HW20: Where Am I?

Problem statement

Implement a smart contract for location tracking.

• The creator of the contract is the owner.
• The owner can add or remove participants.
• Locations are referred to using unique identifiers (e.g. the id 8 is the library, etc). The meaning of each id is defined elsewhere (off-chain).
• Id 0 represents an undefined/unknown location.
• Participants can announce changing their location to a new one using the new location's identifier.
• Anyone can query the number of participants at a given location.

Example scenario

BME wants a tracking system for the location of its professors so that each of them can be found easily when they are needed. For this, the administration creates an instance of this contract and registers all the professors. The administration also creates a list of all the rooms they want to keep track of together with the corresponding identifiers, which they publish on their website.

There is a large meeting in room IE007 (let's say the id of this room is 192). Each professor entering the room registers this event on the contract. 10 professors attend the meeting. When someone queries the number of participant in room IE007, they get 10. When someone queries the current location of Professor Bob (who attends the meeting), they will get 192 (i.e. room IE007).

Note that the registering process could be automated. E.g. to open a classroom, the professor has to scan the QR code of their Ethereum address. After this, the scanner at the door could automatically register the new location for the professor. For now, however, let's assume that participants register this for themselves.

Contract interface

• constructor: function WhereAmI()

  • Creates an instance of this contract.
  • The sender of the transaction (msg.sender) is the owner.

• register person: function registerPerson(address person)

  • Registers person, unless they're already registered.
  • Only the owner of the contract can register new people.
  • The location of newly registered people is undefined (i.e. id 0).

• remove person: function removePerson(address person)

  • Removes person.
  • person must have been registered before.
  • Only the owner of the contract can remove people.

• change location: function changeLocation(uint16 newLocation)

  • Change the location of the sender of the transaction (msg.sender) from their old location to a new one, represented by newLocation.
  • Locations are represented by 16-bit unsigned integers. 0 represents undefined/unknown. The meaning of other ids are defined by the contract's creator.

• get location: function locationOf(address person) view returns (uint16)

  • Get the id of the current location of person.

• get number of people: function numPeopleAtLocation(uint16 id) view returns (uint16)

  • Get the number of people currently registered at location id.

Contract skeleton

pragma solidity ^0.4.21;

contract WhereAmI {

    // <contract_variables>

    // </contract_variables>

    function WhereAmI() public {
        // TODO
    }

    function registerPerson(address person) public {
        // TODO
    }

    function removePerson(address person) public {
        // TODO
    }

    function changeLocation(uint16 newLocation) public {
        // TODO
    }

    function locationOf(address person) public view returns (uint16) {
        // TODO
    }

    function numPeopleAtLocation(uint16 id) public view returns (uint16) {
        // TODO
    }

}