HW09: Tokenized gym membership

Problem statement

Implement a smart contract for tokenized gym membership management.

• The contract should have exactly one owner (the gym's management).
• The owner issues a certain number of monthly passes when they create the contract.
• Initially, all passes belong to the owner.
• Users can transfer gym passes, i.e. transfer ownership to other addresses.
• Each pass is valid for 30 days from its first use.
• Anyone can check the validity of each pass (i.e. whether it has expired or not).
• The contract does not store any ether.

Example scenario

Alice owns a gym. She creates an instance of this contract with 100 monthly pass tokens and transfers these tokens to the lucky winners of their recent contest.

The winners can trade with these passes, transferring them to other addresses if they want to.

Bob bought one of these passes from his friend, Claire. After this, every time Bob visits the gym, he shows his public address QR code to the receptionist, who scans it and validates Bob's membership through the smart contract. Bob's membership is valid for 30 days from the first time he went to the gym.

Contract interface

• constructor: function GymMembership(uint16 numPasses)

  • The creator of the contract (msg.sender) is the owner.
  • numPasses is the number of monthly pass tokens issued.
  • Passes are assigned unique identifiers (from 0 to numPasses - 1).

• register: function transfer(uint16 passId, address to)

  • The sender gives ownership of pass passId to the owner of the address to.
  • Succeeds only if passId is a valid pass id and the sender of the transaction (msg.sender) is the current owner of the associated pass. (Note: initially, all passes belong to the contract's owner.)

• validate: function validate(uint16 passId)

  • This transaction succeeds only if passId is a valid pass identifier, the sender of the transaction (msg.sender) is the owner of the associated pass, and the pass has not yet expired.
  • If this is the first time validate is called with the given passId, we set the expiry date of the associated pass to 30 days from the time of the transaction (i.e. now).

Contract skeleton

pragma solidity ^0.4.21;

contract GymMembership {

    // <contract_variables>

    // </contract_variables>

    function GymMembership(uint16 numPasses) public {
        // TODO
    }

    function transfer(uint16 passId, address to) public {
        // TODO
    }

    function validate(uint16 passId) public {
        // TODO
    }

}