Contract Source Code:

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.7.0) (utils/Address.sol)

pragma solidity ^0.8.1;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     *
     * [IMPORTANT]
     * ====
     * You shouldn't rely on `isContract` to protect against flash loan attacks!
     *
     * Preventing calls from contracts is highly discouraged. It breaks composability, breaks support for smart wallets
     * like Gnosis Safe, and does not provide security since it can be circumvented by calling from a contract
     * constructor.
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize/address.code.length, which returns 0
        // for contracts in construction, since the code is only stored at the end
        // of the constructor execution.

        return account.code.length > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        (bool success, ) = recipient.call{value: amount}("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain `call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(
        address target,
        bytes memory data,
        uint256 value
    ) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(
        address target,
        bytes memory data,
        uint256 value,
        string memory errorMessage
    ) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        (bool success, bytes memory returndata) = target.call{value: value}(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal view returns (bytes memory) {
        (bool success, bytes memory returndata) = target.staticcall(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal returns (bytes memory) {
        (bool success, bytes memory returndata) = target.delegatecall(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Tool to verify that a low level call to smart-contract was successful, and revert (either by bubbling
     * the revert reason or using the provided one) in case of unsuccessful call or if target was not a contract.
     *
     * _Available since v4.8._
     */
    function verifyCallResultFromTarget(
        address target,
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal view returns (bytes memory) {
        if (success) {
            if (returndata.length == 0) {
                // only check isContract if the call was successful and the return data is empty
                // otherwise we already know that it was a contract
                require(isContract(target), "Address: call to non-contract");
            }
            return returndata;
        } else {
            _revert(returndata, errorMessage);
        }
    }

    /**
     * @dev Tool to verify that a low level call was successful, and revert if it wasn't, either by bubbling the
     * revert reason or using the provided one.
     *
     * _Available since v4.3._
     */
    function verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal pure returns (bytes memory) {
        if (success) {
            return returndata;
        } else {
            _revert(returndata, errorMessage);
        }
    }

    function _revert(bytes memory returndata, string memory errorMessage) private pure {
        // Look for revert reason and bubble it up if present
        if (returndata.length > 0) {
            // The easiest way to bubble the revert reason is using memory via assembly
            /// @solidity memory-safe-assembly
            assembly {
                let returndata_size := mload(returndata)
                revert(add(32, returndata), returndata_size)
            }
        } else {
            revert(errorMessage);
        }
    }
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC20/IERC20.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);

    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `to`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address to, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `from` to `to` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC20/extensions/IERC20Metadata.sol)

pragma solidity ^0.8.0;

import "./IERC20.sol";

/**
 * @dev Interface for the optional metadata functions from the ERC20 standard.
 *
 * _Available since v4.1._
 */
interface IERC20Metadata is IERC20 {
    /**
     * @dev Returns the name of the token.
     */
    function name() external view returns (string memory);

    /**
     * @dev Returns the symbol of the token.
     */
    function symbol() external view returns (string memory);

    /**
     * @dev Returns the decimals places of the token.
     */
    function decimals() external view returns (uint8);
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC20/extensions/IERC20Permit.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in
 * https://eips.ethereum.org/EIPS/eip-2612[EIP-2612].
 *
 * Adds the {permit} method, which can be used to change an account's ERC20 allowance (see {IERC20-allowance}) by
 * presenting a message signed by the account. By not relying on {IERC20-approve}, the token holder account doesn't
 * need to send a transaction, and thus is not required to hold Ether at all.
 */
interface IERC20Permit {
    /**
     * @dev Sets `value` as the allowance of `spender` over ``owner``'s tokens,
     * given ``owner``'s signed approval.
     *
     * IMPORTANT: The same issues {IERC20-approve} has related to transaction
     * ordering also apply here.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `deadline` must be a timestamp in the future.
     * - `v`, `r` and `s` must be a valid `secp256k1` signature from `owner`
     * over the EIP712-formatted function arguments.
     * - the signature must use ``owner``'s current nonce (see {nonces}).
     *
     * For more information on the signature format, see the
     * https://eips.ethereum.org/EIPS/eip-2612#specification[relevant EIP
     * section].
     */
    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    /**
     * @dev Returns the current nonce for `owner`. This value must be
     * included whenever a signature is generated for {permit}.
     *
     * Every successful call to {permit} increases ``owner``'s nonce by one. This
     * prevents a signature from being used multiple times.
     */
    function nonces(address owner) external view returns (uint256);

