rvzstd []

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~~NOTOC~~
====== RVZstd ======

This specification defines the implementation of ZIP archives as used by the RVZstd standard. This standard is RomVault's implementation of deterministic ZIP archives compressed with zstd. 

===== Archive format =====
==== General format of an RVZstd .zip archive with n files: ====

^Archive Start^
|[local file header 1]|
|[file data 1]|
|[local file header 2]|
|[file data 2]|
|...|
|[local file header n]|
|[file data n]|
^Central Directory - Start (SOCD file offset)^
|[central directory file 1]|
|[central directory file 2]|
|...|
|[central directory file n]|
^Central Directory - End (EOCD file offset)^
|[end of central directory record]|
^Archive End^


==== Local file header x: (Showing RVZstd default values): ====
^Type ^Attribute ^Value ^Description ^ 
|UInt32|Local file header signature|04034B50|Static signature value|
|UInt16|Version needed to extract|3F00|63: Used for modern compression algorithms like LZMA and zstd|
|UInt16|General purpose bit flag| |Bit 2: (0x200) Maximum compression option was used. Bit 11: (0x800) is set for unicode filename|
|UInt16|Compression method|5D00|93: The file is compressed with zstd|
|UInt16|Last mod file time|0000|zeroed time, 1/1/0001|
|UInt16|Last mod file date|0000|zeroed date, 12:00:00 AM|
|UInt32|CRC32| |File CRC32|
|UInt32|Compressed size| |File Compressed Size|
|UInt32|Uncompressed size| |File Uncompressed Size|
|UInt16|Filename length| |Filename length|
|UInt16|Extra field length| |Normally 0, Length of Extra field data if zip64 extra field information is included|
|Byte[]|Filename (variable size)| |Byte array of filename|

<WRAP info>The default values are required to ensure consistent RVZstd archives. Unlike torrentzip, RVZstd uses zeroed values for date and time instead of the date/time of the first MAME release.</WRAP>

==== File data x: ====
The data compression must be exactly zstd version 1.5.5 using level 19 without long distance matching, training, or any-other parameters altered. All files and empty directories must be compressed with the zstd method, which is method 93.


==== Central Directory file x: (Showing RVZstd default values): ====

^Type ^Attribute ^Value ^Description ^ 
|UInt32|Central file header signature|02014B50|Static signature value|
|UInt16|Version made by|0000|MS_DOS and OS/2 (FAT/FAT32 file systems)|
|UInt16|Version needed to extract|3F00|63: Used for modern compression algorithms like LZMA and zstd|
|UInt16|General purpose bit flag| |Bit 2: (0x200) Maximum compression option was used. Bit 11: (0x800) is set for unicode filename|
|UInt16|Compression method|5D00|93: The file is compressed with zstd|
|UInt16|Last mod file time|0000|zeroed time, 1/1/0001|
|UInt16|Last mod file date|0000|zeroed date, 12:00:00 AM|
|UInt32|CRC32| |File CRC32|
|UInt32|Compressed size| |File Compressed Size|
|UInt32|Uncompressed size| |File Uncompressed Size|
|UInt16|File name length| |Filename length|
|UInt16|Extra field length| |Normally 0, Length of Extra field data if zip64 extra field information is included|
|UInt16|File comment length|0000|No file comment|
|UInt16|Disk number start|0000|Multi disk storage not used so set to disk 0|
|UInt16|Internal file attributes|0000|No internal attributes|
|UInt32|External file attributes|0000|No external attributes|
|UInt32|Relative offset of local header| |File offset of this files Local Header|
|Byte[]|File name (variable size)| |Byte array of filename|

