contract VestedToken is StandardToken, LimitedTransferToken {

    uint256 MAX_GRANTS_PER_ADDRESS = 20;

    struct TokenGrant {
        address granter;     // 20 bytes
        uint256 value;       // 32 bytes
        uint64 cliff;
        uint64 vesting;
        uint64 start;        // 3 * 8 = 24 bytes
        bool revokable;
        bool burnsOnRevoke;  // 2 * 1 = 2 bits? or 2 bytes?
    } // total 78 bytes = 3 sstore per operation (32 per sstore)

    mapping (address => TokenGrant[]) public grants;

    event NewTokenGrant(address indexed from, address indexed to, uint256 value, uint256 grantId);

    /**
     * @dev Grant tokens to a specified address
     * @param _to address The address which the tokens will be granted to.
     * @param _value uint256 The amount of tokens to be granted.
     * @param _start uint64 Time of the beginning of the grant.
     * @param _cliff uint64 Time of the cliff period.
     * @param _vesting uint64 The vesting period.
     */
    function grantVestedTokens(
        address _to,
        uint256 _value,
        uint64 _start,
        uint64 _cliff,
        uint64 _vesting,
        bool _revokable,
        bool _burnsOnRevoke
    ) public {

        // Check for date inconsistencies that may cause unexpected behavior
        require(_cliff >= _start && _vesting >= _cliff);

        require(tokenGrantsCount(_to) < MAX_GRANTS_PER_ADDRESS);   // To prevent a user being spammed and have his balance locked (out of gas attack when calculating vesting).

        uint256 count = grants[_to].push(
            TokenGrant(
                _revokable ? msg.sender : 0, // avoid storing an extra 20 bytes when it is non-revokable
                _value,
                _cliff,
                _vesting,
                _start,
                _revokable,
                _burnsOnRevoke
            )
        );

        transfer(_to, _value);

        NewTokenGrant(msg.sender, _to, _value, count - 1);
    }

    /**
     * @dev Revoke the grant of tokens of a specifed address.
     * @param _holder The address which will have its tokens revoked.
     * @param _grantId The id of the token grant.
     */
    function revokeTokenGrant(address _holder, uint256 _grantId) public {
        TokenGrant storage grant = grants[_holder][_grantId];

        require(grant.revokable);
        require(grant.granter == msg.sender); // Only granter can revoke it

        address receiver = grant.burnsOnRevoke ? 0xdead : msg.sender;

        uint256 nonVested = nonVestedTokens(grant, uint64(now));

        // remove grant from array
        delete grants[_holder][_grantId];
        grants[_holder][_grantId] = grants[_holder][grants[_holder].length.sub(1)];
        grants[_holder].length -= 1;

        balances[receiver] = balances[receiver].add(nonVested);
        balances[_holder] = balances[_holder].sub(nonVested);

        Transfer(_holder, receiver, nonVested);
    }


    /**
     * @dev Calculate the total amount of transferable tokens of a holder at a given time
     * @param holder address The address of the holder
     * @param time uint64 The specific time.
     * @return An uint256 representing a holder's total amount of transferable tokens.
     */
    function transferableTokens(address holder, uint64 time) public constant returns (uint256) {
        uint256 grantIndex = tokenGrantsCount(holder);

        if (grantIndex == 0) return super.transferableTokens(holder, time); // shortcut for holder without grants

        // Iterate through all the grants the holder has, and add all non-vested tokens
        uint256 nonVested = 0;
        for (uint256 i = 0; i < grantIndex; i++) {
            nonVested = SafeMath.add(nonVested, nonVestedTokens(grants[holder][i], time));
        }

        // Balance - totalNonVested is the amount of tokens a holder can transfer at any given time
        uint256 vestedTransferable = SafeMath.sub(balanceOf(holder), nonVested);

        // Return the minimum of how many vested can transfer and other value
        // in case there are other limiting transferability factors (default is balanceOf)
        return Math.min256(vestedTransferable, super.transferableTokens(holder, time));
    }

    /**
     * @dev Check the amount of grants that an address has.
     * @param _holder The holder of the grants.
     * @return A uint256 representing the total amount of grants.
     */
    function tokenGrantsCount(address _holder) public constant returns (uint256 index) {
        return grants[_holder].length;
    }