    /**
     * @dev Returns the domain separator used in the encoding of the signature for {permit}, as defined by {EIP712}.
     */
    // solhint-disable-next-line func-name-mixedcase
    function DOMAIN_SEPARATOR() external view returns (bytes32);
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.3) (token/ERC20/utils/SafeERC20.sol)

pragma solidity ^0.8.0;

import "./IERC20.sol";
import "./IERC20Permit.sol";
import "./Address.sol";

/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using Address for address;

    /**
     * @dev Transfer `value` amount of `token` from the calling contract to `to`. If `token` returns no value,
     * non-reverting calls are assumed to be successful.
     */
    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    /**
     * @dev Transfer `value` amount of `token` from `from` to `to`, spending the approval given by `from` to the
     * calling contract. If `token` returns no value, non-reverting calls are assumed to be successful.
     */
    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(IERC20 token, address spender, uint256 value) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        require(
            (value == 0) || (token.allowance(address(this), spender) == 0),
            "SafeERC20: approve from non-zero to non-zero allowance"
        );
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
    }

    /**
     * @dev Increase the calling contract's allowance toward `spender` by `value`. If `token` returns no value,
     * non-reverting calls are assumed to be successful.
     */
    function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 oldAllowance = token.allowance(address(this), spender);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, oldAllowance + value));
    }

    /**
     * @dev Decrease the calling contract's allowance toward `spender` by `value`. If `token` returns no value,
     * non-reverting calls are assumed to be successful.
     */
    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        unchecked {
            uint256 oldAllowance = token.allowance(address(this), spender);
            require(oldAllowance >= value, "SafeERC20: decreased allowance below zero");
            _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, oldAllowance - value));
        }
    }

    /**
     * @dev Set the calling contract's allowance toward `spender` to `value`. If `token` returns no value,
     * non-reverting calls are assumed to be successful. Meant to be used with tokens that require the approval
     * to be set to zero before setting it to a non-zero value, such as USDT.
     */
    function forceApprove(IERC20 token, address spender, uint256 value) internal {
        bytes memory approvalCall = abi.encodeWithSelector(token.approve.selector, spender, value);

        if (!_callOptionalReturnBool(token, approvalCall)) {
            _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, 0));
            _callOptionalReturn(token, approvalCall);
        }
    }

    /**
     * @dev Use a ERC-2612 signature to set the `owner` approval toward `spender` on `token`.
     * Revert on invalid signature.
     */
    function safePermit(
        IERC20Permit token,
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) internal {
        uint256 nonceBefore = token.nonces(owner);
        token.permit(owner, spender, value, deadline, v, r, s);
        uint256 nonceAfter = token.nonces(owner);
        require(nonceAfter == nonceBefore + 1, "SafeERC20: permit did not succeed");
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address-functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
        require(returndata.length == 0 || abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     *
     * This is a variant of {_callOptionalReturn} that silents catches all reverts and returns a bool instead.
     */
    function _callOptionalReturnBool(IERC20 token, bytes memory data) private returns (bool) {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We cannot use {Address-functionCall} here since this should return false
        // and not revert is the subcall reverts.

        (bool success, bytes memory returndata) = address(token).call(data);
        return
            success && (returndata.length == 0 || abi.decode(returndata, (bool))) && Address.isContract(address(token));
    }
}

// SPDX-License-Identifier: AGPL-3.0
pragma solidity ^0.8.22;

import {IERC20, SafeERC20} from "./SafeERC20.sol";
import {IERC20Metadata} from "./IERC20Metadata.sol";

contract YearnBoostedStaker {
    using SafeERC20 for IERC20;

    uint public immutable MAX_STAKE_GROWTH_WEEKS;
    uint8 public immutable MAX_WEEK_BIT;
    uint public immutable START_TIME;
    IERC20 public immutable stakeToken;

    // Account weight tracking state vars.
    mapping(address account => AccountData data) public accountData;
    mapping(address account => mapping(uint week => uint weight)) private accountWeeklyWeights;
    mapping(address account => mapping(uint week => ToRealize weight)) public accountWeeklyToRealize;
    mapping(address account => mapping(uint week => uint amount)) public accountWeeklyMaxStake;

