Guests

Guests contain functions for Jolt to prove. Making a function provable is as easy as ensuring it is inside the guest package and adding the jolt::provable macro above it.

Let's take a look at a simple guest program to better understand it.

#![allow(unused)]
#![cfg_attr(feature = "guest", no_std)]

fn main() {
#[jolt::provable]
fn add(x: u32, y: u32) -> u32 {
    x + y
}
}

As we can see, the guest looks like a normal no_std Rust library. The only major change is the addition of the jolt::provable macro, which lets Jolt know of the function's existence. The only requirement of these functions is that its inputs are serializable and outputs are deserializable with serde. Fortunately serde is prevalent throughout the Rust ecosystem, so most types will support it by default.

There is no requirement that just a single function lives within the guest, and we are free to add as many as we need. Additionally, we can import any no_std compatible library just as we normally would in Rust.

#![allow(unused)]
#![cfg_attr(feature = "guest", no_std)]

fn main() {
use sha2::{Sha256, Digest};
use sha3::{Keccak256, Digest};

#[jolt::provable]
fn sha2(input: &[u8]) -> [u8; 32] {
    let mut hasher = Sha256::new();
    hasher.update(input);
    let result = hasher.finalize();
    Into::<[u8; 32]>::into(result)
}

#[jolt::provable]
fn sha3(input: &[u8]) -> [u8; 32] {
    let mut hasher = Keccak256::new();
    hasher.update(input);
    let result = hasher.finalize();
    Into::<[u8; 32]>::into(result)
}
}

Standard Library

Jolt supports the Rust standard library. To enable support, simply add the guest-std feature to the Jolt import in the guest's Cargo.toml file and remove the #![cfg_attr(feature = "guest", no_std)] directive from the guest code.

Example

#![allow(unused)]
fn main() {
[package]
name = "guest"
version = "0.1.0"
edition = "2021"

[features]
guest = []

[dependencies]
jolt = { package = "jolt-sdk", git = "https://github.com/a16z/jolt", features = ["guest-std"] }
}

#![allow(unused)]

fn main() {
#[jolt::provable]
fn int_to_string(n: i32) -> String {
    n.to_string()
}
}

alloc

Jolt provides an allocator which supports most containers such as Vec and Box. This is useful for Jolt users who would like to write no_std code rather than using Jolt's standard library support. To use these containers, they must be explicitly imported from alloc. The alloc crate is automatically provided by rust and does not need to be added to the Cargo.toml file.

Example

#![allow(unused)]
#![cfg_attr(feature = "guest", no_std)]

fn main() {
extern crate alloc;
use alloc::vec::Vec;

#[jolt::provable]
fn alloc(n: u32) -> u32 {
    let mut v = Vec::<u32>::new();
    for i in 0..100 {
        v.push(i);
    }

    v[n as usize]
}
}

Print statements

Jolt provides utilities emulating print! and println! in a guest program.

Example

#![allow(unused)]
fn main() {
use jolt::{jolt_print, jolt_println};

#[jolt::provable(memory_size = 10240, max_trace_length = 65536)]
fn int_to_string(n: i32) -> String {
    jolt_print!("Hello, ")
    jolt_println!("from int_to_string({n})!");
    n.to_string()
}
}

Note that jolt_print and jolt_println support format strings. The printed strings are written to stdout during RISC-V emulation of the guest.