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I'm trying to develop an ERC20 token that mints new tokens to holders over time, however I would like to do this in a way that does not require me to make new transactions each time a users balance increases since this would be prohibitively expensive in terms of gas.

I figured that the balanceOf() function could query something to show an increased balance.

Does anyone have any idea about best practices for this?

  • Can you clarify your scenario? In general, all changes to the state of the ethereum network must be initiated by a transaction. There is no passive or scheduled updating beyond the use of a oracle. – Shawn Tabrizi Jan 8 at 21:52
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What you want to achieve is possible. Consider:

function balanceOf(address _user) public{
    uint256 rawBalance = balances[_user];
    if (rawBalance == 0) {
        return 0;
    }

    uint256 startLevel = compoundedInterestFactor[_user];
    uint256 currentLevel = getInterestRate().getCurrentCompoundingLevel(); // defined in another contract
    return _rawBalance.mul(currentLevel).div(startLevel);
}

Assume that the getCurrentCompoundingLevel returns a number depending on the time. If this number is rising as a function of time, this will correspond to a positive interest rate; if it is falling, it will correspond to a negative interest rate.

Each time you update a balance (send or receive a balance), you need to update both the balances mapping and the compoundedInterestFactor mapping for this user. The compoundedInterestFactor[_owner] must be set to the current level when the transaction occurs.

If a user receives 1000 units when the getInterestRate().getCurrentCompoundingLevel() function call returns 1000, and you want to apply a positive interest rate of 0.1 % per day, the function call must return 1001 the next day. The number returned from balanceOf(_user) will then be 1000 * 1001 / 1000 = 1001 which corresponds to the desired 0.1 % interest rate per day.

This way, you can enforce both a negative and a positive interest rate without looping over all balances to have them updated.

  • This is not equivalent to an interest rate. If I gain 1% every day and start with 10 token, day 2 I have 10.1, day 3 10.201, and so on. Using the simple system you describe day two you have 10.2 (not 10.201). This is a simplification. – Rick Park Jan 9 at 13:27
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    The getCurrentCompoundingLevel() is located in another contract and the rate of change can be defined as you please. You can make a linear "interest rate" going like 1000 -> 1001 -> 1002, or you can make a more precise approximation of exponential interest rate that goes like 1.000.000.000 -> 1.001.000.000. -> 1.002.001.000 -> 1.003.003.001 etc. – Thorkil Værge Jan 9 at 13:34

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