    // Global weight tracking stats vars.
    uint112 public globalGrowthRate;
    uint16 public globalLastUpdateWeek;
    mapping(uint week => uint weight) private globalWeeklyWeights;
    mapping(uint week => ToRealize weight) public globalWeeklyToRealize;
    mapping(uint week => uint amount) public globalWeeklyMaxStake;

    // Generic token interface.
    uint public totalSupply;
    uint8 public immutable decimals;

    // Permissioned roles
    address public owner;
    address public pendingOwner;
    mapping(address account => mapping(address caller => ApprovalStatus approvalStatus)) public approvedCaller;
    mapping(address staker => bool approved) public approvedWeightedStaker;

    struct ToRealize {
        uint128 weightPersistent;
        uint128 weight;
    }

    struct AccountData {
        uint112 realizedStake;  // Amount of stake that has fully realized weight.
        uint112 pendingStake;   // Amount of stake that has not yet fully realized weight.
        uint16 lastUpdateWeek;  // Week of last sync.

        // One byte member to represent weeks in which an account has pending weight changes.
        // A bit is set to true when the account has a non-zero token balance to be realized in
        // the corresponding week. We use this as a "map", allowing us to reduce gas consumption
        // by avoiding unnecessary lookups on weeks which an account has zero pending stake.
        //
        // Example: 01000001
        // The left-most bit represents the final week of pendingStake.
        // Therefore, we can see that account has stake updates to process only in weeks 7 and 1.
        uint8 updateWeeksBitmap;
    }

    enum ApprovalStatus {
        None,               // 0. Default value, indicating no approval
        StakeOnly,          // 1. Approved for stake only
        UnstakeOnly,        // 2. Approved for unstake only
        StakeAndUnstake     // 3. Approved for both stake and unstake
    }

    event Staked(address indexed account, uint indexed week, uint amount, uint newUserWeight, uint weightAdded);
    event Unstaked(address indexed account, uint indexed week, uint amount, uint newUserWeight, uint weightRemoved);
    event ApprovedCallerSet(address indexed account, address indexed caller, ApprovalStatus status);
    event OwnershipTransferred(address indexed newOwner);
    event WeightedStakerSet(address indexed staker, bool approved);

    /**
        @param _token The token to be staked.
        @param _max_stake_growth_weeks The number of weeks a stake will grow for.
                            Not including desposit week.
        @param _start_time  allows deployer to optionally set a custom start time.
                            useful if needed to line up with week count in another system.
                            Passing a value of 0 will start at block.timestamp.
        @param _owner       Owner is able to grant access to stake with max boost.
    */
    constructor(address _token, uint _max_stake_growth_weeks, uint _start_time, address _owner) {
        owner = _owner;
        emit OwnershipTransferred(_owner);
        stakeToken = IERC20(_token);
        decimals = IERC20Metadata(_token).decimals();
        require(
            _max_stake_growth_weeks > 0 &&
            _max_stake_growth_weeks <= 7,
            "Invalid weeks"
        );
        MAX_STAKE_GROWTH_WEEKS = _max_stake_growth_weeks;
        MAX_WEEK_BIT = uint8(1 << MAX_STAKE_GROWTH_WEEKS);
        if (_start_time == 0){
            START_TIME = block.timestamp;
        }
        else {
            require(_start_time <= block.timestamp, "!Past");
            START_TIME = _start_time;
        }
    }

    /**
        @notice Stake tokens into the staking contract.
        @param _amount Amount of tokens to stake.
    */
    function stake(uint _amount) external returns (uint) {
        return _stake(msg.sender, _amount);
    }

    function stakeFor(address _account, uint _amount) external returns (uint) {
        if (msg.sender != _account) {
            ApprovalStatus status = approvedCaller[_account][msg.sender];
            require(
                status == ApprovalStatus.StakeAndUnstake ||
                status == ApprovalStatus.StakeOnly,
                "!Permission"
            );
        }
        
        return _stake(_account, _amount);
    }

    function _stake(address _account, uint _amount) internal returns (uint) {
        require(_amount > 1 && _amount < type(uint112).max, "invalid amount");

        // Before going further, let's sync our account and global weights
        uint systemWeek = getWeek();
        (AccountData memory acctData, uint accountWeight) = _checkpointAccount(_account, systemWeek);
        uint112 globalWeight = uint112(_checkpointGlobal(systemWeek));

        uint weight = _amount >> 1;
        _amount = weight << 1; // This helps prevent balance/weight discrepencies.
        
