stebansaa / prediction-market

a solidity contract for a prediction market on EVMs

Prediction Market Contract

This Solidity contract represents a prediction market where participants can buy shares for different outcomes and potentially earn profits based on the resolved outcome.

Contract Details

• Contract Name: PredictionMarket
• SPDX License Identifier: MIT
• Solidity Version: 0.8.0

Contract Structure

The contract consists of the following components:

Enum

• Outcome: An enumeration representing the possible outcomes of the prediction market. It includes the following values:
  • NotSet: Default value indicating that the outcome is not set.
  • OutcomeOne: The first outcome.
  • OutcomeTwo: The second outcome.

Struct

• Market: A structure containing the information about the prediction market. It includes the following fields:
  • totalShares: The total number of shares available in the market.
  • sharesSold: The total number of shares sold in the market.
  • outcomeOneShares: The number of shares bought for outcome one.
  • outcomeTwoShares: The number of shares bought for outcome two.
  • closed: A flag indicating if the market is closed for trading.
  • resolved: A flag indicating if the market outcome is resolved.
  • outcome: The resolved outcome of the market, represented using the Outcome enum.

Storage Variables

• balances: A mapping that stores the balances of participants. It maps an address to the corresponding balance.
• shares: A nested mapping that stores the shares bought by participants. It maps an address to a mapping of Outcome to the number of shares bought.
• market: A public variable representing the current state of the market, including the totalShares and other market details.

Events

The contract emits the following events:

• MarketCreated: Triggered when the market is created, and it includes the totalShares value as an argument.
• MarketClosed: Triggered when the market is closed for trading.
• MarketResolved: Triggered when the market outcome is resolved, and it includes the outcome value as an argument.

Modifiers

The contract includes the following modifiers:

• onlyOracle: Ensures that only the trusted oracle can call the function using this modifier.
• marketOpen: Checks that the market is open for trading and not yet resolved.
• marketClosed: Checks that the market is closed for trading.
• marketResolved: Checks that the market outcome is resolved.

Constructor

• constructor(uint256 _totalShares): Initializes the contract by setting the totalShares value provided as an argument and emits the MarketCreated event.

Functions

The contract includes the following functions:

• buyShares(Outcome _outcome, uint256 _amount): Allows participants to buy shares for a specific outcome. It takes the outcome and the amount of shares to buy as arguments. Participants must send the correct amount of Ether as payment.
• closeMarket(): Closes the market for trading if all shares are sold.
• resolveMarket(Outcome _outcome): Resolves the market outcome by the trusted oracle. Only the oracle can call this function and provide the resolved outcome as an argument.
• withdraw(): Allows participants to withdraw their earnings after the market outcome is resolved.

Usage

To use the contract, follow these steps:

1. Deploy the contract to a compatible Ethereum blockchain network using a suitable development environment or a blockchain platform.
2. Pass the desired totalShares value to the constructor during deployment to set the total number of shares available in the market.
3. Use the appropriate account or contract to interact with the contract functions:
   • Call the buyShares function to buy shares for a specific outcome by providing the outcome (Outcome.OutcomeOne or Outcome.OutcomeTwo) and the amount of shares to buy. Make sure to include the correct Ether value matching the cost of the shares.
   • If necessary, call the closeMarket function to close the market for trading.
   • Use the trusted oracle account to call the resolveMarket function and provide the resolved outcome as an argument.
   • After the market outcome is resolved, participants can call the withdraw function to withdraw their earnings based on the outcome.

License

This contract is released under the MIT License.