SnarkyJS Merkle tree example

merkle-tree.ts

/*
Description: 

This example describes how developers can use Merkle Trees as a basic off-chain storage tool.

zkApps on Mina can only store a small amount of data on-chain, but many use cases require your application to at least reference big amounts of data.
Merkle Trees give developers the power of storing large amounts of data off-chain, but proving its integrity to the on-chain smart contract!


! Unfamiliar with Merkle Trees? No problem! Check out https://blog.ethereum.org/2015/11/15/merkling-in-ethereum/
*/

import {
  SmartContract,
  Poseidon,
  Field,
  State,
  state,
  PublicKey,
  Mina,
  method,
  UInt32,
  PrivateKey,
  AccountUpdate,
  MerkleTree,
  MerkleWitness,
  Struct,
} from 'snarkyjs';

const doProofs = true;

class MyMerkleWitness extends MerkleWitness(8) {}

class Account extends Struct({
  publicKey: PublicKey,
  points: UInt32,
}) {
  hash(): Field {
    return Poseidon.hash([
      ...this.publicKey.toFields(),
      ...this.points.toFields(),
    ]);
  }

  addPoints(points: UInt32) {
    return new Account({
      publicKey: this.publicKey,
      points: this.points.add(points),
    });
  }
}
// we need the initiate tree root in order to tell the contract about our off-chain storage
let initialCommitment: Field = Field(0);
/*
  We want to write a smart contract that serves as a leaderboard,
  but only has the commitment of the off-chain storage stored in an on-chain variable.
  The accounts of all participants will be stored off-chain!
  If a participant can guess the preimage of a hash, they will be granted one point :)
*/

class Leaderboard extends SmartContract {
  // a commitment is a cryptographic primitive that allows us to commit to data, with the ability to "reveal" it later
  @state(Field) commitment = State<Field>();

  @method init() {
    super.init();
    this.commitment.set(initialCommitment);
  }

  @method
  guessPreimage(guess: Field, account: Account, path: MyMerkleWitness) {
    // this is our hash! its the hash of the preimage "22", but keep it a secret!
    let target = Field(
      '17057234437185175411792943285768571642343179330449434169483610110583519635705'
    );
    // if our guess preimage hashes to our target, we won a point!
    Poseidon.hash([guess]).assertEquals(target);

    // we fetch the on-chain commitment
    let commitment = this.commitment.get();
    this.commitment.assertEquals(commitment);

    // we check that the account is within the committed Merkle Tree
    path.calculateRoot(account.hash()).assertEquals(commitment);

    // we update the account and grant one point!
    let newAccount = account.addPoints(UInt32.from(1));

    // we calculate the new Merkle Root, based on the account changes
    let newCommitment = path.calculateRoot(newAccount.hash());

    this.commitment.set(newCommitment);
  }
}

type Names = 'Bob' | 'Alice' | 'Charlie' | 'Olivia';

let Local = Mina.LocalBlockchain({ proofsEnabled: doProofs });
Mina.setActiveInstance(Local);
let initialBalance = 10_000_000_000;

let feePayerKey = Local.testAccounts[0].privateKey;
let feePayer = Local.testAccounts[0].publicKey;

// the zkapp account
let zkappKey = PrivateKey.random();
let zkappAddress = zkappKey.toPublicKey();

// this map serves as our off-chain in-memory storage
let Accounts: Map<string, Account> = new Map<Names, Account>(
  ['Bob', 'Alice', 'Charlie', 'Olivia'].map((name: string, index: number) => {
    return [
      name as Names,
      new Account({
        publicKey: Local.testAccounts[index].publicKey,
        points: UInt32.from(0),
      }),
    ];
  })
);

// we now need "wrap" the Merkle tree around our off-chain storage
// we initialize a new Merkle Tree with height 8
const Tree = new MerkleTree(8);

Tree.setLeaf(0n, Accounts.get('Bob')!.hash());
Tree.setLeaf(1n, Accounts.get('Alice')!.hash());
Tree.setLeaf(2n, Accounts.get('Charlie')!.hash());
Tree.setLeaf(3n, Accounts.get('Olivia')!.hash());

// now that we got our accounts set up, we need the commitment to deploy our contract!
initialCommitment = Tree.getRoot();

let leaderboardZkApp = new Leaderboard(zkappAddress);
console.log('Deploying leaderboard..');
if (doProofs) {
  await Leaderboard.compile();
}
let tx = await Mina.transaction(feePayer, () => {
  AccountUpdate.fundNewAccount(feePayer).send({
    to: zkappAddress,
    amount: initialBalance,
  });
  leaderboardZkApp.deploy();
});
await tx.prove();
await tx.sign([feePayerKey, zkappKey]).send();

console.log('Initial points: ' + Accounts.get('Bob')?.points);

console.log('Making guess..');
await makeGuess('Bob', 0n, 22);

console.log('Final points: ' + Accounts.get('Bob')?.points);

async function makeGuess(name: Names, index: bigint, guess: number) {
  let account = Accounts.get(name)!;
  let w = Tree.getWitness(index);
  let witness = new MyMerkleWitness(w);

  let tx = await Mina.transaction(feePayer, () => {
    leaderboardZkApp.guessPreimage(Field(guess), account, witness);
  });
  await tx.prove();
  await tx.sign([feePayerKey, zkappKey]).send();

  // if the transaction was successful, we can update our off-chain storage as well
  account.points = account.points.add(1);
  Tree.setLeaf(index, account.hash());
  leaderboardZkApp.commitment.get().assertEquals(Tree.getRoot());
}