[UniswapV2] UniswapV2ERC20.sol

This is quite similar to OpenZepplin’s ERC20.sol, but with some unique features.

---

Variables

nonces

To prevent replay attack.

constructor

Having problem understanding it...

Functions

permit

Give permission.

1. If time passed deadline, revert.
   

   require(deadline >= block.timestamp, 'UniswapV2: EXPIRED');
   

2. Recover address from digest and signature. (Honestly, I don’t get it.)
   

   bytes32 digest = keccak256(
               abi.encodePacked(
                   '\x19\x01',
                   DOMAIN_SEPARATOR,
                   keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, value, nonces[owner]++, deadline))
               )
           );
   address recoveredAddress = ecrecover(digest, v, r, s);
   

3. If address matches the owner approve it.
   

   require(recoveredAddress != address(0) && recoveredAddress == owner, 'UniswapV2: INVALID_SIGNATURE');
   _approve(owner, spender, value);