LZS format - Revision history

FireFly: This is not markdown

2017-02-26T17:48:59Z

This is not markdown

FireFly: Point out lack of relation to LZS/LZSS

2017-02-26T17:47:16Z

Point out lack of relation to LZS/LZSS

FireFly: Document LZS archives as found in D2PC

2017-02-26T17:45:26Z

Document LZS archives as found in D2PC

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The '''LZS format''' appears to be a compression format used for compressed archives. Here, both the compression and the underlying archive format are described.

It appears in at least Disgaea 2 PC.

== Magic ==
The file starts with a <code>char magic[4]</code> ''"dat\0"'', not part of the compressed data.

== Compression ==
LZ77-based compression (that is, has literal data and backreferences, the latter referring back to a portion of data that has already been output but occurs again).

<pre>
struct header {
u32 compressed_size; // including this 12-byte header
u32 decompressed_size;
u32 marker; // marker used to denote backrefs in the compressed data. 0x00-0xFF.
};
</pre>

This header is followed by the compressed data, which consists of literal bytes (copied straight to output) or backrefs. Backrefs are encoded as

<pre>
struct backref { u8 marker, dist, count; };
</pre>

representing that ''count'' bytes should be copied from ''dist'' back into the output history. If ''dist > marker'', then 1 should be subtracted from it.

Why this extra complication? Because two ''marker'' bytes in a row denote an escaped literal ''marker'' byte. Thus, a decompression loop looks something like this:

<pre>
while (read_ctr < header.compressed_size - sizeof(struct header)) {
u8 v = READ();
if (v == header.marker) {
u8 dist = READ();
if (dist == header.marker) {
WRITE(header.marker); // marker repeated twice = an escaped marker byte
} else {
u8 count = READ();
if (dist > header.marker) dist--;
for (size_t i = 0; i < count; i++) WRITE(HISTORY(dist));
}
} else {
WRITE(v);
}
}
</pre>

After this step, ''header.decompressed_size'' bytes should have been written.

== Archive format ==
The underlying archive consists of a header with the number of files, a table of offsets and filenames, and file data; nothing weird here.

<pre>
struct header {
u32 count; // #files in the archive
u32 unk1; // 0
u32 unk2; // 0
u32 unk3; // 0
};
</pre>

The header is followed by `header.count` file entries.

<pre>
struct file_entry {
u32 end_offset;
char filename[0x1C];
};
</pre>

After this comes the file data. The first file runs from this point for `entries[0].end_offset` bytes. That is, all of the ''end_offset''s are counted starting from after the file metadata table.

[[Category:File formats]]

@@ Line 61: / Line 61: @@
 </pre>
-The header is followed by `header.count` file entries.
+The header is followed by ''header.count'' file entries.
 <pre>
@@ Line 70: / Line 70: @@
 </pre>
-After this comes the file data. The first file runs from this point for `entries[0].end_offset` bytes.  That is, all of the ''end_offset''s are counted starting from after the file metadata table.
+After this comes the file data. The first file runs from this point for ''entries[0].end_offset'' bytes.  That is, all of the ''end_offset''s are counted starting from after the file metadata table.
 [[Category:File formats]]

@@ Line 1: / Line 1: @@
 The '''LZS format''' appears to be a compression format used for compressed archives. Here, both the compression and the underlying archive format are described.
-It appears in at least Disgaea 2 PC.
+Somewhat confusingly, it appears to not use either of [https://en.wikipedia.org/wiki/Lempel–Ziv–Stac LZS] or [https://en.wikipedia.org/wiki/Lempel–Ziv–Storer–Szymanski LZSS] compression, despite the name.
 == Magic ==