DBPF/Compression - Revision history

Kiwi tea at 19:40, 17 September 2012

2012-09-17T19:40:49Z

Afr0 at 17:42, 9 April 2012

2012-04-09T17:42:16Z

Show changes

Jfade at 19:11, 3 January 2007

2007-01-03T19:11:46Z

Delphy at 09:10, 31 December 2006

2006-12-31T09:10:21Z

Delphy: DBPF Compression moved to DBPF/Compression: Moved to subpage

2006-12-31T09:09:56Z

DBPF Compression moved to DBPF/Compression: Moved to subpage

Curlybap: Reverted the part about confusing control characters

2006-12-29T20:08:00Z

Reverted the part about confusing control characters

Curlybap: Fixed some grammar and simplified the language used.

2006-12-28T14:15:18Z

Fixed some grammar and simplified the language used.

Delphy: /* Example Code */

2006-12-28T13:29:14Z

‎Example Code

Delphy at 13:13, 28 December 2006

2006-12-28T13:13:25Z

Delphy at 13:12, 28 December 2006

2006-12-28T13:12:56Z

@@ Line 477: / Line 477: @@
 *[[DBPF]]
-[[Category:Modding]]
+[[Category:Sims 2 Modding]]
 [[Category:Source Code]]

@@ Line 10: / Line 10: @@
 Thus, the algorithm to decompress these files goes like this:
- Read the file size
+Read the 9 byte header, which is formatted like so:
-  Go to offset 9 (the actual data starts at the 9th byte)
-  While not at the end of the file
+ Offset 00 - Compressed Size of file
+  {
   	- Read the next control character.

← Older revision		Revision as of 09:10, 31 December 2006
Line 261:		Line 261:

	[[Category:Modding]]		[[Category:Modding]]
		+	[[Category:Source Code]]

@@ Line 43: / Line 43: @@
 :*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.  This would result in you going back 5 steps, then copying 10 ahead.  Obviously there aren't 10 characters to copy, only 5.
+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.
-−
-The solution is to read and write one character at a time. In this example, you read the 10th character from the end, and write it to the end before reading the 9th.  By doing this, even offset 0 is possible and would result in duplicating the last character a number of times.  This is used by the compression to create repeating text.
 This is the simplest form of control character. The only thing it does is tell how many plain text characters follow. 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.

@@ Line 1: / Line 1: @@
 =Overview=
-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 idea behind the compression is to reuse previously decoded strings. For example, if the word "heureka" occurs twice in a file, the second occurrence would be encoded by pointing to the first, thus lowering the size of the file.
-The compression is done by defining control characters that tells three things:
+The compression is done by defining control characters that tell three things:
-#How many characters of plain text that follows that should be appended to the output.
+#How many characters of plain text that follow should be appended to the output.
-#How many characters that should be read from the already decoded text (and appended to the output)
+#How many characters should be read from the already decoded text (and appended to the output)
-#At which offset in the already decoded text to read the characters.
+#Where to read the characters from in the already decoded text.
 Thus, the algorithm to decompress these files goes like this:
-  Read file size at offset 0
+  Read the file size
-  Seek to offset 9
+  Go to offset 9 (the actual data starts at the 9th byte)
-  while not end of file is reached do
+  While not at the end of the file
+  {
-  	- Read next control character.
+  	- Read the next control character.
-  	- (Depending on control character read 0-3 more bytes that are a part of the control character.)
+  	- Depending on the 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.
+  	- Inspect the control character.  From this, find out ''how many'' characters should be read and ''where from''.
-  	- Read 0-n characters from source and append them to the output.
+  	- Read 0-''n'' characters from source and append them to the output. (''n'' being the "how many" data from above)
-  	- Copy 0-n characters from somewhere in the output to the end of the output.
+  	- Copy 0-''n'' characters from somewhere in the output to the end of the output. (''n'' in this case is the
+  }
 =Control Characters=
-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:
+There are 4 types of control characters. These are used with different restrictions on 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.
+:Number of characters 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.
@@ Line 43: / Line 43: @@
 :*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
+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.  This would result in you going back 5 steps, then copying 10 ahead.  Obviously there aren't 10 characters to copy, only 5.
-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.
+The solution is to read and write one character at a time. In this example, you read the 10th character from the end, and write it to the end before reading the 9th.  By doing this, even offset 0 is possible and would result in duplicating the last character a number of times.  This is used by the compression to create repeating text.
 ==0x00 - 0x7F==
@@ Line 74: / Line 76: @@
 ==0xC0 - 0xDF==
-This format differes depending on the game.
+This format differs depending on the game.
 ===Sims 2===
@@ Line 124: / Line 126: @@
 =Example Code=
-This is written in PHP, converted from Perl code by  dmchess [http://hullabaloo.simshost.com/forum/viewtopic.php?t=6578&postdays=0&postorder=asc]
+This is written in PHP, converted from Perl code by dmchess [http://hullabaloo.simshost.com/forum/viewtopic.php?t=6578&postdays=0&postorder=asc]
   // First, we read in the length of the total compressed data

@@ Line 126: / Line 126: @@
 This is written in PHP, converted from Perl code by  dmchess [http://hullabaloo.simshost.com/forum/viewtopic.php?t=6578&postdays=0&postorder=asc]
-  // Read a 4 byte unsigned number from the file
+  // First, we read in the length of the total compressed data
   // read_UL4 is a php function in my DBPF class that grabs the next 4 bytes and uses unpack to convert to a integer
   $len = $this->read_UL4($handle);

@@ Line 256: / Line 256: @@
+         }
-=Related Pages=
+=See Also=
 *[[DBPF]]

← Older revision		Revision as of 13:12, 28 December 2006
Line 255:		Line 255:
	return $answer;		return $answer;
	}		}
		+
		+	=Related Pages=
		+
		+	*[[DBPF]]