ERC20.sol

// SPDX-License-Identifier: AGPL-3.0-only
pragma solidity ^0.8.7;

import "https://github.com/maple-labs/erc20/blob/main/contracts/interfaces/IERC20.sol";
/*
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    ██╔════╝██╔══██╗██╔════╝    ╚════██╗██╔═████╗
    █████╗  ██████╔╝██║          █████╔╝██║██╔██║
    ██╔══╝  ██╔══██╗██║         ██╔═══╝ ████╔╝██║
    ███████╗██║  ██║╚██████╗    ███████╗╚██████╔╝
    ╚══════╝╚═╝  ╚═╝ ╚═════╝    ╚══════╝ ╚═════╝
*/

/**
 *  @title Modern ERC-20 implementation.
 *  @dev   Acknowledgements to Solmate, OpenZeppelin, and DSS for inspiring this code.
 */
contract MyToken is IERC20 {

    /**************************************************************************************************************************************/
    /*** ERC-20                                                                                                                         ***/
    /**************************************************************************************************************************************/

    string public override name;
    string public override symbol;

    uint8 public  override decimals;

    uint256 public override totalSupply;

    mapping(address => uint256) public override balanceOf;

    mapping(address => mapping(address => uint256)) public override allowance;

    /**************************************************************************************************************************************/
    /*** ERC-2612                                                                                                                       ***/
    /**************************************************************************************************************************************/

    // PERMIT_TYPEHASH = keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32 public constant override PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;

    mapping(address => uint256) public override nonces;

    /**
     *  @param name_     The name of the token.
     *  @param symbol_   The symbol of the token.
     *  @param decimals_ The decimal precision used by the token.
     */
    constructor(string memory name_, string memory symbol_, uint8 decimals_) {
        name     = name_;
        symbol   = symbol_;
        decimals = decimals_;
    }

    /**************************************************************************************************************************************/
    /*** External Functions                                                                                                             ***/
    /**************************************************************************************************************************************/

    function approve(address spender_, uint256 amount_) public virtual override returns (bool success_) {
        _approve(msg.sender, spender_, amount_);
        return true;
    }

    function decreaseAllowance(address spender_, uint256 subtractedAmount_) public virtual override returns (bool success_) {
        _decreaseAllowance(msg.sender, spender_, subtractedAmount_);
        return true;
    }

    function increaseAllowance(address spender_, uint256 addedAmount_) public virtual override returns (bool success_) {
        _approve(msg.sender, spender_, allowance[msg.sender][spender_] + addedAmount_);
        return true;
    }

    function permit(address owner_, address spender_, uint256 amount_, uint256 deadline_, uint8 v_, bytes32 r_, bytes32 s_)
        public virtual override
    {
        require(deadline_ >= block.timestamp, "ERC20:P:EXPIRED");

        // Appendix F in the Ethereum Yellow paper (https://ethereum.github.io/yellowpaper/paper.pdf), defines
        // the valid range for s in (301): 0 < s < secp256k1n ÷ 2 + 1, and for v in (302): v ∈ {27, 28}.
        require(
            uint256(s_) <= uint256(0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) &&
            (v_ == 27 || v_ == 28),
            "ERC20:P:MALLEABLE"
        );

        // Nonce realistically cannot overflow.
        unchecked {
            bytes32 digest_ = keccak256(
                abi.encodePacked(
                    "\x19\x01",
                    DOMAIN_SEPARATOR(),
                    keccak256(abi.encode(PERMIT_TYPEHASH, owner_, spender_, amount_, nonces[owner_]++, deadline_))
                )
            );

            address recoveredAddress_ = ecrecover(digest_, v_, r_, s_);

            require(recoveredAddress_ == owner_ && owner_ != address(0), "ERC20:P:INVALID_SIGNATURE");
        }

        _approve(owner_, spender_, amount_);
    }

    function transfer(address recipient_, uint256 amount_) public virtual override returns (bool success_) {
        _transfer(msg.sender, recipient_, amount_);
        return true;
    }

    function transferFrom(address owner_, address recipient_, uint256 amount_) public virtual override returns (bool success_) {
        _decreaseAllowance(owner_, msg.sender, amount_);
        _transfer(owner_, recipient_, amount_);
        return true;
    }

    /**************************************************************************************************************************************/
    /*** View Functions                                                                                                                 ***/
    /**************************************************************************************************************************************/

    function DOMAIN_SEPARATOR() public view override returns (bytes32 domainSeparator_) {
        return keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes(name)),
                keccak256(bytes("1")),
                block.chainid,
                address(this)
            )
        );
    }

    /**************************************************************************************************************************************/
    /*** Internal Functions                                                                                                             ***/
    /**************************************************************************************************************************************/

    function _approve(address owner_, address spender_, uint256 amount_) internal {
        emit Approval(owner_, spender_, allowance[owner_][spender_] = amount_);
    }

    function _burn(address owner_, uint256 amount_) internal {
        balanceOf[owner_] -= amount_;

        // Cannot underflow because a user's balance will never be larger than the total supply.
        unchecked { totalSupply -= amount_; }

        emit Transfer(owner_, address(0), amount_);
    }

    function _decreaseAllowance(address owner_, address spender_, uint256 subtractedAmount_) internal {
        uint256 spenderAllowance = allowance[owner_][spender_];  // Cache to memory.

        if (spenderAllowance != type(uint256).max) {
            _approve(owner_, spender_, spenderAllowance - subtractedAmount_);
        }
    }

    function _mint(address recipient_, uint256 amount_) internal {
        totalSupply += amount_;

        // Cannot overflow because totalSupply would first overflow in the statement above.
        unchecked { balanceOf[recipient_] += amount_; }

        emit Transfer(address(0), recipient_, amount_);
    }

    function _transfer(address owner_, address recipient_, uint256 amount_) internal {
        balanceOf[owner_] -= amount_;

        // Cannot overflow because minting prevents overflow of totalSupply, and sum of user balances == totalSupply.
        unchecked { balanceOf[recipient_] += amount_; }

        emit Transfer(owner_, recipient_, amount_);
    }

}