This ERC-20 'stake and earn' smart contract (source code), distributes token rewards to stakers over time. Stakers must first approve() this smart contract to interact with their ERC-20 tokens. Token staking may only happen after the approve() transaction is mined on Ethereum.

An ERC-20 creator/owner with a balance of tokens may deposit tokens into this staking contracts rewards pool. As time goes by, stakers earn reward tokens and, after the staker-pledged lock-up term expires, stakers may claim rewards and withdraw their deposits.

The longer tokens are staked for, the higher the earned reward. If tokens are withdrawn prior to the staker-pledged lock-up term, then only the original deposit-amount of tokens is returned, without reward. Token reward-givers must be designated by the original ERC-20 token contract creator, and reward-givers determine the annual percentage yield (APY) per ERC-20 token instance.

Staking is only allowed when there is sufficient collateral for token-rewards deposited in the contract by a reward-giver. Any ERC-20 smart contract may use this contract, free of charge or commission, to provide a 'stake and earn' service to hodlers.

          APY formula is (1+i)^(n/12)^a

          where:
          i=target 12 month APY
          n=number of months locked
          a=scaling factor 1 <= a <= 2