        acctData.pendingStake += uint112(weight);
        globalGrowthRate += uint112(weight);

        uint realizeWeek = systemWeek + MAX_STAKE_GROWTH_WEEKS;
        ToRealize memory toRealize = accountWeeklyToRealize[_account][realizeWeek];
        toRealize.weight += uint128(weight);
        toRealize.weightPersistent += uint128(weight);
        accountWeeklyToRealize[_account][realizeWeek] = toRealize;

        toRealize = globalWeeklyToRealize[realizeWeek];
        toRealize.weight += uint128(weight);
        toRealize.weightPersistent += uint128(weight);
        globalWeeklyToRealize[realizeWeek] = toRealize;
        
        accountWeeklyWeights[_account][systemWeek] = accountWeight + weight;
        globalWeeklyWeights[systemWeek] = globalWeight + weight;

        acctData.updateWeeksBitmap |= 1; // Use bitwise or to ensure bit is flipped at least weighted position.
        accountData[_account] = acctData;
        totalSupply += _amount;
        
        stakeToken.safeTransferFrom(msg.sender, address(this), uint(_amount));
        emit Staked(_account, systemWeek, _amount, accountWeight + weight, weight);
        
        return _amount;
    }

    /**
        @notice Allows an option for an approved helper to stake to any account at any weight week.
        @dev A stake using this method only effects weight in current and future weeks. It does not backfill prior weeks.
        @param _amount Amount to stake
        @return amount of tokens staked
    */
    function stakeAsMaxWeighted(address _account, uint _amount) external returns (uint) {
        require(
            approvedWeightedStaker[msg.sender],
            "!approvedStaker"
        );
        require(_amount > 1 && _amount < type(uint112).max, "invalid amount");

        // Before going further, let's sync our account and global weights
        uint systemWeek = getWeek();
        (AccountData memory acctData, uint accountWeight) = _checkpointAccount(_account, systemWeek);
        uint112 globalWeight = uint112(_checkpointGlobal(systemWeek));

        uint weight = _amount >> 1;
        _amount = weight << 1;
        acctData.realizedStake += uint112(weight);
        weight = weight * (MAX_STAKE_GROWTH_WEEKS + 1);

        // Note: The usage of `stakeAsMaxWeighted` breaks an ability to reliably derive account + global
        // amount deposited at any week using `weeklyToRealize` variables.
        // To make up for this, we introduce the following two variables that are meant to recover that same
        // ability for any on-chain integrators. They may combine this new data with `weeklyToRealize`.
        accountWeeklyMaxStake[_account][systemWeek] += _amount;
        globalWeeklyMaxStake[systemWeek] += _amount;

        accountWeeklyWeights[_account][systemWeek] = accountWeight + weight;
        globalWeeklyWeights[systemWeek] = globalWeight + weight;

        

        accountData[_account] = acctData;
        totalSupply += _amount;

        stakeToken.safeTransferFrom(msg.sender, address(this), uint(_amount));
        emit Staked(_account, systemWeek, _amount, accountWeight + weight, weight);

        return _amount;
    }

    /**
        @notice Unstake tokens from the contract.
        @dev During partial unstake, this will always remove from the least-weighted first.
    */
    function unstake(uint _amount, address _receiver) external returns (uint) {
        return _unstake(msg.sender, _amount, _receiver);
    }

    /**
        @notice Unstake tokens from the contract on behalf of another user.
        @dev During partial unstake, this will always remove from the least-weighted first.
    */
    function unstakeFor(address _account, uint _amount, address _receiver) external returns (uint) {
        if (msg.sender != _account) {
            ApprovalStatus status = approvedCaller[_account][msg.sender];
            require(
                status == ApprovalStatus.StakeAndUnstake ||
                status == ApprovalStatus.UnstakeOnly,
                "!Permission"
            );
        }
        return _unstake(_account, _amount, _receiver);
    }

    function _unstake(address _account, uint _amount, address _receiver) internal returns (uint) {
        require(_amount > 1 && _amount < type(uint112).max, "invalid amount");
        uint systemWeek = getWeek();

        // Before going further, let's sync our account and global weights
        (AccountData memory acctData, ) = _checkpointAccount(_account, systemWeek);
        _checkpointGlobal(systemWeek);

