HW08: Gym membership

Problem statement

Implement a smart contract for gym membership management.

• The contract should have exactly one owner (the gym's management).
• Clients can buy membership for a certain duration of time.
• Anyone can check if a given membership is valid or has expired.
• The owner can withdraw the profit.

Example scenario

Alice owns a gym. She creates an instance of this contract to manage memberships. Bob soon buys a membership for 30 days. 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.

Contract interface

• constructor: function GymMembership(uint256 dailyPrice)

  • The creator of the contract (msg.sender) is the owner.
  • dailyPrice is the price of buying membership for one day in wei.

• register: function register(uint256 numDays) payable

  • The sender purchases membership for numDays days starting from the time of the transaction (i.e. now).
  • Membership is associated with the sender's Ethereum address.
  • The sender must pay the corresponding price in ether.
  • Clients who are already members should not be able to buy a new membership until the old has expired.

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

  • Returns true if client currently has a valid membership.

• withdraw: function withdraw()

  • Transfer all funds on the contract to the sender.
  • Only the owner of the contract can call it.

Contract skeleton

pragma solidity ^0.4.21;

contract GymMembership {

    // <contract_variables>

    // </contract_variables>

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

    function register(uint256 numDays) public payable {
        // TODO
    }

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

    function withdraw() public {
        // TODO
    }

}