HW12: Gym machine rental

Problem statement

Implement a smart contract for gym machine rental.

• The contract should have exactly one owner (the gym's management).
• Clients can buy access to machines in the gym for time slots of 30 minutes.
• The owner sets the minimum price per time slot.
• Clients can reserve each other's slots if they offer more ether.
• Anyone can check if a client has access to the machine at any given time.
• The owner can withdraw the profits.

Example scenario

Alice owns a gym. She creates an instance of this contract to manage access to one of the popular machines in her gym, setting the minimum price per time slot to 0.005 ether.

Bob reserves the time slot between 11:00am and 11:30am this Sunday for 0.005 ether using the time slot's number since contract creation. After 11:00am he scans the QR code of his Ethereum address on the machine. The machine checks if Bob has access and unlocks.

Alternatively, Claire reserves the same time slot at 10:30am for 0.01 ether. Bob gets his money back, and Claire can use the machine from 11:00am the same way Bob would have used it.

Contract interface

• constructor: function GymMachineRental(uint256 minimumPricePerSlot)

  • The creator of the contract (msg.sender) is the owner.
  • minimumPricePerSlot is the minimum price for renting the machine for a single time slot.

• reserve: function reserve(uint24 slotId) payable

  • Reserve the slotId'th time slot since contract creation. For example, if the contract was created today at 00:00, then slot number 16 would be the one from 07:30 to 08:00, while slot number 64 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 at least the minimum amount of ether required, or
    • the time slot has been reserved, but the current sender offers more ether than the original client who reserved it. In this case, the original client gets his/her money back.

• validate: function validate(address client) view returns (bool)

  • Returns true if client is entitled to use the machine at the time of the validation.
  • Can be called by anyone. (In our use case, this will be called by the machine itself.)

• withdraw: function withdraw()

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

Contract skeleton

pragma solidity ^0.4.21;

contract GymMachineRental {

    // <contract_variables>

    // </contract_variables>

    function GymMachineRental(uint256 minimumPricePerSlot) public {
        // TODO
    }

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

    function validate(address client) public view returns (bool) {
        // TODO
    }

    function withdraw() public {
        // TODO
    }

}