        // Here we do work to pull from most recent (least weighted) stake first
        uint8 bitmap = acctData.updateWeeksBitmap;
        uint128 weightToRemove;

        uint128 amountNeeded = uint128(_amount >> 1);
        _amount = amountNeeded << 1; // This helps prevent balance/weight discrepencies.

        if (bitmap > 0) {
            for (uint128 weekIndex; weekIndex < MAX_STAKE_GROWTH_WEEKS;) {
                // Move right to left, checking each bit if there's an update for corresponding week.
                uint8 mask = uint8(1 << weekIndex);
                if (bitmap & mask == mask) {
                    uint weekToCheck = systemWeek + MAX_STAKE_GROWTH_WEEKS - weekIndex;
                    uint128 pending = accountWeeklyToRealize[_account][weekToCheck].weight;
                    if (amountNeeded > pending){
                        weightToRemove += pending * (weekIndex + 1);
                        accountWeeklyToRealize[_account][weekToCheck].weight = 0;
                        globalWeeklyToRealize[weekToCheck].weight -= pending;
                        if (weekIndex == 0) { // Current system week
                            accountWeeklyToRealize[_account][weekToCheck].weightPersistent = 0;
                            globalWeeklyToRealize[weekToCheck].weightPersistent -= pending;
                        }
                        bitmap = bitmap ^ mask;
                        amountNeeded -= pending;
                    }
                    else { 
                        // handle the case where we have more pending than needed
                        weightToRemove += amountNeeded * (weekIndex + 1);
                        accountWeeklyToRealize[_account][weekToCheck].weight -= amountNeeded;
                        globalWeeklyToRealize[weekToCheck].weight -= amountNeeded;
                        if (weekIndex == 0) { // Current system week
                            accountWeeklyToRealize[_account][weekToCheck].weightPersistent -= amountNeeded;
                            globalWeeklyToRealize[weekToCheck].weightPersistent -= amountNeeded;
                        }
                        if (amountNeeded == pending) bitmap = bitmap ^ mask;
                        amountNeeded = 0;
                        break;
                    }
                }
                unchecked{weekIndex++;}
            }
            acctData.updateWeeksBitmap = bitmap;
        }
        
        uint pendingRemoved = (_amount >> 1) - amountNeeded;
        if (amountNeeded > 0) {
            weightToRemove += amountNeeded * uint128(1 + MAX_STAKE_GROWTH_WEEKS);
            acctData.realizedStake -= uint112(amountNeeded);
            acctData.pendingStake = 0;
        }
        else{
            acctData.pendingStake -= uint112(pendingRemoved);
        }
        
        accountData[_account] = acctData;

        globalGrowthRate -= uint112(pendingRemoved);
        globalWeeklyWeights[systemWeek] -= weightToRemove;
        uint newAccountWeight = accountWeeklyWeights[_account][systemWeek] - weightToRemove;
        accountWeeklyWeights[_account][systemWeek] = newAccountWeight;
        
        totalSupply -= _amount;

        emit Unstaked(_account, systemWeek, _amount, newAccountWeight, weightToRemove);
        
        stakeToken.safeTransfer(_receiver, _amount);
        
        return _amount;
    }
    
    /**
        @notice Get the current realized weight for an account
        @param _account Account to checkpoint.
        @return acctData Most recent account data written to storage.
        @return weight Most current account weight.
        @dev Prefer to use this function over it's view counterpart for
             contract -> contract interactions.
    */
    function checkpointAccount(address _account) external returns (AccountData memory acctData, uint weight) {
        (acctData, weight) = _checkpointAccount(_account, getWeek());
        accountData[_account] = acctData;
    }

    /**
        @notice Checkpoint an account using a specified week limit.
        @dev    To use in the event that significant number of weeks have passed since last 
                heckpoint and single call becomes too expensive.
        @param _account Account to checkpoint.
        @param _week Week which we want to checkpoint to.
        @return acctData Most recent account data written to storage.
        @return weight Account weight for provided week.
    */
    function checkpointAccountWithLimit(address _account, uint _week) external returns (AccountData memory acctData, uint weight) {
        uint systemWeek = getWeek();
        if (_week >= systemWeek) _week = systemWeek;
        (acctData, weight) = _checkpointAccount(_account, _week);
        accountData[_account] = acctData;
    }

