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UserManager.sol

pragma solidity ^0.4.11;

contract UserManager {
    address                     public owner;
    uint                        public numUsers = 0;
    mapping(address => address) private linkedUsers;

    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }
    
    function UserManager () {
        owner = msg.sender;
    }
    
    function noUsers () constant private returns(bool) {
        return linkedUsers[owner] == 0;
    }
    
    function userExists (address searchUser) constant public returns(bool) {
        if (searchUser != 0) {
            bool foundUser = false;
            address currUser = owner;
            while (!foundUser && currUser != 0) {
                currUser = linkedUsers[currUser];
                foundUser = currUser == searchUser;
            }
            return foundUser;
        } else {
            return false;
        }
    }
    
    function getUsers () constant public returns(address[]) {
        // it is not possible to resize memory arrays
        address[] memory users = new address[](numUsers);
        address currUser = linkedUsers[owner];
        for (uint i = 0; i < numUsers; i++) {
            users[i] = currUser;
            currUser = linkedUsers[currUser];
        }
        return users;
    }
    
    function _addUser (address newUser) private {
        address lastUser = linkedUsers[owner];
        while (linkedUsers[lastUser] != newUser) {
            if (linkedUsers[lastUser] == 0) {
                linkedUsers[lastUser] = newUser;
            } else {
                lastUser = linkedUsers[lastUser];
            }
        }
    }
    
    function addUser (address newUser) onlyOwner {
        require(newUser != owner);
        require(!userExists(newUser));
        if (noUsers()) {
            linkedUsers[owner] = newUser;
        } else {
            _addUser(newUser);
        }
        numUsers += 1;
    }
    
    function _removeUser (address oldUser) private {
        bool userRemoved;
        address lastUser = owner;
        while (!userRemoved) {
            if (linkedUsers[lastUser] == oldUser) {
                linkedUsers[lastUser] = linkedUsers[oldUser];
                userRemoved = true;
            } else {
                lastUser = linkedUsers[lastUser];
            }
        }
    }
    
    function removeUser (address oldUser) onlyOwner {
        require(userExists(oldUser));
        _removeUser(oldUser);
        numUsers -= 1;
    }
}