If then else

This example demonstrates how to write a simple contract with conditions using FHEVM, in comparison to a simple counter.

To run this example correctly, make sure the files are placed in the following directories:

• .sol file → <your-project-root-dir>/contracts/

• .ts file → <your-project-root-dir>/test/

This ensures Hardhat can compile and test your contracts as expected.

FHEIfThenElse.sol

// SPDX-License-Identifier: BSD-3-Clause-Clear
pragma solidity ^0.8.24;

import { FHE, ebool, euint8, externalEuint8 } from "@fhevm/solidity/lib/FHE.sol";
import { SepoliaConfig } from "@fhevm/solidity/config/ZamaConfig.sol";

contract FHEIfThenElse is SepoliaConfig {
  euint8 private _a;
  euint8 private _b;
  euint8 private _max;

  // solhint-disable-next-line no-empty-blocks
  constructor() {}

  function setA(externalEuint8 inputA, bytes calldata inputProof) external {
    _a = FHE.fromExternal(inputA, inputProof);
    FHE.allowThis(_a);
  }

  function setB(externalEuint8 inputB, bytes calldata inputProof) external {
    _b = FHE.fromExternal(inputB, inputProof);
    FHE.allowThis(_b);
  }

  function computeMax() external {
    // a >= b
    // solhint-disable-next-line var-name-mixedcase
    ebool _a_ge_b = FHE.ge(_a, _b);

    // a >= b ? a : b
    _max = FHE.select(_a_ge_b, _a, _b);

    // For more information about FHE permissions in this case,
    // read the `computeAPlusB()` commentaries in `FHEAdd.sol`.
    FHE.allowThis(_max);
    FHE.allow(_max, msg.sender);
  }

  function result() public view returns (euint8) {
    return _max;
  }
}

FHEIfThenElse.ts

import { FHEIfThenElse, FHEIfThenElse__factory } from "../../../types";
import type { Signers } from "../../types";
import { FhevmType, HardhatFhevmRuntimeEnvironment } from "@fhevm/hardhat-plugin";
import { HardhatEthersSigner } from "@nomicfoundation/hardhat-ethers/signers";
import { expect } from "chai";
import { ethers } from "hardhat";
import * as hre from "hardhat";

async function deployFixture() {
  // Contracts are deployed using the first signer/account by default
  const factory = (await ethers.getContractFactory("FHEIfThenElse")) as FHEIfThenElse__factory;
  const fheIfThenElse = (await factory.deploy()) as FHEIfThenElse;
  const fheIfThenElse_address = await fheIfThenElse.getAddress();

  return { fheIfThenElse, fheIfThenElse_address };
}

/**
 * This trivial example demonstrates the FHE encryption mechanism
 * and highlights a common pitfall developers may encounter.
 */
describe("FHEIfThenElse", function () {
  let contract: FHEIfThenElse;
  let contractAddress: string;
  let signers: Signers;
  let bob: HardhatEthersSigner;

  before(async function () {
    // Check whether the tests are running against an FHEVM mock environment
    if (!hre.fhevm.isMock) {
      throw new Error(`This hardhat test suite cannot run on Sepolia Testnet`);
    }

    const ethSigners: HardhatEthersSigner[] = await ethers.getSigners();
    signers = { owner: ethSigners[0], alice: ethSigners[1] };
    bob = ethSigners[2];
  });

  beforeEach(async function () {
    // Deploy a new contract each time we run a new test
    const deployment = await deployFixture();
    contractAddress = deployment.fheIfThenElse_address;
    contract = deployment.fheIfThenElse;
  });

  it("a >= b ? a : b should succeed", async function () {
    const fhevm: HardhatFhevmRuntimeEnvironment = hre.fhevm;

    let tx;

    // Let's compute `a >= b ? a : b`
    const a = 80;
    const b = 123;

    // Alice encrypts and sets `a` as 80
    const inputA = await fhevm.createEncryptedInput(contractAddress, signers.alice.address).add8(a).encrypt();
    tx = await contract.connect(signers.alice).setA(inputA.handles[0], inputA.inputProof);
    await tx.wait();

    // Alice encrypts and sets `b` as 203
    const inputB = await fhevm.createEncryptedInput(contractAddress, signers.alice.address).add8(b).encrypt();
    tx = await contract.connect(signers.alice).setB(inputB.handles[0], inputB.inputProof);
    await tx.wait();

    // Why Bob has FHE permissions to execute the operation in this case ?
    // See `computeAPlusB()` in `FHEAdd.sol` for a detailed answer
    tx = await contract.connect(bob).computeMax();
    await tx.wait();

    const encryptedMax = await contract.result();

    const clearMax = await fhevm.userDecryptEuint(
      FhevmType.euint8, // Specify the encrypted type
      encryptedMax,
      contractAddress, // The contract address
      bob, // The user wallet
    );

    expect(clearMax).to.equal(a >= b ? a : b);
  });
});