    function _checkpointAccount(address _account, uint _systemWeek) internal returns (AccountData memory acctData, uint weight){
        acctData = accountData[_account];
        uint lastUpdateWeek = acctData.lastUpdateWeek;

        if (_systemWeek == lastUpdateWeek) {
            return (acctData, accountWeeklyWeights[_account][lastUpdateWeek]);
        }

        require(_systemWeek > lastUpdateWeek, "specified week is older than last update.");

        uint pending = uint(acctData.pendingStake);
        uint realized = acctData.realizedStake;

        if (pending == 0) {
            if (realized != 0) {
                weight = accountWeeklyWeights[_account][lastUpdateWeek];
                while (lastUpdateWeek < _systemWeek) {
                    unchecked{lastUpdateWeek++;}
                    // Fill in any missing weeks
                    accountWeeklyWeights[_account][lastUpdateWeek] = weight;
                }
            }
            accountData[_account].lastUpdateWeek = uint16(_systemWeek);
            acctData.lastUpdateWeek = uint16(_systemWeek);
            return (acctData, weight);
        }

        weight = accountWeeklyWeights[_account][lastUpdateWeek];
        uint8 bitmap = acctData.updateWeeksBitmap;
        uint targetSyncWeek = min(_systemWeek, lastUpdateWeek + MAX_STAKE_GROWTH_WEEKS);

        // Populate data for missed weeks
        while (lastUpdateWeek < targetSyncWeek) {
            unchecked{ lastUpdateWeek++; }
            weight += pending; // Increment weights by weekly growth factor.
            accountWeeklyWeights[_account][lastUpdateWeek] = weight;

            // Shift left on bitmap as we pass over each week.
            bitmap = bitmap << 1;
            if (bitmap & MAX_WEEK_BIT == MAX_WEEK_BIT){ // If left-most bit is true, we have something to realize; push pending to realized.
                // Do any updates needed to realize an amount for an account.
                uint toRealize = accountWeeklyToRealize[_account][lastUpdateWeek].weight;
                pending -= toRealize;
                realized += toRealize;
                if (pending == 0) break; // All pending has been realized. No need to continue.
            }
        }

        // Fill in any missed weeks.
        while (lastUpdateWeek < _systemWeek){
            unchecked{lastUpdateWeek++;}
            accountWeeklyWeights[_account][lastUpdateWeek] = weight;
        }   

        // Write new account data to storage.
        acctData = AccountData({
            updateWeeksBitmap: bitmap,
            pendingStake: uint112(pending),
            realizedStake: uint112(realized),
            lastUpdateWeek: uint16(_systemWeek)
        });
    }

    /**
        @notice View function to get the current weight for an account
    */
    function getAccountWeight(address account) external view returns (uint) {
        return getAccountWeightAt(account, getWeek());
    }

    /**
        @notice Get the weight for an account in a given week
    */
    function getAccountWeightAt(address _account, uint _week) public view returns (uint) {
        if (_week > getWeek()) return 0;
        
        AccountData memory acctData = accountData[_account];
        
        uint16 lastUpdateWeek = acctData.lastUpdateWeek;

        if (lastUpdateWeek >= _week) return accountWeeklyWeights[_account][_week]; 

        uint weight = accountWeeklyWeights[_account][lastUpdateWeek];

        uint pending = uint(acctData.pendingStake);
        if (pending == 0) return weight;

        uint8 bitmap = acctData.updateWeeksBitmap;

        while (lastUpdateWeek < _week) { // Populate data for missed weeks
            unchecked{lastUpdateWeek++;}
            weight += pending; // Increment weight by 1 week

            // Our bitmap is used to determine if week has any amount to realize.
            bitmap = bitmap << 1;
            if (bitmap & MAX_WEEK_BIT == MAX_WEEK_BIT){ // If left-most bit is true, we have something to realize; push pending to realized.
                pending -= accountWeeklyToRealize[_account][lastUpdateWeek].weight;
                if (pending == 0) break; // All pending has now been realized, let's exit.
            }            
        }
        
        return weight;
    }

    /**
        @notice Get the current total system weight
        @dev Also updates local storage values for total weights. Using
             this function over it's `view` counterpart is preferred for
             contract -> contract interactions.
    */
    function checkpointGlobal() external returns (uint) {
        uint systemWeek = getWeek();
        return _checkpointGlobal(systemWeek);
    }

