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?
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.
