pragma solidity ^0.4.11;

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
  function mul(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal constant returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  function sub(uint256 a, uint256 b) internal constant returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  function add(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}


/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
  uint256 public totalSupply;
  function balanceOf(address who) constant returns (uint256);
  function transfer(address to, uint256 value) returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);
}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances. 
 */
contract BasicToken is ERC20Basic {
  using SafeMath for uint256;

  mapping(address => uint256) balances;

  /**
  * @dev transfer token for a specified address
  * @param _to The address to transfer to.
  * @param _value The amount to be transferred.
  */
  function transfer(address _to, uint256 _value) returns (bool) {
    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    Transfer(msg.sender, _to, _value);
    return true;
  }

  /**
  * @dev Gets the balance of the specified address.
  * @param _owner The address to query the the balance of. 
  * @return An uint256 representing the amount owned by the passed address.
  */
  function balanceOf(address _owner) constant returns (uint256 balance) {
    return balances[_owner];
  }

}


/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
  function allowance(address owner, address spender) constant returns (uint256);
  function transferFrom(address from, address to, uint256 value) returns (bool);
  function approve(address spender, uint256 value) returns (bool);
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is ERC20, BasicToken {

  mapping (address => mapping (address => uint256)) allowed;


  /**
   * @dev Transfer tokens from one address to another
   * @param _from address The address which you want to send tokens from
   * @param _to address The address which you want to transfer to
   * @param _value uint256 the amout of tokens to be transfered
   */
  function transferFrom(address _from, address _to, uint256 _value) returns (bool) {
    var _allowance = allowed[_from][msg.sender];

    // Check is not needed because sub(_allowance, _value) will already throw if this condition is not met
    // require (_value <= _allowance);

    balances[_to] = balances[_to].add(_value);
    balances[_from] = balances[_from].sub(_value);
    allowed[_from][msg.sender] = _allowance.sub(_value);
    Transfer(_from, _to, _value);
    return true;
  }

  /**
   * @dev Aprove the passed address to spend the specified amount of tokens on behalf of msg.sender.
   * @param _spender The address which will spend the funds.
   * @param _value The amount of tokens to be spent.
   */
  function approve(address _spender, uint256 _value) returns (bool) {

    // To change the approve amount you first have to reduce the addresses`
    //  allowance to zero by calling `approve(_spender, 0)` if it is not
    //  already 0 to mitigate the race condition described here:
    //  https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
    require((_value == 0) || (allowed[msg.sender][_spender] == 0));

    allowed[msg.sender][_spender] = _value;
    Approval(msg.sender, _spender, _value);
    return true;
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param _owner address The address which owns the funds.
   * @param _spender address The address which will spend the funds.
   * @return A uint256 specifing the amount of tokens still avaible for the spender.
   */
  function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
  }
}


// Migration Agent interface
contract MigrationAgent {
    function migrateFrom(address _from, uint _value);
}

contract GVToken is StandardToken {
    
    // Constants
    string public constant name = "Genesis Vision Token";
    string public constant symbol = "GVT";
    uint   public constant decimals = 18;
    uint   constant TOKEN_LIMIT = 44 * 1e6 * 1e18; 
    
    address public ico;

    // GVT transfers are blocked until ICO is finished.
    bool public isFrozen = true;

    // Token migration variables
    address public migrationMaster;
    address public migrationAgent;
    uint public totalMigrated;

    event Migrate(address indexed _from, address indexed _to, uint _value);

    // Constructor
    function GVToken(address _ico, address _migrationMaster) {
        require(_ico != 0);
        require(_migrationMaster != 0);
        ico = _ico;
        migrationMaster = _migrationMaster;
    }

    // Create tokens
    function mint(address holder, uint value) {
        require(msg.sender == ico);
        require(value > 0);
        require(totalSupply + value <= TOKEN_LIMIT);

        balances[holder] += value;
        totalSupply += value;
        Transfer(0x0, holder, value);
    }

    // Allow token transfer.
    function unfreeze() {
        require(msg.sender == ico);
        isFrozen = false;
    }

    // ERC20 functions
    // =========================

    function transfer(address _to, uint _value) public returns (bool) {
        require(_to != address(0));
        require(!isFrozen);
        return super.transfer(_to, _value);
    }

    function transferFrom(address _from, address _to, uint _value) public returns (bool) {
        require(!isFrozen);
        return super.transferFrom(_from, _to, _value);
    }

    function approve(address _spender, uint _value) public returns (bool) {
        require(!isFrozen);
        return super.approve(_spender, _value);
    }

    // Token migration
    function migrate(uint value) external {
        require(migrationAgent != 0);
        require(value > 0);
        require(value <= balances[msg.sender]);

        balances[msg.sender] -= value;
        totalSupply -= value;
        totalMigrated += value;
        MigrationAgent(migrationAgent).migrateFrom(msg.sender, value);
        Migrate(msg.sender, migrationAgent, value);
    }

    // Set address of migration contract
    function setMigrationAgent(address _agent) external {
        require(migrationAgent == 0);
        require(msg.sender == migrationMaster);
        migrationAgent = _agent;
    }

    function setMigrationMaster(address _master) external {
        require(msg.sender == migrationMaster);
        require(_master != 0);
        migrationMaster = _master;
    }
}