`ToActorId` function in FilAddresses utils

[lordshashank]

It would be great utility to have a function like ToActorId in our FilAddresses utils which converts filAddress type to actorId if it exists.

[snissn]

Thanks for the suggestion @lordshashank — I can see how a ToActorId helper could be useful, but I’m a bit skeptical we can add it to this repo in a compliant and robust way.

How should it handle malformed input — revert, return a sentinel value, or fail silently? And what should it do if there’s no actor ID yet — is that an error case or just return 0?

Also, how would the lookup actually work? Would it rely on a system call or runtime-level resolution?

Do you have an implementation sketch in mind? That might help clarify what this would really involve.

I’m open to being convinced.. Also it may make sense to initially land your feature idea in a separate utility tool — especially given this repo has already gone through audits. We could eventually bundle it back into here too.

[lordshashank]

@snissn , ig we would need a function decodeUnsignedLeb128FromUInt64 , sort of reverse to [encodeUnsignedLeb128FromUInt64](https://github.com/filecoin-project/filecoin-solidity/blob/c72ec4bf8213f38ca5cd6f553abff5cab0709488/contracts/v0.8/utils/Leb128.sol#L31).
Code would be something like following (very raw implementation)

function toActorID(CommonTypes.FilAddress memory filAddress) external pure returns (uint64) {
    // Make sure the address has data and starts with the protocol ID
    require(filAddress.data.length > 1, "Invalid FilAddress: empty data");
    require(filAddress.data[0] == CommonTypes.PROTOCOL_ID, "Invalid FilAddress: wrong protocol");
    
    // Skip the protocol ID byte and decode the remaining bytes as Leb128
    bytes memory encodedActorID = new bytes(filAddress.data.length - 1);
    for (uint i = 1; i < filAddress.data.length; i++) {
        encodedActorID[i-1] = filAddress.data[i];
    }
    
    return decodeUnsignedLeb128ToUInt64(encodedActorID);
}

/// @notice decode bytes into a unsigned integer 64bits
/// @param data the encoded Leb128 bytes
/// @return value the decoded uint64 value
function decodeUnsignedLeb128ToUInt64(bytes memory data) internal pure returns (uint64 value) {
    uint64 result = 0;
    uint64 shift = 0;
    
    for (uint i = 0; i < data.length; i++) {
        uint64 byte_ = uint64(uint8(data[i]));
        result |= (byte_ & 0x7f) << shift;
        
        if (byte_ & 0x80 == 0) {
            break;
        }
        
        shift += 7;
        require(shift < 64, "Invalid Leb128: overflow");
    }
    
    return result;
}

we can revert with specific error codes for various cases here or even put exit codes, since we use exit codes for actor calls and this is not one of them ig error codes could work.
I guess we can just ignore the case of actor being intialized or not. This is simple leb128 decode, and fromActorId also don't care about this imo as I can put any number there and get filAddress type.
Audit maybe issue, but would it be too hard to ask them to see this one function only.

I am just surprised why this didn't existed already as I myself have been in need of this for a lot of my work and have been doing workaround since like forever which is not nice.