HW21: Ticket sale (*)

Problem statement

Implement a smart contract for selling tickets to events.

• The creator of the contract is the owner.
• The owner specifies the number and price of the tickets.
• People can buy tickets directly from the contract.
• Anyone can validate whether a given address owns a ticket.
• One person can only buy one ticket.
• People can sell their tickets through the contract. For this, they need to submit an offer stating the price they're willing to sell for. This price must be within +-20% of the original price (i.e. at least 80%, but at most 120%). Then, when someone accepts the offer, they pay the required amount, which is forwarded to the seller, and the buyer gets the ticket.
• The owner can withdraw any profits.

Example scenario

The A38 concert venue organizes a concert. For this, they create an instance of this contract for 200 tickets, setting the price at the wei equivalent of 2000HUF.

Alice buys a ticket through the contract for the wei equivalent of 2000HUF, her ticket's id is 72. Unfortunately, she finds out she cannot attend, so she decides to sell the ticket. She submits a sale-offer with the wei equivalent of 2400HUF as the price. Bob really wants to attend the concert, but all the tickets are sold already, so he decides to accept Alice's offer. Bob transfers the wei equivalent of 2400HUF to the contract, which is forwarded to Alice, and from this point on, Bob owns the ticket.

On the day of the event, the A38 staff scan Bob's QR code (representing his Ethereum address) and check if Bob has a ticket.

(Note: This is a good way to control the secondary market of tickets. Using this system, the only way to illegally sell tickets is to sell the associated Ethereum private keys.)

Contract interface

• constructor: function TicketSale(uint16 numTickets, uint256 basePrice)

  • Creates a ticket sale with numTickets tickets, sold for basePrice.
  • The sender of the transaction (msg.sender) is the owner.

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

  • Returns true if person has a ticket, false otherwise.

• buy ticket: function buyTicket() payable

  • The sender of the transaction (msg.sender) buys one ticket for the base price.
  • Succeeds only if there are tickets left, the sender has not bought a ticket yet, and he/she sends the correct amount of ether.
  • New tickets are assigned a unique identifier (uint16).

• get ticket id: function getTicketOf(address person) view returns (uint16)

  • Returns the ticket id associated with the address person.
  • Should succeed only if person has a ticket.

• submit offer: function submitOffer(uint16 ticketId, uint256 price)

  • The sender of the transaction (msg.sender) submits an offer for selling their ticket (ticketId) for price wei.
  • The sender must own ticketId.
  • The proposed price must be in the 80-120% range of the original price.

• accept offer: function acceptOffer(uint16 ticketId) payable

  • The sender of the transaction (the buyer) accepts the sale offer for the ticket ticketId.
  • The buyer must send the correct amount of ether, which is transferred to the seller.
  • After the transaction, ticketId belongs to the buyer, and the offer is destroyed.

• withdraw: function withdraw()

  • Withdraw any funds on the contract.
  • Only the owner can call this method.

Contract skeleton

pragma solidity ^0.4.21;

contract TicketSale {

    // <contract_variables>

    // </contract_variables>

    function TicketSale(uint16 numTickets, uint256 basePrice) public {
        // TODO
    }

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

    function buyTicket() public payable {
        // TODO
    }

    function getTicketOf(address person) public view returns (uint16) {
        // TODO
    }

    function submitOffer(uint16 ticketId, uint256 price) public {
        // TODO
    }

    function acceptOffer(uint16 ticketId) public payable {
        // TODO
    }

    function withdraw() public {
        // TODO
    }

}