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test_assertAndrequire.sol

pragma solidity ^0.5.0;

contract Sharer {
    function sendHalf(address payable addr) public payable returns (uint balance){
        require(msg.value % 2 == 0, "Even value reqired.");
        uint balanceBeforeTransfer = address(this).balance;
        addr.transfer(msg.value / 2);
        // Since transfer throws an exception on failure and
        // cannot call back here, there should be no way for us to
        // still have half of the money.
        assert(address(this).balance == balanceBeforeTransfer - msg.value/2);
        return address(this).balance;
    }
    
    function getBalance() public view returns(uint){
        return address(this).balance;
    }
}

contract VendingMachine {
    function buy(uint amount) public payable {
        if(amount > msg.value / 1 ether)
        revert("Not enough Ether provided");
    // Alternative way to do it:
    require(
        amount <= msg.value / 1 ether,
        "Not enough Ether provided."
        );
        //
    }
}