Arithmetic Error risk in VotingEscrow::_find_block_epoch() function

Vulnerability Detail

The problem here is that integer division truncates the result towards zero. In the binary search algorithm, it's essential to calculate the midpoint (_mid) by rounding up, not down, to ensure that the algorithm converges correctly. This error can lead to inaccurate estimates of the timestamp for a given block number. Specifically, the binary search algorithm used in this function is designed to find the closest epoch in the point_history array based on the provided block number. If there are arithmetic errors in the calculations, it can result in the function returning an incorrect epoch value.

1002            uint _mid = (_min + _max + 1) / 2;

Impact

Incorrect epoch value is returned.

Code Snippet

https://github.com/sherlock-audit/2024-06-velocimeter/blob/main/v4-contracts/contracts/VotingEscrow.sol#L1002

POC : Ref : https://ethereum.stackexchange.com/questions/7169/binary-search-in-solidity-arrays

Tool used

Manual Review

Recommendation

This modification ensures that the midpoint _mid is calculated correctly by rounding up, which is crucial for the binary search algorithm to work accurately. modify it like this :
```
1002            uint _mid = (_min + _max) / 2 + 1; 
```

Duplicate of #339

sherlock-audit / 2024-06-velocimeter-judging