// SPDX-License-Identifier: GPL-3.0

pragma solidity ^0.6.12;

import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/utils/Address.sol";

import "../external/maker/DaiInterface.sol";

contract Dai is DaiInterface {
  using SafeMath for uint256;
  using Address for address;

  mapping (address => uint256) private _balances;

  mapping (address => mapping (address => uint256)) private _allowances;

  uint256 private _totalSupply;

  string private _name;
  string private _symbol;
  uint8 private _decimals;

  /**
    * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
    * a default value of 18.
    *
    * To select a different value for {decimals}, use {_setupDecimals}.
    *
    * All three of these values are immutable: they can only be set once during
    * construction.
    */
  constructor (uint256 chainId_) public {
    string memory version = "1";

    _name = "Dai Stablecoin";
    _symbol = "DAI";
    _decimals = 18;

    DOMAIN_SEPARATOR = keccak256(
      abi.encode(
        keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
        keccak256(bytes(_name)),
        keccak256(bytes(version)),
        chainId_,
        address(this)
      )
    );
  }

  /**
    * @dev Returns the name of the token.
    */
  function name() public view returns (string memory) {
      return _name;
  }

  /**
    * @dev Returns the symbol of the token, usually a shorter version of the
    * name.
    */
  function symbol() public view returns (string memory) {
      return _symbol;
  }

  /**
    * @dev Returns the number of decimals used to get its user representation.
    * For example, if `decimals` equals `2`, a balance of `505` tokens should
    * be displayed to a user as `5,05` (`505 / 10 ** 2`).
    *
    * Tokens usually opt for a value of 18, imitating the relationship between
    * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
    * called.
    *
    * NOTE: This information is only used for _display_ purposes: it in
    * no way affects any of the arithmetic of the contract, including
    * {IERC20-balanceOf} and {IERC20-transfer}.
    */
  function decimals() public view returns (uint8) {
      return _decimals;
  }

  /**
    * @dev See {IERC20-totalSupply}.
    */
  function totalSupply() public view override returns (uint256) {
      return _totalSupply;
  }

  /**
    * @dev See {IERC20-balanceOf}.
    */
  function balanceOf(address account) public view override returns (uint256) {
      return _balances[account];
  }

  /**
    * @dev See {IERC20-transfer}.
    *
    * Requirements:
    *
    * - `recipient` cannot be the zero address.
    * - the caller must have a balance of at least `amount`.
    */
  function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
      _transfer(msg.sender, recipient, amount);
      return true;
  }

  /**
    * @dev See {IERC20-allowance}.
    */
  function allowance(address owner, address spender) public view virtual override returns (uint256) {
      return _allowances[owner][spender];
  }

  /**
    * @dev See {IERC20-approve}.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    */
  function approve(address spender, uint256 amount) public virtual override returns (bool) {
      _approve(msg.sender, spender, amount);
      return true;
  }

  /**
    * @dev See {IERC20-transferFrom}.
    *
    * Emits an {Approval} event indicating the updated allowance. This is not
    * required by the EIP. See the note at the beginning of {ERC20};
    *
    * Requirements:
    * - `sender` and `recipient` cannot be the zero address.
    * - `sender` must have a balance of at least `amount`.
    * - the caller must have allowance for ``sender``'s tokens of at least
    * `amount`.
    */
  function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
      _transfer(sender, recipient, amount);
      _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount, "ERC20: transfer amount exceeds allowance"));
      return true;
  }

  /**
    * @dev Atomically increases the allowance granted to `spender` by the caller.
    *
    * This is an alternative to {approve} that can be used as a mitigation for
    * problems described in {IERC20-approve}.
    *
    * Emits an {Approval} event indicating the updated allowance.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    */
  function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
      _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
      return true;
  }

  /**
    * @dev Atomically decreases the allowance granted to `spender` by the caller.
    *
    * This is an alternative to {approve} that can be used as a mitigation for
    * problems described in {IERC20-approve}.
    *
    * Emits an {Approval} event indicating the updated allowance.
    *
    * Requirements:
    *
    * - `spender` cannot be the zero address.
    * - `spender` must have allowance for the caller of at least
    * `subtractedValue`.
    */
  function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
      _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
      return true;
  }

