PseudonymPairs.sol

contract PseudonymPairs {

    mapping(address => uint) pseudonymID;
    mapping(uint => address) pseudonymIndex;

    struct PairUtility {
        mapping(uint => uint) index;
        uint counter;
    }
    PairUtility[2] pairingUtility;
    mapping(uint => uint) pairLookup;
    
    // sortingHat() is invoked after ring signature key transfer is complete, and will cost you a tiny amount of GAS
    // can be invoked continously at any time from the end of the previous event, up until the next event, and forms
    // complete pairs regardless of wether or not every single person "commits" to the next event
    
    function sortingHat() internal {
        uint8 idx;
        uint totalSorted = pairingUtility[0].counter + pairingUtility[1].counter;
    
        // every other person is sorted in pairingUtility[1], instead of pairingUtility[0], 
        // the two combined form a complete list of pairs when the event begins
    
        idx = totalSorted % 2;
        totalSorted++;
        pseudonymID[msg.sender] = totalSorted;
        pseudonymIndex[totalSorted] = msg.sender;

        // the pairingUtility counts how many have been sorted in the pairingUtility you are
        // assigned to, and then randomly places you in the position another person had, moving
        // that other person to the end of the list. this shuffles the pairs with each person sorted
    
        pairingUtility[idx].counter++;
        uint counter = pairingUtility[idx].counter;
        uint randomNumber = getRandomNumber() % counter;
        uint personShuffled = pairingUtility[idx].index[randomNumber];
        pairingUtility[idx].index[randomNumber] = pseudonymID[msg.sender];
        pairLookup[totalSorted] = randomNumber;
        pairingUtility[idx].index[counter] = personShuffled;
        pairLookup[personShuffled] = counter;
    }   
}