Gist

Contract address - 0xfbf9ae92ecb05bc436d9b1e6c1d928824aedb2f1 - Self destructed

This contract allows participants to guess when they believe the price of ether (ETH) will next reach 500 USD. The contract is loaded with 0.5 ETH (though anyone can add more), and there are two possibilities to receive these funds.

1. 10% of balance: The participant to first call claimCheckPriceReward() once the oracle has set priceConfirmedOver500 to true.
2. 90% of balance: The participant whose timestamp guess is closest to when ETH hits the 500 USD mark according to the Kraken API.

So bottomline, if you want to participate, call makeGuess with your UNIX timestamp, and if you think your guess is strong relative to when we hit 500, call nominateSelfAsWinner with your address. If you want to win the 10% prize, keep an eye on this API call (specifically, the first value of the c array) as we approach 500 and call checkPrice followed by claimCheckPriceReward.

Functionality and Contract Logic

1. When making a guess, participants include a UNIX timestamp. msg.sender is the address that is making the guess and would receive the ETH.

• Guessing period is open for 14 days following contract creation.
• One guess per address, and participant can't change their guess once made. This is done to prevent people from changing their guess if price starts approaching 500USD during the 14 day guessing period.

2. When checkPrice is called, and assuming the price is greater than 500 USD based on Kraken's last trade between the ETHUSD pairing on their API, the oracle callback function sets priceConfirmedOver500 to true and winningTimestamp is set to the current block timestamp.

• To call checkPrice, msg.value of the transaction must be >= oraclePrice (the return value of oraclize_getPrice("URL"), currently about .004 ETH). This is because the Oraclize callback requires a fee, which is taken out of the contract balance. First call is free.
• winningTimestamp is determined by the blockchain and not the instant price breaks 500 on Kraken. Thus, all functions (except selfdestruct) can be called by anyone.
• As an incentive mechanism to call checkPrice as close as possible to when we actually pass 500USD, the first person to call claimCheckPriceReward AFTER priceConfirmedOver500 is set to true by the oracle is rewarded 10% of the contract's current totalAmount. To be clear, the person who gets the reward is not the person who called checkPrice but the person who called claimCheckPriceReward after priceConfirmedOver500 is set to true by the oracle. The reason for this is because there are possible gas errors by having the oracle __callback call other functions that change state.

3. Now the nomination period begins and lasts for seven days. Instead of iterating over the myriad guesses that may exist, creating possible gas issues, people who think they have a chance of winning nominate themselves, creating a smaller subset of possible winners who effectively compete for the lowest difference between their guess and winningTimestamp.

• The nomination period lasts for seven days following when winningTimestamp is established by the oracle. payout can be called by anyone following this seven day period, and the address who got closest to winningTimestamp takes all that remains in totalAmount. It doesn't matter if you guessed over or below winningTimestamp, it's all about who is closest.
• Note, the contract does not account for the unlikely scenario where two or more addresses nominate themselves with the same lowestDiff relative to winningTimestamp (i.e., there will not be multiple winners or a tie). I experimented a little with a few ways of checking for/implementing this and just let it be. In this unique situation, the contract logic is such that the last address nominated during the nomination period would be set as winner.

4. Self Destruct - I have included an escape hatch that the owner can call in the event that there is some unforeseen logic error or if Kraken's API changes. The address hard-coded in the contract is for the nonprofit Give Directly (proof), who would receive the contract balance if this function is called.

How to deploy

• Remix with Metamask. Copy the contract into a new file, compile with Solidity 0.4.24, then go under the Run tab paste in the contract address where it says at Address. This should allow you to interact with the various functions
• MyEtherWallet: Copy the abi.json file in this repo and paste in the contract address
• Etherscan: Because the contract is verified, there is functionality in the block explorer that allow people to read and write to the contract using Metamask.
• There are other ways, to include Mist and the Geth command line. Submit a PR to include your preferred method!

Test contracts

• Rinkeby test
• Main net test
• There are many others self-destructed on Rinkeby

These contracts evolved during the testing period, and differed (in terms of the variables) only by setting the threshold lower for ETHUSD price (relative to current market price) and eliminating the amount of time to elapse to call certain functions.

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Contributing to the project

This is an open source project. Contributions are welcomed & encouraged! 😄

TODO

• Allow for multiple winners

References

• Oracalize
• Kraken API
• UNIX Timestamp converter
• Compiled with Solidity 0.4.24