Revision as of 04:44, 12 July 2006

This article describes the specifics of the package format used in SimCity 4 and The Sims 2.

Format

(see DBPF)

Compression

The idea behind the compression is to reuse previously decoded strings. For example, if the word "heureka" occurs twice in a file, the second occurence would be encoded by pointing to the first.

The compression is done by defining control characters that tells three things:

1. How many characters of plain text that follows that should be appended to the output.
2. How many characters that should be read from the already decoded text (and appended to the output)
3. At which offset in the already decoded text to read the characters.

Thus, the algorithm to decompress these files goes like this:

Read file size at offset 0 
Seek to offset 9 
while not end of file is reached do 
{ 
	- Read next control character. 
	- (Depending on control character read 0-3 more bytes that are a part of the control character.) 
	- Figure out how many characters that should be read and from where by inspecting the control character. 
	- Read 0-n characters from source and append them to the output. 
	- Copy 0-n characters from somewhere in the output to the end of the output. 
} 

There are 4 types of control characters which are used with different restrictions of how many characters that can be read and from how far behind these can be read. The following conventions are used to describe them:

CC length

Length of control character.

Num plain text

Number of chars immediately after the control character that should be read and appended to output.

Num to copy

Number of chars that should be copied from somewhere in the already decoded output and added to the end of the output.

Copy offset

Where to start reading characters when copying from somewhere in the already decoded output.

This is given as an offset from the current end of the output buffer, i.e. an offset of 0 means that you should copy the last character in the output and append it to the output. And offset of 1 means that you should copy the second-to-last character.

byte0

first byte of control character.

Bits

Bits of the control character.

• p - num plain text
• c - num to copy
• o - copy offset
• i - identifier.

Note: It can sometimes be confusing when a control character states that you should copy for example 10 characters 5 steps from the end of the output. Clearly, you cannot read more than 5 characters before you reach the end of the buffer. The solution is to read and write one character at the time. Each time you read a character you copy it to the end thereby increasing the size of the output. By doing this, even offset 0 is possible and would result in duplicating the last character a number of times. This is utilized by the compression to recreate repeating text, for example bars of repeating dashes

This is the simplest form of control character. The only thing it does is telling how many plain text characters that follows. The formula for this is: (C - 0x7F) * 4. Thus a value of 0xE0 means that you should read 4 characters of plain text and append to the output.

0x00 - 0x7F

CC length: 2 bytes
Num plain text: byte0 & 0x03
Num to copy: ( (byte0 & 0x1C) > > 2) + 3
Copy offset: ( (byte0 & 0x60) < < 3) + byte1 + 1

Bits: 0oocccpp oooooooo
Num plain text limit: 0-3
Num to copy limit: 3-11
Maximum Offset: 1023

0x80 - 0xBF

CC length: 3 bytes
Num plain text: ((byte1 & 0xC0) > > 6 ) & 0x03
Num to copy: (byte0 & 0x3F) + 4
Copy offset: ( (byte1 & 0x3F) < < 8 ) + byte2 + 1

Bits: 10cccccc ppoooooo oooooooo
Num plain text limit: 0-3
Num to copy limit: 4-67
Maximum Offset: 16383

0xC0 - 0xDF

This format differes depending on the game.

SimCity 4

CC length: 4 bytes
Num plain text: byte0 & 0x03
Num to copy: ( (byte0 & 0x1C) < < 6 )  + byte3 + 5
Copy offset: (byte1 < < 8) + byte2

Bits: 110cccpp oooooooo oooooooo cccccccc
Num plain text limit: 0-3
Num to copy limit: 5-2047
Maximum Offset: 65535

Sims 2

CC length: 4 bytes
Num plain text: byte0 & 0x03
Num to copy: ( (byte0 & 0x0C) < < 6 )  + byte3 + 5
Copy offset: ((byte0 & 0x10) < < 12 ) + (byte1 < < 8 ) + byte2 + 1

Bits: 110occpp oooooooo oooooooo cccccccc
Num plain text limit: 0-3
Num to copy limit: 5-1028
Maximum Offset: 131072

0xE0 - 0xFF

CC length: 1 byte 
Num plain text: ((byte0 & 0x1F) < < 2 ) + 4
Num to copy: 0 
Copy offset: - 

Bits: 111ppppp 
Num plain text limit: 4-128 
Num to copy limit: 0 
Maximum Offset: - 

Directory Files

Directory files are one of the newer filetypes I found about the beginning of May. Their purpose is to spead up the loading of a DAT file by showing exactly what is compressed inside it. They are directories of all the compressed files in an archive. This luckily makes their structure fairly simple.

repeated

DWORD

Type ID of the file

DWORD

Group ID of the file

DWORD

Instance ID of the file

DWORD

Instance2/Resource ID (only in new index format dbpfs)

DWORD

Size of the decompressed file in Hex

These files are found as DIR/Directory by the reader and are automatically modified by it during changes to dat files.

It is HIGHLY reccommended that you modify the Directory after editing any compressed file before saving the dat if you are working manually in hex or making your own program.

See E86B1EEF.

This article is imported from the old MTS2 wiki. It's original page, with comments, can be found at http://old_wiki.modthesims2.com/Packages