HW13: Smart parking

Problem statement

Implement a smart contract for smart parking.

• The contract is managing access to a single parking spot.
• The contract should have exactly one owner (the parking lot's management).
• Clients can buy access to the parking spot for time slots of 60 minutes. To do this, they have to provide their car's registration number (license plate).
• The owner sets the price per time slot.
• Anyone can check if a car has access to the parking spot at any given time.
• Clients can transfer reservations to each other.
• The owner can withdraw the profits.

Example scenario

Alice owns a parking lot. She creates an instance of this contract for each parking spot, setting the price per time slot to 0.01 ether.

Bob reserves one of the parking spots for the time slot between 10:00am and 11:00am this Sunday using his license plate number "ABC123". When the time arrives, Bob parks his car at the spot and leaves to do his business. The cameras in the parking lot detect his license plate number ("ABC123") and check if he has access to the parking spot at that time. If not, they might send a payment notice to the address associated with the plate number.

Contract interface

• constructor: function SmartParking(uint256 pricePerSlot)

  • The creator of the contract (msg.sender) is the owner.
  • pricePerSlot is the price for renting the parking spot for a single time slot.

• reserve: function reserve(uint24 slotId, string licensePlate) payable

  • Reserve the slotId'th time slot since contract creation for the car identified by licensePlate. For example, if the contract was created today at 00:00, then slot number 8 would be the one from 08:00 to 09:00, while slot number 32 would be the same time slot on the next day.
  • This transaction succeeds only if the time slot has not yet been reserved, and the sender transfers the correct amount of ether.

• transfer: function transfer(uint24 slotId, address to, string newLicensePlate)

  • Transfer ownership of the slotId'th time slot since contract creation to the address to (with the license plate number newLicensePlate).
  • Succeeds only if the previous owner of the time slot is the sender of the transaction (msg.sender).

• validate: function validate(string licensePlate) view returns (bool)

  • Returns true if the car identified by licensePlate is entitled to use the parking spot at the time of the validation.
  • Can be called by anyone. (In our use case, this will be called by the parking lot monitoring system.)
  • Note: you can compare strings a and b like this: keccak256(a) == keccak256(b).

• withdraw: function withdraw()

  • Transfer profit to the sender.
  • Only the owner of the contract can call it.

Contract skeleton

pragma solidity ^0.4.21;

contract SmartParking {

    // <contract_variables>

    // </contract_variables>

    function SmartParking(uint256 pricePerSlot) public {
        // TODO
    }

    function reserve(uint24 slotId, string licensePlate) public payable {
        // TODO
    }

    function transfer(uint24 slotId, address to, string newLicensePlate) public {
        // TODO
    }

    function validate(string licensePlate) public view returns (bool) {
        // TODO
    }

    function withdraw() public {
        // TODO
    }

}