Staking

Based on Synthetix's staking contract: 
​https://docs.synthetix.io/incentives/​
Description
Frax users are able to stake in select Uniswap liquidity pool tokens in exchange for FXS rewards. Future pools and incentives can be added by governance. 
Deployment
Liquidity Pool Tokens (LP)
Uniswap FRAX/WETH LP: 0xFD0A40Bc83C5faE4203DEc7e5929B446b07d1C76
Uniswap FRAX/USDC LP: 0x97C4adc5d28A86f9470C70DD91Dc6CC2f20d2d4D
Uniswap FRAX/FXS LP: 0xE1573B9D29e2183B1AF0e743Dc2754979A40D237
Uniswap FXS/WETH LP: 0xecBa967D84fCF0405F6b32Bc45F4d36BfDBB2E81
Staking Contracts 
Uniswap FRAX/WETH staking: 0xD875628B942f8970De3CcEaf6417005F68540d4f
Uniswap FRAX/USDC staking: 0xa29367a3f057F3191b62bd4055845a33411892b6
Uniswap FRAX/FXS staking: 0xda2c338350a0E59Ce71CDCED9679A3A590Dd9BEC
Uniswap FXS/WETH staking (deprecated): 0xDc65f3514725206Dd83A8843AAE2aC3D99771C88
State Variables
FRAXStablecoin private FRAX
Instance of the FRAX contract.
ERC20 public rewardsToken
Instance for the reward token.
ERC20 public stakingToken
Instance for the staking token.
uint256 public periodFinish
Block when the staking period will finish.
uint256 public rewardRate
Maximum reward per second.
uint256 public rewardsDuration
Reward period, in seconds.
uint256 public lastUpdateTime
Last timestamp where the contract was updated / state change.
uint256 public rewardPerTokenStored
Actual reward per token in the current period.
uint256 public locked_stake_max_multiplier
Maximum boost / weight multiplier for locked stakes.
uint256 public locked_stake_time_for_max_multiplier
The time, in seconds, to reach locked_stake_max_multiplier .
uint256 public locked_stake_min_time
Minimum staking time for a locked staked, in seconds.
string public locked_stake_min_time_str
String version is locked_stake_min_time_str .
uint256 public cr_boost_max_multiplier
Maximum boost / weight multiplier from the collateral ratio (CR). This is applied to both locked and unlocked stakes.
mapping(address => uint256) public userRewardPerTokenPaid
Keeps track of when an address last collected a reward.  If they collect it some time later, they will get the correct amount because rewardPerTokenStored is constantly varying.
mapping(address => uint256) public rewards
Current rewards balance for a given address.
uint256 private _staking_token_supply
Total amount of pool tokens staked .
uint256 private _staking_token_boosted_supply
_staking_token_supply with the time and CR boosts accounted for. This is not an actual amount of pool tokens, but rather a 'weighed denominator'.
mapping(address => uint256) private _balances
Balance of pool tokens staked for a given address.
mapping(address => uint256) private _boosted_balances
_balances , but with the time and CR boosts accounted for, like _staking_token_boosted_supply.
mapping(address => LockedStake[]) private lockedStakes
Gives a list of locked stake lots for a given address.
struct LockedStake {
        bytes32 kek_id;
        uint256 start_timestamp;
        uint256 amount;
        uint256 ending_timestamp;
        uint256 multiplier; // 6 decimals of precision. 1x = 1000000
    }
A locked stake 'lot'. 
View Functions
totalSupply
totalSupply() external override view returns (uint256)
Get the total number of staked liquidity pool tokens.
stakingMultiplier
stakingMultiplier(uint256 secs) public view returns (uint256)
Get the time-based staking multiplier, given the secs length of the stake.
crBoostMultiplier
crBoostMultiplier() public view returns (uint256)
Get the collateral ratio (CR) - based staking multiplier.
stakingTokenSupply
stakingTokenSupply() external view returns (uint256)
same as totalSupply().
balanceOf
balanceOf(address account) external override view returns (uint256)
Get the amount of staked liquidity pool tokens for a given account.
boostedBalanceOf
boostedBalanceOf(address account) external view returns (uint256)
Get the boosted amount of staked liquidity pool tokens for a given account. Boosted accounts for the CR and time-based multipliers.
lockedBalanceOf
lockedBalanceOf(address account) public view returns (uint256)
Get the amount of locked staked liquidity pool tokens for a given account .
