PHB-Staking

1. Constructor parameters：

owner address
phb token address
phb token address

2. After the contract is deployed, owner should set the inflation rate：

 function setInflationSpeed(uint256 speed) external;
 // reward amount per second

3. staking:

 function stake(uint256 amount) external;

4. return withdrawable staking Amount

 function withdrawableAmount(address account)external view returns(uint256);

5. withdraw staking token

function applyWithdraw(uint256 amount) external nonReentrant; //apply withdraw
function withdraw(uint256 amount) external; //can withdraw token after the locking time

6. return the reward amount

function getUserRewards(address account) external view returns(uint256);

7. claim reward

function claimReward() external;

8. this staking contract reward calculation is based Compound algorithm

we maintains 2 (kinds) index:
globalIndex: means how many rewards per staking token,has 2 fields: index and last updated timestamp
userIndex: user share from the total staking
every time the total staking amount changed ,user claimed rewards and inflation speed changed, we need to update the globalIndex
initial status: we ignore the decimals of tokens 
globalindex = 1, total staking = 0, speed = 0, we do the operation every 10 seconds

1) admin set speed to 100 (100 phb per seconds), since still now staking, we only update the globalindex timestamp to now.
2) user A stakes 10000 phb, previous total staking  = 0, globalindex = 1, previous balance(A) = 0,distributed reward = 0, then set user A's index = 1
3) user B stakes 10000 phb, previous total staking = 10000 , globalindex = 1 + delta(time) * speed/ 10000 = 1.1, set user B's index = 1 ,previous balance(b) = 0,distributed reward = 0, then set user B's index = 1.1
4) query/claim user A's reward now(also 10 seconds past) ,the new globalindex = 1.1 + 10 * 100/20000 = 1.15
    (if for the getUserReward function, the globalindex is not saved to storage since it's a 'view' function)   
    so the rewards is delta(index) * balance(A) = (1.15-1)*10000 = 1500 phb    
5) at the same time user B's reward is delta(index) * balance(B) = (1.15-1.1) * 10000 = 500 phb
now let's check the result(***NOTE since no one staked in the 1st 10 seconds, so the 1st 1000 phb is wasted!!!!!)

time	total reward	user A staking	user B staking	user A reward	user B reward	gIdex	AIdx	BIdx
0	0	0	0	0	0	1	0	0
10	1000	10000	0	0	0	1	1	0
20	2000	10000	10000	1000	0	1.1	1	1.1
30	3000	10000	10000	500	500	1.15	1	1.1
40	4000	10000	10000	500	500	1.2	1	1.1
500	6000	10000	10000	1000	1000	1.3	1	1.1

for refresh speed is the same
6) admin change speed to 200, then global index = 1.1(assume step 5 is query operation, didn't change the storage) + delta(time) * oldspeed / 20000 = 1.1+20 *100/20000 = 1.2
7) user A query reward: current global index = 1.2+ delta(time) * newspeed / 20000 = 1.2 + 10 * 200/20000 = 1.3, so the reward should be (1.3 - 1)* 10000 = 3000
user B query reward: so the reward should be (1.3 - 1.1)* 10000 = 2000