.cats File Format

.cats ((c)atharsis (a)rchive (t)ool (s)tructure) is our custom file format to store resourcepacks a bit more efficiently than a simple zip file and loading it faster.

File Format Repo - Pack Squash Tool - Pack Squash Website

Publishing to Modrinth

Since Modrinth only allows .zip files to be uploaded, you can put the .cats file inside a .zip file and naming the zip <>.cats.zipwhen publishing to Modrinth. You can also add pack.mcmeta and the icon alongside the .cats in the zip which appear when catharsis isn't installed, the pack.mcmeta and the icon inside the .cats get prioritized.

YourPack.cats.zip
├── YourPack.cats
│   ├── pack.png      # Shows when catharsis is installed
│   └── pack.mcmeta   # Shows when catharsis is installed
├── pack.png          # Shows when catharsis is NOT installed, shows in the Modrinth/Prism Launcher
└── pack.mcmeta       # Shows when catharsis is NOT installed, shows in the Modrinth/Prism Launcher

Format

Field	Type	Description
Header	Header	The file header
Data	Byte[]	The raw data of files/directories

Header

Field	Type	Description
Magic Number	Int	Always 0x43415453 = "CATS"
Version	UByte	Version (currently 0x01)
Root Directory	Directory	The root directory

Directory

Field	Type	Description
Entry Count	UShort	Number of entries in the directory
Entries	Entry[]	Array of entries

File

Field	Type	Description
Offset	Int	Offset from the header to data
Size	Int	Size of the file data
Compression	UByte	Compression type (0xFF = None, 0xFE = GZIP)

Entry

Field	Type	Description
Entry Type	UByte	Type of entry (0x00 = File, 0x01 = Directory)
Name Length	UByte	Length of the name string
Name	UByte[]	ASCII name of the entry, Only 0x21-0x7E excluding / and \ are allowed.
Entry Data	Directory or File	Data specific to the entry type

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