HW10: Top-up gym membership

Problem statement

Implement a smart contract for top-up gym membership management.

• The contract should have exactly one owner (the gym's management).
• The owners can set a daily price.
• Clients can top-up their balance by sending ether to the contract.
• The owner can check if the client has sufficient balance for using the facilities.
• The owner can withdraw the profit.

Example scenario

Alice owns a gym. She creates an instance of this contract to manage memberships, setting the daily price to 0.1 ether. Bob uploads 1 ether to his account. Every time Bob visits the gym, he shows his public address QR code to the receptionist who scans it and checks Bob's membership through the smart contract. When Bob enters, his balance is charged with the daily price. If Bob's last payment was at most 12 hours ago (e.g. he goes out during his training session to have a phone call), he does not have to pay again to enter.

Contract interface

• constructor: function GymMembership(uint256 dailyPrice)

  • The creator of the contract (msg.sender) is the owner.
  • dailyPrice is the price of using the gym once.

• top-up: function topUp() payable

  • Add the sent amount of ether (msg.value) to the balance of the sender (msg.sender).

• validate: function validate(address client)

  • Checks whether client has sufficient balance to enter the gym.
  • Can only be called by the owner of the contract.
  • Succeeds if
    • client has entered the gym (paying the daily price) in the last 12 hours. In this case, client is not charged again. Or
    • client has not entered the gym in the last 12 hours, and client has sufficient balance. In this case, client is charged with the daily price and the transaction succeeds.

• withdraw: function withdraw()

  • Transfer profit to the sender.
  • Succeeds only if the sender of the transaction (msg.sender) is the owner of the contract.
  • Note: the owner cannot withdraw balances that have not been charged yet!

Contract skeleton

pragma solidity ^0.4.21;

contract GymMembership {

    // <contract_variables>

    // </contract_variables>

    function GymMembership(uint256 dailyPrice) public {
        // TODO
    }

    function topUp() public payable {
        // TODO
    }

    function validate(address client) public {
        // TODO
    }

    function withdraw() public {
        // TODO
    }

}