balance-checker.solidity

/*
  Check the token balances of a wallet for multiple tokens.
  Pass 0x0 as a "token" address to get ETH balance.

  Possible error throws:
    - extremely large arrays for user and or tokens (gas cost too high) 

  Returns a one-dimensional that's user.length * tokens.length long. The
  array is ordered by all of the 0th users token balances, then the 1th
  user, and so on.
*/
function balances(address[] users, address[] tokens) external view returns (uint[]) {
  uint[] memory addrBalances = new uint[](tokens.length * users.length);

  for(uint i = 0; i < users.length; i++) {
    for (uint j = 0; j < tokens.length; j++) {
      uint addrIdx = j + tokens.length * i;
      if (tokens[j] != address(0x0)) { 
        addrBalances[addrIdx] = tokenBalance(users[i], tokens[j]);
      } else {
        addrBalances[addrIdx] = users[i].balance; // ETH balance    
      }
    }  
  }

  return addrBalances;
}