Agent skill

dojo-vrf

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npx add-skill https://github.com/dojoengine/book/tree/main/skills/dojo-vrf

SKILL.md

Cartridge VRF Integration

Integrate Cartridge's Verifiable Random Function (VRF) for provably fair, atomic randomness in Dojo games.

Overview

Cartridge VRF provides cheap, atomic verifiable randomness for fully onchain games. The VRF request and response are processed within the same transaction, enabling synchronous and immediate randomness.

Contract Addresses

Network Contract Address
Mainnet 0x051fea4450da9d6aee758bdeba88b2f665bcbf549d2c61421aa724e9ac0ced8f
Sepolia 0x051fea4450da9d6aee758bdeba88b2f665bcbf549d2c61421aa724e9ac0ced8f

Installation

Add to your Scarb.toml:

toml
[dependencies]
cartridge_vrf = { git = "https://github.com/cartridge-gg/vrf" }

Cairo Integration

1. Import the VRF Provider

cairo
use cartridge_vrf::IVrfProviderDispatcher;
use cartridge_vrf::IVrfProviderDispatcherTrait;
use cartridge_vrf::Source;

2. Define VRF Provider Address

cairo
// Use the deployed VRF provider address
const VRF_PROVIDER_ADDRESS: felt252 = 0x051fea4450da9d6aee758bdeba88b2f665bcbf549d2c61421aa724e9ac0ced8f;

3. Consume Random Values

cairo
#[dojo::contract]
mod game_system {
    use starknet::{ContractAddress, get_caller_address};
    use cartridge_vrf::{IVrfProviderDispatcher, IVrfProviderDispatcherTrait, Source};

    const VRF_PROVIDER_ADDRESS: felt252 = 0x051fea4450da9d6aee758bdeba88b2f665bcbf549d2c61421aa724e9ac0ced8f;

    #[abi(embed_v0)]
    impl GameImpl of IGame<ContractState> {
        fn roll_dice(ref self: ContractState) -> u8 {
            let vrf_provider = IVrfProviderDispatcher { 
                contract_address: VRF_PROVIDER_ADDRESS.try_into().unwrap() 
            };
            
            let player = get_caller_address();
            
            // Consume random value using player's nonce
            let random_value: felt252 = vrf_provider.consume_random(Source::Nonce(player));
            
            // Convert to dice roll (1-6)
            let random_u256: u256 = random_value.into();
            let dice_roll: u8 = (random_u256 % 6 + 1).try_into().unwrap();
            
            dice_roll
        }
    }
}

Source Types

Two source types for randomness:

Source::Nonce(ContractAddress)

  • Uses the address's internal nonce
  • Each request generates a unique seed
  • Recommended for most use cases
cairo
let random = vrf_provider.consume_random(Source::Nonce(player_address));

Source::Salt(felt252)

  • Uses a provided salt value
  • Same salt = same random value
  • Useful for deterministic scenarios
cairo
let random = vrf_provider.consume_random(Source::Salt(game_id));

Client-Side Integration (JavaScript/TypeScript)

When executing transactions that use VRF, prefix with request_random:

typescript
import { Account, CallData } from "starknet";

const VRF_PROVIDER = "0x051fea4450da9d6aee758bdeba88b2f665bcbf549d2c61421aa724e9ac0ced8f";

async function rollDice(account: Account, gameContract: string) {
  const call = await account.execute([
    // First: request_random
    {
      contractAddress: VRF_PROVIDER,
      entrypoint: "request_random",
      calldata: CallData.compile({
        caller: gameContract,
        source: [0, account.address], // Source::Nonce(address)
      }),
    },
    // Then: your game action
    {
      contractAddress: gameContract,
      entrypoint: "roll_dice",
      calldata: [],
    },
  ]);
  
  return call;
}

Using Dojo SDK

typescript
import { buildVrfCalls } from "@cartridge/vrf";

const calls = await buildVrfCalls({
  account,
  call: {
    contractAddress: GAME_CONTRACT,
    entrypoint: "roll_dice",
    calldata: [],
  },
  vrfProviderAddress: VRF_PROVIDER,
});

await account.execute(calls);

How It Works

  1. Game calls request_random(caller, source) as first call in multicall
  2. Game contract calls consume_random(source) internally
  3. VRF server generates random value using VRF algorithm
  4. Cartridge Paymaster wraps multicall with submit_random and assert_consumed
  5. Proof verified onchain, random value immediately available
  6. assert_consumed ensures consume_random was called and resets storage

Testing with Mock Provider

For local development/testing, use the mock provider:

cairo
#[dojo::contract]
mod vrf_provider_mock {
    use cartridge_vrf::PublicKey;
    use cartridge_vrf::vrf_provider::vrf_provider_component::VrfProviderComponent;
    use openzeppelin::access::ownable::OwnableComponent;

    component!(path: OwnableComponent, storage: ownable, event: OwnableEvent);
    component!(path: VrfProviderComponent, storage: vrf_provider, event: VrfProviderEvent);

    #[abi(embed_v0)]
    impl VrfProviderImpl = VrfProviderComponent::VrfProviderImpl<ContractState>;

    #[storage]
    pub struct Storage {
        #[substorage(v0)]
        ownable: OwnableComponent::Storage,
        #[substorage(v0)]
        vrf_provider: VrfProviderComponent::Storage,
    }

    fn dojo_init(ref self: ContractState, pubkey_x: felt252, pubkey_y: felt252) {
        self.ownable.initializer(starknet::get_caller_address());
        self.vrf_provider.initializer(PublicKey { x: pubkey_x, y: pubkey_y });
    }
}

Common Patterns

Random Number in Range

cairo
fn random_in_range(random: felt252, min: u32, max: u32) -> u32 {
    let random_u256: u256 = random.into();
    let range = max - min + 1;
    let result: u32 = (random_u256 % range.into()).try_into().unwrap();
    result + min
}

Weighted Random Selection

cairo
fn weighted_random(random: felt252, weights: Span<u32>) -> u32 {
    let total_weight: u32 = weights.iter().sum();
    let random_u256: u256 = random.into();
    let threshold: u32 = (random_u256 % total_weight.into()).try_into().unwrap();
    
    let mut cumulative: u32 = 0;
    let mut i: u32 = 0;
    loop {
        cumulative += *weights.at(i);
        if cumulative > threshold {
            break i;
        }
        i += 1;
    }
}

Shuffle Array (Fisher-Yates)

cairo
fn shuffle<T, impl TCopy: Copy<T>, impl TDrop: Drop<T>>(
    ref arr: Array<T>,
    vrf_provider: IVrfProviderDispatcher,
    player: ContractAddress,
) {
    let mut i = arr.len();
    loop {
        if i <= 1 {
            break;
        }
        i -= 1;
        let random = vrf_provider.consume_random(Source::Nonce(player));
        let j: u32 = (random.into() % (i + 1).into()).try_into().unwrap();
        // Swap arr[i] and arr[j]
        let temp = *arr.at(i);
        arr.set(i, *arr.at(j));
        arr.set(j, temp);
    };
}

Important Notes

  • Always match Source in request_random and consume_random
  • consume_random must be called within the same transaction
  • The Paymaster handles proof submission automatically
  • Works with Cartridge Controller out of the box

Resources

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