How to increase balance constantly?

Question

I'm trying to create a token that holders can earn some amount contantly by a certain rate. Don't want to depend on transactions to give tokens to holders. An idle contract must constantly do it.

Is there any way to do this without running out of gas ?

Just an example:

• You buy 100 tokens and go sleep. The contract will reward you at 0.0001 token / hour

Yea I know I can't write anything to the constract storage without spend gas and fees so I can't see how I can do it.

Answer 1

An approach is to store the time of the last transaction and when needed calculate the balance based on the time elapsed. Those changes aren't stored in the contract until a transfer modifies the balance.

mapping(address => uint256) _balances;
// Last transaction's timestamp 
mapping(address => uint256) _lastBlock;
function balanceOf(address owner) public view returns (uint256) {
   uint256 balance = _balances[owner];
   if (balance > 0) {
       uint256 elapsed = block.timestamp - _lastBlock[owner];
       balance += rate * elapsed;
   }
   return balance;
}
function transfer(address recipient, uint256 amount) public returns (bool) {
   // Adjust balance
   uint256 balance = _balances[msg.sender];
   if (balance > 0) {
       uint256 elapsed = block.timestamp - _lastBlock[msg.sender];
       balance += rate * elapsed;
   }
[..] // implement transfer
// Update last transaction timestamp
   _lastBlock[msg.sender] = block.timestamp;
   _lastBlock[recipient] = block.timestamp;
}