contract Ledger {
    uint public total;      // Total amount in Ledger

    struct Entry {          // Doubly linked list tracks amount per address
        uint balance;
        address next;
        address prev;
    }
    mapping (address => Entry) public entries;

    address public owner;
    modifier fromOwner() { require(msg.sender==owner); _; }

    // Constructor sets the owner
    constructor(address _owner)
        public
    {
        owner = _owner;
    }


    /******************************************************/
    /*************** OWNER METHODS ************************/
    /******************************************************/

    function add(address _address, uint _amt)
        fromOwner
        public
    {
        if (_address == address(0) || _amt == 0) return;
        Entry storage entry = entries[_address];

        // If new entry, replace first entry with this one.
        if (entry.balance == 0) {
            entry.next = entries[0x0].next;
            entries[entries[0x0].next].prev = _address;
            entries[0x0].next = _address;
        }
        // Update stats.
        total += _amt;
        entry.balance += _amt;
    }

    function subtract(address _address, uint _amt)
        fromOwner
        public
        returns (uint _amtRemoved)
    {
        if (_address == address(0) || _amt == 0) return;
        Entry storage entry = entries[_address];

        uint _maxAmt = entry.balance;
        if (_maxAmt == 0) return;
        
        if (_amt >= _maxAmt) {
            // Subtract the max amount, and delete entry.
            total -= _maxAmt;
            entries[entry.prev].next = entry.next;
            entries[entry.next].prev = entry.prev;
            delete entries[_address];
            return _maxAmt;
        } else {
            // Subtract the amount from entry.
            total -= _amt;
            entry.balance -= _amt;
            return _amt;
        }
    }


    /******************************************************/
    /*************** PUBLIC VIEWS *************************/
    /******************************************************/

    function size()
        public
        view
        returns (uint _size)
    {
        // Loop once to get the total count.
        Entry memory _curEntry = entries[0x0];
        while (_curEntry.next > 0) {
            _curEntry = entries[_curEntry.next];
            _size++;
        }
        return _size;
    }

    function balanceOf(address _address)
        public
        view
        returns (uint _balance)
    {
        return entries[_address].balance;
    }

    function balances()
        public
        view
        returns (address[] _addresses, uint[] _balances)
    {
        // Populate names and addresses
        uint _size = size();
        _addresses = new address[](_size);
        _balances = new uint[](_size);
        uint _i = 0;
        Entry memory _curEntry = entries[0x0];
        while (_curEntry.next > 0) {
            _addresses[_i] = _curEntry.next;
            _balances[_i] = entries[_curEntry.next].balance;
            _curEntry = entries[_curEntry.next];
            _i++;
        }
        return (_addresses, _balances);
    }
}
/**
 This is a simple class that maintains a doubly linked list of
 address => uint amounts. Address balances can be added to 
 or removed from via add() and subtract(). All balances can
 be obtain by calling balances(). If an address has a 0 amount,
 it is removed from the Ledger.
 
 Note: THIS DOES NOT TEST FOR OVERFLOWS, but it's safe to
 use to track Ether balances.
 
 Public methods:
 - [fromOwner] add()
 - [fromOwner] subtract()
 Public views:
 - total()
 - size()
 - balanceOf()
 - balances()
 - entries() [to manually iterate]
 */
function size()
    public
    view
    returns (uint _size)
{
    // Loop once to get the total count.
    Entry memory _curEntry = entries[0x0];
    while (_curEntry.next > 0) {
        _curEntry = entries[_curEntry.next];
        _size++;
    }
    return _size;
}

/******************************************************/