    /**
        @notice Get the current total system weight
        @dev Also updates local storage values for total weights. Using
             this function over it's `view` counterpart is preferred for
             contract -> contract interactions.
    */
    function _checkpointGlobal(uint systemWeek) internal returns (uint) {
        // These two share a storage slot.
        uint16 lastUpdateWeek = globalLastUpdateWeek;
        uint rate = globalGrowthRate;

        uint weight = globalWeeklyWeights[lastUpdateWeek];

        if (weight == 0) {
            globalLastUpdateWeek = uint16(systemWeek);
            return 0;
        }

        if (lastUpdateWeek == systemWeek){
            return weight;
        }

        while (lastUpdateWeek < systemWeek) {
            unchecked{lastUpdateWeek++;}
            weight += rate;
            globalWeeklyWeights[lastUpdateWeek] = weight;
            rate -= globalWeeklyToRealize[lastUpdateWeek].weight;
        }

        globalGrowthRate = uint112(rate);
        globalLastUpdateWeek = uint16(systemWeek);

        return weight;
    }

    /**
        @notice Get the system weight for current week.
    */
    function getGlobalWeight() external view returns (uint) {
        return getGlobalWeightAt(getWeek());
    }

    /**
        @notice Get the system weight for a specified week in the past.
        @dev querying a week in the future will always return 0.
        @param week the week number to query global weight for.
    */
    function getGlobalWeightAt(uint week) public view returns (uint) {
        uint systemWeek = getWeek();
        if (week > systemWeek) return 0;

        // Read these together since they are packed in the same slot.
        uint16 lastUpdateWeek = globalLastUpdateWeek;
        uint rate = globalGrowthRate;

        if (week <= lastUpdateWeek) return globalWeeklyWeights[week];

        uint weight = globalWeeklyWeights[lastUpdateWeek];
        if (rate == 0) {
            return weight;
        }

        while (lastUpdateWeek < week) {
            unchecked {lastUpdateWeek++;}
            weight += rate;
            rate -= globalWeeklyToRealize[lastUpdateWeek].weight;
        }

        return weight;
    }

    /**
        @notice Returns the balance of underlying staked tokens for an account
        @param _account Account to query balance.
        @return balance of account.
    */
    function balanceOf(address _account) external view returns (uint) {
        AccountData memory acctData = accountData[_account];
        return 2 * (acctData.pendingStake + acctData.realizedStake);
    }

    /**
        @notice Allow another address to stake or unstake on behalf of. Useful for zaps and other functionality.
        @param _caller Address of the caller to approve or unapprove.
        @param _status Enum representing various approval status states.
    */
    function setApprovedCaller(address _caller, ApprovalStatus _status) external {
        approvedCaller[msg.sender][_caller] = _status;
        emit ApprovedCallerSet(msg.sender, _caller, _status);
    }

    /**
        @notice Allow owner to specify an account which has ability to stakeAsWeighted.
        @param _staker Address of account with staker permissions.
        @param _approved Approve or unapprove the staker.
    */
    function setWeightedStaker(address _staker, bool _approved) external {
        require(msg.sender == owner, "!authorized");
        approvedWeightedStaker[_staker] = _approved;
        emit WeightedStakerSet(_staker, _approved);
    }

    /**
        @notice Set a pending owner which can later be accepted.
        @param _pendingOwner Address of the new owner.
    */
    function transferOwnership(address _pendingOwner) external {
        require(msg.sender == owner, "!authorized");
        pendingOwner = _pendingOwner;
    }

    /**
        @notice Allow pending owner to accept ownership
    */
    function acceptOwnership() external {
        require(msg.sender == pendingOwner, "!authorized");
        owner = msg.sender;
        pendingOwner = address(0);
        emit OwnershipTransferred(msg.sender);
    }

    function sweep(address _token) external {
        require(msg.sender == owner, "!authorized");
        uint amount = IERC20(_token).balanceOf(address(this));
        if (_token == address(stakeToken)) {
            amount = amount - totalSupply;
        }
        if (amount > 0) IERC20(_token).safeTransfer(owner, amount);
    }

    function getWeek() public view returns (uint week) {
        unchecked{
            return (block.timestamp - START_TIME) / 1 weeks;
        }
    }

    function min(uint a, uint b) internal pure returns (uint) {
        return a < b ? a : b;
    }
}