cbnf

The CBNF neural network header format.

What is CBNF?

CBNF is a neural network header format for use with efficiently updatable neural networks for chess. It is in a very early stage of development, and is currently subject to change.

Data Format

CBNF is a fixed-size 64-byte header format, shown here with C++ and Rust structs:

struct __attribute__((packed)) CBNFHeader {
    std::array<char, 4> magic;
    std::uint16_t version;
    std::uint16_t flags;
    std::uint8_t padding;
    std::uint8_t arch;
    std::uint8_t activation;
    std::uint16_t hiddenSize;
    std::uint8_t inputBuckets;
    std::uint8_t outputBuckets;
    std::uint8_t nameLen;
    std::array<char, 48> name;
};

#[repr(C, packed)]
pub struct CBNFHeader {
    pub magic: [u8; 4],
    pub version: u16,
    pub flags: u16,
    pub padding: u8,
    pub arch: u8,
    pub activation: u8,
    pub hidden_size: u16,
    pub input_buckets: u8,
    pub output_buckets: u8,
    pub name_len: u8,
    pub name: [u8; 48],
}

The magic bytes must equal CBNF, or the header is considered ill-formed, no diagnostic required, instant UB, nasal demons, etc etc.

Header Fields

Magic

The magic bytes are the first four bytes of the header, and must equal CBNF. This is used to verify that the header is well-formed.

Version

The version field is a 16-bit unsigned integer that specifies the version of the CBNF format. The current version is 1.

Flags

TBD

Padding

The padding field is currently unused, and must be zero.

Architecture

The architecture field is an 8-bit unsigned integer that specifies the architecture of the neural network. The currently defined architectures are: TBD

Activation

The activation field is an 8-bit unsigned integer that specifies the activation function of the neural network. The currently defined activation functions are: 0: Clipped ReLU 1: Squared Clipped ReLU

Hidden Size

The hidden size field is a 16-bit unsigned integer that specifies the number of neurons in the hidden layer of the neural network.

Input Buckets

The input buckets field is an 8-bit unsigned integer that specifies the number of input buckets in the neural network. This should be 1 for bucketless neural networks, and would be 64 for a HalfKA neural network.

Output Buckets

The output buckets field is an 8-bit unsigned integer that specifies the number of output buckets in the neural network. This should be 1 for bucketless neural networks.

Name Length

The name length field is an 8-bit unsigned integer that specifies the length of the name of the neural network, in bytes. The name length must be less than or equal to 48.

Name

A buffer of 48 bytes that contains the name of the neural network. By this specification the name must be valid ASCII, but implementations are free and encouraged to support UTF-8 names.

Existing Implementations

cbnf-rs

This repository contains a Rust implementation of CBNF as a crate that provides rudimentary convenience methods, such as header parsing and utf-8 name validation.