. These programs should be able to be translated into any programming language which can read and write single bytes (built-in methods for reading and writing single bits are not necessary). Be aware, though, that the algorithms I am using are slow when handling large files, and could use some optimization. These programs are intended as a starting point for anyone who would like to write a Lemmings graphics editor. Which, I imagine, will inevitably happen sometime (see my other post - Instructions for editing graphics files).

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
                       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
                             end if
                       else //1
                             if ReadBits(1) = 0 then //10
                                   if ReadBits(1) = 0 then //100
                                         repeat=2
                                   else //101
                                         repeat=3
                                   end if
                             else //11
                                   if ReadBits(1) = 0 then //110
                                         repeat=ReadBits(8)
                                   else //111
                                         length=ReadBits(8)+8
                                         stop=true
                                   end if
                             end if
                       end if

                       if not stop then
                             if repeat <= 3 then
                                   offset=ReadBits(7+repeat)
                             else
                                   offset=ReadBits(12)
                             end if
                             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

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

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

     return bin
end function

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

     source=right(str1, maxDistance*8)
     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 curInStr=0

     return pos
end function

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

file=GetOpenFolderItem("type") //displays "Open File" dialog box
if file <> nil then
     in=file.OpenAsBinaryFile(false)
     in.position=in.length-1

     while not stop
           count=0
           do
                 byte=in.ReadByte
                 if in.position=1 then
                       stop=true
                 else
                       in.position=in.position-2
                 end if
                 newBytes=newBytes+Binary(byte)
                 if len(raw) < max*8 then
                       offset=len(raw)\8-position
                 else
                       offset=max-position
                 end if
                 if len(newBytes) <= 32 then
                       max=pow(2, 6+len(newBytes)\8)
                 else
                       max=4096
                 end if
                 position=FindRepeatedBytes(raw, newBytes, max)
                 count=count+1
           loop until position=0 or count > 256 or stop

           if len(newBytes) > 16 then
                 if not stop then
                       in.position=in.position+1
                       count=count-1
                       newBytes=left(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

                 select case count
                 case 2
                       compressed=compressed+"01"+Binary(offset)
                 case 3
                       compressed=compressed+"100"+right(Binary(offset\256), 1)+Binary(offset mod 256)
                 case 4
                       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 select
           else
                 if length+count <= 264 then
                       compressed=compressed+newBytes
                       length=length+count
                 else
                       compressed=left(compressed, len(compressed)-length*8)+"111"+Binary(length-9)+right(compressed, length*8)
                       compressed=compressed+newBytes
                       length=count
                 end if
           end if

           raw=raw+newBytes
           if len(raw) > 32768 then
                 raw=right(raw, 32768)
           end if
           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
           length=0
     end if

     out=GetFolderItem(file.name+" Compressed").CreateBinaryFile("type") //creates a binary file with the original file name plus "Compressed"
     j=len(compressed) mod 8
     out.WriteByte j

     if left(compressed, 1) = "0" then
           if j > 4 then
                 for i=7 downto j
                       newBytes=newBytes+"0"
                 next
                 compressed=left(compressed, 5)+newBytes+right(compressed, len(compressed)-5)
           elseif j=4 then
                 compressed=left(compressed, 4)+"0000"+right(compressed, len(compressed)-4)
           elseif j=0 then
                 compressed=compressed+"00000000"
           end if
     else
           for i=7 downto j
                 newBytes=newBytes+"0"
           next
           if j=0 then
                 compressed=newBytes+compressed
           elseif j < 3 then
                 compressed=left(compressed, j)+newBytes+right(compressed, len(compressed)-j)
           elseif j < 5 then
                 compressed=left(compressed, 3)+newBytes+right(compressed, len(compressed)-3)
           else
                 compressed=left(compressed, 7)+newBytes+right(compressed, len(compressed)-7)
           end if
     end if

     max=ceil(len(compressed)/8)
     out.WriteByte 0
     out.WriteLong in.length
     out.WriteLong max+10

     for i=1 to max
           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

Can you give a semi-English explanation of what you do for compression?

What language is this code in?  It's not any syntax I recognize... although it looks alot like BASIC.

Edit: After a quick search, it appears to be REALbasic...  :-/

Just curious:  "Lemmingologist", you don't happen to be the author of LemEdit, are you?

I'm guessing this for no reason other than the fact that your knowledge appears to somewhat match the features and limitations of LemEdit, including the significant fact that you don't have info on the VGASPECx.DAT files, just like the last version of LemEdit we get also don't have support for them yet.