==== End of Central Directory: ====
^Type ^Attribute ^Value ^Description ^ 
|UInt32|End of central dir signature|06054B50|Static signature value|
|UInt16|Number of this disk|0000|Multi disk storage not used so set to disk 0|
|UInt16|Number of the disk with the start of the central directory|0000|Multi disk storage not used so set to disk 0|
|UInt16|Total number of entries in the central directory on this disk|n|Total number of files|
|UInt16|Total number of entries in the central directory|n|Total number of files|
|UInt32|Size of the central directory|EOCD-SOCD|length of the central directories|
|UInt32|Offset of start of central directory with respect to the starting disk number|SOCD|Start of central directory|
|UInt16|.ZIP file comment length|0F00|15: RVSztd comment|
|Byte[22]|.ZIP file comment|52565A5354442D...|RVZSTD-XXXXXXXX|

<WRAP info>See above 'General format of an RVZstd .zip file with n files' for SOCD & EOCD</WRAP>


===== The RVZstd archive comment =====
The ZIP file comment in the End of Central Directory is used to check the validity of the RVZstd file.

The comment must be formatted as the 15 bytes of ''RVZSTD-XXXXXXXX''. The ''XXXXXXXX'' is the CRC32 of the central directory records stored as hexadecimal upper case text. (The CRC32 of the bytes in the file between SOCD & EOCD)

This comment ensures that if any change is made to the files within the zip this checksum will no longer match the byte data in the central directory, and in this way we can check the validity of an RVZstd file.
===== Filename Encoding =====
The filenames of the compressed files in a zip archive are stored in the local header and the central directory as byte arrays. Zip was original built on early IBM PCs, and as such uses [[https://en.wikipedia.org/wiki/Code_page_437|code page 437]] to convert a string to a byte array to store the filenames. With the arrival of Unicode, multiple different methods were added to the official zip format to permit Unicode filenames to be stored in a zip file. RVZstd format uses the general purpose bit 11 method. So to store a filename in an RVZstd zip file you must first see if the filename can be stored using code page 437, if not then UTF-8 encoding should be used in the byte arrays, this is then indicated by setting bit 11 of the General Purpose Bit Flags both in the local header and central directory.  


===== File order with an RVZstd archive =====
For the creation of consistent RVZstd files, the file order is also very important. Files must be sorted by filename using a lower case sort.

===== Directory separator character =====
As zips only store files (not directories), files in directories are represented by storing a relative path to the filename. For example file ''test1.rom'' in directory ''set1'' would be stored with a filename of ''set1/test1.rom''. Some zipping programs will store this as ''set1\test1.rom''.

This leads to a possible naming inconsistency. The zip file format states: //"All slashes should be forward slashes ‘/’ as opposed to backwards slashes ‘\’"//. So RomVault will change all ''\'' characters to ''/''. This must be done before sorting, to ensure that the sort is performed correctly.

===== Directory entries and empty directories =====
A directory entry is stored in a zip by adding a file entry ending in a directory separator character with a zero size and CRC. So directory ''set1'' would be stored as a zero length, zero CRC file called ''set1/''. Some zip programs when adding the previously mentioned file ''set1/test1.rom'' will also add the directory ''set1/'', this creates an inconsistency problem. In this example the ''set1/'' directory entry is unnecessary, as the filename ''set1/test1.rom'' implies the existents of the ''set1/'' directory. To resolve this inconsistency un-needed directories should be removed from the zip, the only needed directory entries are empty directories that are not implied by any file entries.

**Example:**
^Filename ^Size ^CRC^
|set1/|0|00000000|
|set1/test1.rom|1024|53AC4D00|
|set2/|0|00000000|

The ''set1/'' entry should be removed, as it is implied by the ''set1/test1.rom'' file. The ''set2/'' entry should be
kept to create the empty directory, as removing it would completely remove the ''set2'' directory.

===== Repeat files =====
Another test that could be performed is checking for repeat file entries inside the zip, most zip programs have a hard time handling this and will just ignore this repeat giving the user no way of knowing there is a repeat filename problem. So it would fix another possible inconsistency if RVZstd scanning at least warned about repeat filename being found inside a zip.

===== Reference =====
  * [[http://www.pkware.com/documents/casestudies/APPNOTE.TXT|Official ZIP file specification]] 
  * [[https://github.com/facebook/zstd|zstd compression]]