function placeBet(uint betMask, uint modulo, uint commitLastBlock, uint commit, bytes32 r, bytes32 s) external payable {

    Bet storage bet = bets[commit];

    require (bet.gambler == address(0), "Bet should be in a 'clean' state.");

    // Validate input data ranges.

    uint amount = msg.value;

    require (modulo > 1 && modulo <= MAX_MODULO, "Modulo should be within range.");

    require (amount >= MIN_BET && amount <= MAX_AMOUNT, "Amount should be within range.");

    require (betMask > 0 && betMask < MAX_BET_MASK, "Mask should be within range.");

    // Check that commit is valid - it has not expired and its signature is valid.

    require (block.number <= commitLastBlock, "Commit has expired.");

    bytes32 signatureHash = keccak256(abi.encodePacked(uint40(commitLastBlock), commit));

    require (secretSigner == ecrecover(signatureHash, 27, r, s), "ECDSA signature is not valid.");

    uint rollUnder;

    uint mask;

    if (modulo <= MAX_MASK_MODULO) {

        // Small modulo games specify bet outcomes via bit mask.

        // rollUnder is a number of 1 bits in this mask (population count).

        // This magic looking formula is an efficient way to compute population

        // count on EVM for numbers below 2**40. For detailed proof consult

        // the dice2.win whitepaper.

        rollUnder = ((betMask * POPCNT_MULT) & POPCNT_MASK) % POPCNT_MODULO;

        mask = betMask;

    } else {

        // Larger modulos specify the right edge of half-open interval of

        // winning bet outcomes.

        require (betMask > 0 && betMask <= modulo, "High modulo range, betMask larger than modulo.");

        rollUnder = betMask;

    }

    // Winning amount and jackpot increase.

    uint possibleWinAmount;

    uint jackpotFee;

    (possibleWinAmount, jackpotFee) = getDiceWinAmount(amount, modulo, rollUnder);

    // Enforce max profit limit.

    require (possibleWinAmount <= amount + maxProfit, "maxProfit limit violation.");

    // Lock funds.

    lockedInBets += uint128(possibleWinAmount);

    jackpotSize += uint128(jackpotFee);

    // Check whether contract has enough funds to process this bet.

    require (jackpotSize + lockedInBets <= address(this).balance, "Cannot afford to lose this bet.");

    // Record commit in logs.

    emit Commit(commit);

    // Store bet parameters on blockchain.

    bet.amount = amount;

    bet.modulo = uint8(modulo);

    bet.rollUnder = uint8(rollUnder);

    bet.placeBlockNumber = uint40(block.number);

    bet.mask = uint40(mask);

    bet.gambler = msg.sender;

}

function settleBet(uint reveal, bytes32 blockHash) external onlyCroupier {

    uint commit = uint(keccak256(abi.encodePacked(reveal)));

    Bet storage bet = bets[commit];

    uint placeBlockNumber = bet.placeBlockNumber;

    // Check that bet has not expired yet (see comment to BET_EXPIRATION_BLOCKS).

    require (block.number > placeBlockNumber, "settleBet in the same block as placeBet, or before.");

    require (block.number <= placeBlockNumber + BET_EXPIRATION_BLOCKS, "Blockhash can't be queried by EVM.");

    require (blockhash(placeBlockNumber) == blockHash);

    // Settle bet using reveal and blockHash as entropy sources.

    settleBetCommon(bet, reveal, blockHash);

}