unlockedBalanceOf
unlockedBalanceOf(address account) external view returns (uint256)
Get the amount of unlocked / free staked liquidity pool tokens for a given account .
lockedStakesOf
lockedStakesOf(address account) external view returns (LockedStake[] memory)
Return an array of all the locked stake 'lots' for 
stakingDecimals
stakingDecimals() external view returns (uint256)
Returns the decimals() for stakingToken .
rewardsFor
rewardsFor(address account) external view returns (uint256)
Get the amount of FXS rewards for a given account .
lastTimeRewardApplicable
lastTimeRewardApplicable() public override view returns (uint256)
Used internally to keep track of rewardPerTokenStored .
rewardPerToken
rewardPerToken() public override view returns (uint256)
The current amount of FXS rewards for staking a liquidity pool token.
earned
earned(address account) public override view returns (uint256)
Returns the amount of unclaimed FXS rewards for a given account .
getRewardForDuration
getRewardForDuration() external override view returns (uint256)
Calculates the FXS reward for a given rewardsDuration period.
Mutative Functions
stake
stake(uint256 amount) external override nonReentrant notPaused updateReward(msg.sender)
Stakes some Uniswap liquidity pool tokens. These tokens are freely withdrawable and are only boosted by the crBoostMultiplier().
stakeLocked
stakeLocked(uint256 amount, uint256 secs) external nonReentrant notPaused updateReward(msg.sender)
Stakes some Uniswap liquidity pool tokens and also locks them for the specified secs . In return for having their tokens locked, the staker's base amount will be multiplied by a linear time-based multiplier, which ranges from 1 at secs = 0 to  locked_stake_max_multiplier at locked_stake_time_for_max_multiplier. The staked value is also multiplied by the crBoostMultiplier() . This multiplied value is added to _boosted_balances and acts as a weighted amount when calculating the staker's share of a given period reward.
withdraw
withdraw(uint256 amount) public override nonReentrant updateReward(msg.sender)
Withdraw unlocked Uniswap liquidity pool tokens.
withdrawLocked
withdrawLocked(bytes32 kek_id) public nonReentrant updateReward(msg.sender)
Withdraw locked Uniswap liquidity pool tokens. Will fail if the staking time for the specific kek_id staking lot has not elapsed yet.
getReward
getReward() public override nonReentrant updateReward(msg.sender)
Claim FXS rewards.
exit
exit() external override
Withdraw all unlocked pool tokens and also collect rewards.
renewIfApplicable
renewIfApplicable() external
Renew a reward period if the period's finish time has completed. Calls retroCatchUp() .
retroCatchUp
retroCatchUp() internal
Renews the period and updates periodFinish , rewardPerTokenStored , and lastUpdateTime .
Restricted Functions
notifyRewardAmount
notifyRewardAmount(uint256 reward) external override onlyRewardsDistribution updateReward(address(0))
This notifies people (via the event RewardAdded ) that the reward is being changed.
recoverERC20
recoverERC20(address tokenAddress, uint256 tokenAmount) external onlyOwner
Added to support recovering LP Rewards from other systems to be distributed to holders.
setRewardsDuration
setRewardsDuration(uint256 _rewardsDuration) external onlyOwner
Set the duration of the rewards period.
setLockedStakeMaxMultiplierUpdated
setLockedStakeMaxMultiplierUpdated(uint256 _locked_stake_max_multiplier) external onlyOwner
Set the maximum multiplier for locked stakes.
setLockedStakeTimeForMaxMultiplier
setLockedStakeTimeForMaxMultiplier(uint256 _locked_stake_time_for_max_multiplier) external onlyOwner
Set the time, in seconds, when the locked stake multiplier reaches locked_stake_max_multiplier .
setLockedStakeMinTime
setLockedStakeMinTime(uint256 _locked_stake_min_time) external onlyOwner
Set the minimum time, in seconds, of a locked stake.
setMaxCRBoostMultiplier
setMaxCRBoostMultiplier(uint256 _max_boost_multiplier) external onlyOwner
aaa
initializeDefault
initializeDefault() external onlyOwner
Intended to only be called once in the lifetime of the contract. Initializes lastUpdateTime and periodFinish .
Modifiers
updateReward
updateReward(address account)
Calls retroCatchUp() , if applicable, and otherwise syncs rewardPerTokenStored and lastUpdateTime . Also, syncs the rewards and userRewardPerTokenPaid for the provided account .
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