ARC0

Filetype: Archive
File Extensions: .fa
Signature: ARC0
Platform(s): Nintendo 3DS
Endianness: Little-endian
Used in: Yo-kai Watch 1, Yo-kai Watch 2, Yo-kai Watch Blasters, Yo-kai Watch 3, Yo-kai Watch Busters 2
Alignment: 4 Byte Alignment
String Encoding: SJIS*

*The default (japanese) encoding used by the 3DS games’ is actually cp932 (an extension of sjis) although the difference is negligible - this was likely just an unintended result of the libraries used by Level5.

ARC0 Archives are the main game archives which hold all of romfs (with the exception of audio) within Level5’s 3DS games. In Yo-kai Watch there are two forms of ARC0 archives:

  • Main game archive
    • Structure: <game>_a.fa
    • This holds most of the game data - in Yo-kai Watch games this archive is called either yw2_a.fa (YW2), yw1_a.fa (YW1), or yw_a.fa (YWB+) depending on the game.
  • Locale archive
    • Structure: <game>_lg_<lg>.fa
    • There are several of these - one for each locale the copy of the game supports, with the player’s locale deciding which one is used. These have priority over the main archive so the game checks to see if the file is in the locale archive first. This is commonly used for text and assets which contain text as they are language-dependant. Some example locale archives can be found below:
      • yw2_lg_engb.fa
      • yw1_lg_fr.fa
      • yw_lg_es.fa

The two forms have the same binary layout shown below:

Header (0x48/72 bytes)

Compressed Directory Entry Table
Compressed Directory Hash Table
Compressed File Entry Table
Compressed Name Table

File Data Section (uncompressed, 4-byte aligned) <- this is likely because basically all of level5s file formats are already compressed; compressing a compressed file won't save much due to the increased entropy

Note that all tables except file data are stored inside Level-5’s Compressed Container structure and all major sections are padded to a 4-byte alignment.

ARC0 Header

Structure

Offset Size Type Name
0x00 4 char[4] magic (ARC0)
0x04 4 u32 directoryEntriesOffset
0x08 4 u32 directoryHashOffset
0x0C 4 u32 fileEntriesOffset
0x10 4 u32 nameOffset
0x14 4 u32 dataOffset
0x18 2 u16 directoryEntriesCount
0x1A 2 u16 directoryHashCount
0x1C 4 u32 fileEntriesCount
0x20 4 u32 tableChunkSize
0x24 4 u32 reserved1 (always 0)
0x28 4 u32 unk2
0x2C 4 u32 unk3
0x30 4 u32 unk4
0x34 4 u32 unk5
0x38 4 u32 directoryCount
0x3C 4 u32 fileCount
0x40 4 u32 unk7
0x44 4 u32 reserved2 (always 0)

Leave the unk entries as-is when parsing ARC0 archives. unk2-unk5 may be hashes. Also note that:

  • fileCount is the number of files in this directory
    tableChunkSize is the total size of all tables before the file data section, aligned to 4 bytes. It is calculated as follows: 
    tableChunkSize = (directoryEntriesCount * 20) + (directoryHashCount * 4) + (fileEntriesCount * 16) + nameTableDecompressedSize + 0x20

Directory Entry Table

This section is compressed with Level5’s Compressed Container format - the decompressed format is shown below:

Structure

Offset Size Type Name
0x00 4 u32 crc32
0x04 2 u16 firstDirectoryIndex
0x06 2 s16 directoryCount
0x08 2 u16 firstFileIndex
0x0A 2 s16 fileCount
0x0C 4 s32 fileNameStartOffset
0x10 4 s32 directoryNameStartOffset

Note that:

  • firstFileIndex is an index into file entry table.
  • firstDirectoryIndex references the index in the sorted directory table.
  • directoryCount = number of direct subdirectories.
  • fileCount = number of files in this directory.
  • directoryNameStartOffset = offset inside name table.
  • fileNameStartOffset = offset inside name table.
  • For the root directory crc32 = 0xFFFFFFFF.

Directory Hash Table

This section is compressed with Level5’s Compressed Container format - the decompressed format is shown below:

  • 4 bytes per entry

  • Contains CRCs of directories where:

    crc32 != 0xFFFFFFFF
  • Order matches sorted directory entry order

File Entry Table

This section is compressed with Level5’s Compressed Container format - the decompressed format is shown below:

Structure

Offset Size Type Name
0x00 4 u32 crc32
0x04 4 u32 nameOffsetInFolder
0x08 4 u32 fileOffset
0x0C 4 u32 fileSize

Note that:

  • Entries are grouped by directory and within a directory, entries are sorted in the order: CRC-32 ascending
  • crc32 is computed from filename only
  • nameOffsetInFolder is relative to the directory’s fileNameStartOffset
  • fileOffset is relative to dataOffset
  • fileSize is uncompressed size
  • File data is 4-byte aligned

Name Table

This section is compressed with Level5’s Compressed Container format - the decompressed format is shown below:

Structure

This section entirely consists of a continuous blob of mull-terminated directory names and null-terminated file names. Directory names must end with a forward slash followed by a null terminator ("/\x00") - with the exception of the root directory which is an empty string ("\x00"). The trailing / is included in the CRC-32 calculation. The CRC-32 of files only contains the file name and not the directory as stated in the File Entry table.

File Data Section

This section begins at dataOffset and requires that files are written sequentially, with each file being aligned to a 32-bit word (4 bytes). Note that NO compression is applied at the ARC0 level - it is left up to the files contained within it to compress themselves as part of their individual file formats.

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