The reason we (well, at least I) want to know is because, well, LemEdit could use some fixes to work better on modern Windows machines......

Sorry to break your theory, but (s)he can't be, since in order to compress .DATs (LemEdit does ;P) you MUST know how to create the correct checksum.

Good point.  I honestly haven't been really trying to thoroughly read whatever (s)he posted.  X_X

Start at the end of the uncompressed file, and repeat the following until reaching the beginning:

Read in bytes, one at a time, backwards toward the beginning, until a string (two bytes or longer) is found which has already been read. At the end of the compressed file, first indicate the number of non-repeating bytes that were read in before the repeated string, minus one (call this number n). This is done in the following manner:
     If n is less than 8, write two 0-bits, then write n as a three-bit integer.
     If n is greater than or equal to 8, write three 1-bits, then write n - 8 as an eight-bit integer.
For example, if 23 non-repeating bytes were read before the repeated string, then n = 23 - 1 = 22. 22 - 8 = 14, so the output to the compressed file would then be 111 followed by 14 in binary, which is 00001110.
Now append the non-repeating bytes to the compressed file.

Then, add two or three bits indicating the number of
repeating bytes, using the codes below:
     01 - 2 repeated bytes
     100 - 3 repeated bytes
     101 - 4 repeated bytes
     110 - anywhere from 2 to 256 repeated bytes. Follow this with the number of repeated bytes minus one, written as an eight-bit integer.

Next, write the offset of the repeated string, which is equal to the total number of bytes read so far from the uncompressed file, minus the number of bytes read before the previous occurrence of the repeated string.
The number of bits used to write the offset depends on the repeat indicator code used above:
     01 - offset is an eight-bit integer
     100 - offset is a nine-bit integer
     101 - offset is a ten-bit integer
     110 - offset is a twelve-bit integer
This means that the maximum offset is 255, 511, 1023, and 4095 for each of the above codes respectively. If the string does not occur within 4096 bytes of the current position in the file, it must not be counted as a repeated string.


After all of the data have been read, reverse the order of the bits (not bytes) in the compressed file. Then, at the beginning of the new compressed file, add the ten-byte header:
Byte 0: the remainder when the number of bits in the compressed file is divided by 8.
Byte 1: checksum. This is ignored by Lemmings, so a zero can be used here.
Bytes 2-5: four-byte big-endian integer indicating the size, in bytes, of the uncompressed file.
Bytes 6-9: four-byte big-endian integer indicating the size, in bytes, of the compressed file, including the ten-byte header.

Finally, some 0-bits need to be added near the end of the new compressed file. The placement of these bits depends on the value of byte 0 of the header:
     00 - add eight 0-bits at the end of the file.
     01 - if the last bit of the file is 1, add seven 0-bits before the last bit of the file. Otherwise, do nothing.
     02 - if the last bit of the file is 1, add six 0-bits before the last two bits of the file. Otherwise, do nothing.
     03 - if the last bit of the file is 1, add five 0-bits before the last three bits of the file. Otherwise, do nothing.
     04 - add four 0-bits before the last four bits of the file.
     05 - if the last bit of the file is 0, add three 0-bits before the last five bits of the file. If the last bit is 1, add three 0-bits before the last seven bits of the file.
     06 - if the last bit of the file is 0, add two 0-bits before the last five bits of the file. If the last bit is 1, add two 0-bits before the last seven bits of the file.
     07 - if the last bit of the file is 0, add one 0-bit before the last five bits of the file. If the last bit is 1, add one 0-bit before the last seven bits of the file.

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?  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.

Hmmm...
Crap... (s)he's right... of course, my Lemmings game is cracked, so they may have the checksum checking code removed...

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

Yeah, interestingly it works apparently.

Then again, I don't think I ever said that it wouldn't work either.  In the code it definitely passes back the result of the checksum comparison, but whether the result was used or not I can't tell since I'm not going to go through all the places where the function is called and see whether the caller bothers to check the return value.

Anyhow, it's just a dirt-simple checksum.  Probably the simplest aspect of the whole thing.

Now of course, whether it works for LemEdit or not is another question which I haven' tested (not that it matters for me).

No checksum mismatch warnings from LemEdit...


Edit: Oooo... wow, I crashed LemEdit.

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 ;)

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

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.

Ooh-er, could've explained myself better there ;P 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:

<?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).



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).

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)

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

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.

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

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.  :-/)

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.

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

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