Venmo

pragma solidity ^0.4.23;


contract Venmo {

  // charge(address _friend, uint256 _amount)
  // pay(uint256 _chargeId)
  // decline(uint256 _chargeId)
  // withdraw()
  // deposit()

  struct Charge {
    address initiator;
    address friend;
    uint amount;
    bool declined;
    bool paid;
  }

  // chargeId => amount
  mapping(address => uint) public balances;

  mapping(uint => Charge) public charges;

  uint public lastChargeId;

  function charge(address _friend, uint _amount) public {
    require(_friend != 0);
    require(_amount != 0);
    charges[lastChargeId] = Charge({
      friend: _friend,
      amount: _amount,
      declined: false,
      paid: false,
      initiator: msg.sender
    });

    lastChargeId += 1;
  }

  function decline(uint _chargeId) public {
    require(charges[_chargeId].friend == msg.sender);
    require(!charges[_chargeId].declined);
    require(!charges[_chargeId].paid);
    charges[_chargeId].declined = true;
  }

  function pay(uint _chargeId) public payable {
    Charge currentCharge = charges[_chargeId];
    require(currentCharge.friend == msg.sender);
    require(!currentCharge.declined);
    require(!currentCharge.paid);
    require(balances[msg.sender] + msg.value >= currentCharge.amount);

    uint amountDue = currentCharge.amount;
    if (msg.value > amountDue) {
      balances[msg.sender] += (msg.value - amountDue);
    } else if (msg.value < amountDue) {
      amountDue -= msg.value;
      balances[msg.sender] -= amountDue;
    }

    currentCharge.paid = true;
    balances[currentCharge.initiator] += currentCharge.amount;
  }

  function() public payable {
    deposit();
  }

  function deposit() public payable {
    balances[msg.sender] += msg.value;
  }

  function withdraw(uint _amount) public {
    require(balances[msg.sender] >= _amount);
    balances[msg.sender] -= _amount;
    require(msg.sender.call.value(_amount)());
  }


}