    /**
     * @dev Calculate amount of vested tokens at a specific time
     * @param tokens uint256 The amount of tokens granted
     * @param time uint64 The time to be checked
     * @param start uint64 The time representing the beginning of the grant
     * @param cliff uint64  The cliff period, the period before nothing can be paid out
     * @param vesting uint64 The vesting period
     * @return An uint256 representing the amount of vested tokens of a specific grant
     *  transferableTokens
     *   |                         _/--------   vestedTokens rect
     *   |                       _/
     *   |                     _/
     *   |                   _/
     *   |                 _/
     *   |                /
     *   |              .|
     *   |            .  |
     *   |          .    |
     *   |        .      |
     *   |      .        |
     *   |    .          |
     *   +===+===========+---------+----------> time
     *      Start       Cliff    Vesting
     */
    function calculateVestedTokens(
        uint256 tokens,
        uint256 time,
        uint256 start,
        uint256 cliff,
        uint256 vesting) public pure returns (uint256)
    {
        // Shortcuts for before cliff and after vesting cases.
        if (time < cliff) return 0;
        if (time >= vesting) return tokens;

        // Interpolate all vested tokens.
        // As before cliff the shortcut returns 0, we can use just calculate a value
        // in the vesting rect (as shown in above's figure)

        // vestedTokens = (tokens * (time - start)) / (vesting - start)
        uint256 vestedTokens = SafeMath.div(
            SafeMath.mul(
                tokens,
                SafeMath.sub(time, start)
            ),
            SafeMath.sub(vesting, start)
        );

        return vestedTokens;
    }

    /**
     * @dev Get all information about a specific grant.
     * @param _holder The address which will have its tokens revoked.
     * @param _grantId The id of the token grant.
     * @return Returns all the values that represent a TokenGrant(address, value, start, cliff,
     * revokability, burnsOnRevoke, and vesting) plus the vested value at the current time.
     */
    function tokenGrant(address _holder, uint256 _grantId) public constant returns (address granter, uint256 value, uint256 vested, uint64 start, uint64 cliff, uint64 vesting, bool revokable, bool burnsOnRevoke) {
        TokenGrant storage grant = grants[_holder][_grantId];

        granter = grant.granter;
        value = grant.value;
        start = grant.start;
        cliff = grant.cliff;
        vesting = grant.vesting;
        revokable = grant.revokable;
        burnsOnRevoke = grant.burnsOnRevoke;

        vested = vestedTokens(grant, uint64(now));
    }

    /**
     * @dev Get the amount of vested tokens at a specific time.
     * @param grant TokenGrant The grant to be checked.
     * @param time The time to be checked
     * @return An uint256 representing the amount of vested tokens of a specific grant at a specific time.
     */
    function vestedTokens(TokenGrant grant, uint64 time) private constant returns (uint256) {
        return calculateVestedTokens(
            grant.value,
            uint256(time),
            uint256(grant.start),
            uint256(grant.cliff),
            uint256(grant.vesting)
        );
    }

    /**
     * @dev Calculate the amount of non vested tokens at a specific time.
     * @param grant TokenGrant The grant to be checked.
     * @param time uint64 The time to be checked
     * @return An uint256 representing the amount of non vested tokens of a specific grant on the
     * passed time frame.
     */
    function nonVestedTokens(TokenGrant grant, uint64 time) private constant returns (uint256) {
        return grant.value.sub(vestedTokens(grant, time));
    }

    /**
     * @dev Calculate the date when the holder can transfer all its tokens
     * @param holder address The address of the holder
     * @return An uint256 representing the date of the last transferable tokens.
     */
    function lastTokenIsTransferableDate(address holder) public constant returns (uint64 date) {
        date = uint64(now);
        uint256 grantIndex = grants[holder].length;
        for (uint256 i = 0; i < grantIndex; i++) {
            date = Math.max64(grants[holder][i].vesting, date);
        }
    }
}
function nonVestedTokens(TokenGrant grant, uint64 time) private constant returns (uint256) {
        return grant.value.sub(vestedTokens(grant, time));
    }
/**
 * @dev Calculate the amount of non vested tokens at a specific time.
 * @param grant TokenGrant The grant to be checked.
 * @param time uint64 The time to be checked
 * @return An uint256 representing the amount of non vested tokens of a specific grant on the
 * passed time frame.
 */