Negative value from BigInt is wrong

[timmornYE]

Description

Negative return value from smart contract results in a wrong number in dart. Checking the same in web3.js results in the correct value. Positive value gets printed correct.

Example

Minimal contract example

pragma solidity >=0.4.24 <0.7.0;
contract Credit {
    int32 public credits;
    constructor() public {
    }
    function getSupply() public view returns (int32) {
	    return credit;
    }
    function chargeCredit(int32 amount) public returns (bool success) {
        credit += amount;
	    return true;
    }

}

Minimal Dart code example

import 'package:http/http.dart'; //You can also import the browser version
import 'package:web3dart/web3dart.dart';
void main() {
  final functionChargeCredit = contract.function('chargeCredit');
  final functionGetSupply = contract.function('getSupply');
  await ethClient.sendTransaction(credentials, Transaction.callContract(
      contract: contract, 
      function: functionChargeCredit, 
      gasPrice: EtherAmount.inWei(BigInt.one),
      maxGas: 100000,
      parameters: [new BigInt.from(-200)]),); 
  
  voucherBalance = await ethClient.call(
      contract: contract, function: functionGetSupply, params: []); 
  BigInt balanceBigInt = voucherBalance.first;
  print( 'Credit: ${balanceBigInt.toString()} ');
}

Minimal web3.js minimal example

        // [...]
        dec = (await instance.getSupply()).toNumber();
        console.log(dec);

Result

Expected output (= web3.js output): -200
Dart output: 115792089237316195423570985008687907853269984665640564039457584007908834672440

[timmornYE]

I checked... invert binary representation of 115792089237316195423570985008687907853269984665640564039457584007908834672440, convert most left bit which is 1 to 0 and add 1 --> you get 200. This means, that dart does not recognize that the binary representation of this number is a negative in two's complement.

[timmornYE]

Ok, solved... the problem is, that i return a 32 bit integer from smart contract. So it is not recognized by dart that this is a negative integer, because the returned most left bit is not the most left in the BigInt. This way it can be solved:

  // [...]
  BigInt balanceBigInt = voucherBalance.first;
  print( 'Credit: ${balanceBigInt.toSigned(32)} ');
}

If this behavior is expected I can not decide. web3.js does recognize it without explicit conversion.

[simolus3]

This is definitely a bug in web3dart. It should report "-200" as well, without any explicit conversion.
I actually had a check for this, but it was buggy and didn't respect sign-extension padding (32 bit ints are encoded as a 256 bit block). I only tested -1 where it didn't make any difference 🤦‍♂️

This will be fixed in 1.2.2, which I'll release after the CI passes. Thanks for the report!

[simolus3]

Just released version 1.2.2 to pub, which contains this fix.