liquid-democracy-3.sol

contract LiquidDemocracy {
  ...

  function delegateTo(address delegate) public isValidVoter {
    require(
      voters[delegate].state != State.Invalid,
      'Delegating to invalid voter'
    );
    require(
      delegate != msg.sender && !delegationCycleExists(delegate, msg.sender),
      'Forming a delegation cycle'
    );
    Voter storage voter = voters[msg.sender];
    voter.delegatedTo = voters[delegate].index;
    voter.state = State.Delegated;
  }

  function delegationCycleExists(address delegate, address sender) internal view returns(bool) {
    // traverse the delegation cycle until we find a voter that has not delegated
    while(voters[delegate].state == State.Delegated) {
      delegate = validVoters[voters[delegate].delegatedTo];
      if (delegate == sender) return true; // delegation cycle found
    }
    return false;
  }
}