  /**
    * @dev Moves tokens `amount` from `sender` to `recipient`.
    *
    * This is internal function is equivalent to {transfer}, and can be used to
    * e.g. implement automatic token fees, slashing mechanisms, etc.
    *
    * Emits a {Transfer} event.
    *
    * Requirements:
    *
    * - `sender` cannot be the zero address.
    * - `recipient` cannot be the zero address.
    * - `sender` must have a balance of at least `amount`.
    */
  function _transfer(address sender, address recipient, uint256 amount) internal virtual {
      require(sender != address(0), "ERC20: transfer from the zero address");
      require(recipient != address(0), "ERC20: transfer to the zero address");

      _beforeTokenTransfer(sender, recipient, amount);

      _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
      _balances[recipient] = _balances[recipient].add(amount);
      emit Transfer(sender, recipient, amount);
  }

  /** @dev Creates `amount` tokens and assigns them to `account`, increasing
    * the total supply.
    *
    * Emits a {Transfer} event with `from` set to the zero address.
    *
    * Requirements
    *
    * - `to` cannot be the zero address.
    */
  function _mint(address account, uint256 amount) internal virtual {
      require(account != address(0), "ERC20: mint to the zero address");

      _beforeTokenTransfer(address(0), account, amount);

      _totalSupply = _totalSupply.add(amount);
      _balances[account] = _balances[account].add(amount);
      emit Transfer(address(0), account, amount);
  }

  /**
    * @dev Destroys `amount` tokens from `account`, reducing the
    * total supply.
    *
    * Emits a {Transfer} event with `to` set to the zero address.
    *
    * Requirements
    *
    * - `account` cannot be the zero address.
    * - `account` must have at least `amount` tokens.
    */
  function _burn(address account, uint256 amount) internal virtual {
      require(account != address(0), "ERC20: burn from the zero address");

      _beforeTokenTransfer(account, address(0), amount);

      _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
      _totalSupply = _totalSupply.sub(amount);
      emit Transfer(account, address(0), amount);
  }

  /**
    * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
    *
    * This internal function is equivalent to `approve`, and can be used to
    * e.g. set automatic allowances for certain subsystems, etc.
    *
    * Emits an {Approval} event.
    *
    * Requirements:
    *
    * - `owner` cannot be the zero address.
    * - `spender` cannot be the zero address.
    */
  function _approve(address owner, address spender, uint256 amount) internal virtual {
      require(owner != address(0), "ERC20: approve from the zero address");
      require(spender != address(0), "ERC20: approve to the zero address");

      _allowances[owner][spender] = amount;
      emit Approval(owner, spender, amount);
  }

  /**
    * @dev Sets {decimals} to a value other than the default one of 18.
    *
    * WARNING: This function should only be called from the constructor. Most
    * applications that interact with token contracts will not expect
    * {decimals} to ever change, and may work incorrectly if it does.
    */
  function _setupDecimals(uint8 decimals_) internal {
      _decimals = decimals_;
  }

  /**
    * @dev Hook that is called before any transfer of tokens. This includes
    * minting and burning.
    *
    * Calling conditions:
    *
    * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
    * will be to transferred to `to`.
    * - when `from` is zero, `amount` tokens will be minted for `to`.
    * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
    * - `from` and `to` are never both zero.
    *
    * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
    */
  function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }

  mapping (address => uint)                      public nonces;

  // --- EIP712 niceties ---
  bytes32 public DOMAIN_SEPARATOR;
  // bytes32 public constant PERMIT_TYPEHASH = keccak256("Permit(address holder,address spender,uint256 nonce,uint256 expiry,bool allowed)");
  bytes32 public constant PERMIT_TYPEHASH = 0xea2aa0a1be11a07ed86d755c93467f4f82362b452371d1ba94d1715123511acb;

  // --- Approve by signature ---
  function permit(
    address holder, address spender, uint256 nonce, uint256 expiry,
    bool allowed, uint8 v, bytes32 r, bytes32 s) external override
  {
    bytes32 digest = keccak256(
      abi.encodePacked(
        "\x19\x01",
        DOMAIN_SEPARATOR,
        keccak256(
          abi.encode(
            PERMIT_TYPEHASH,
            holder,
            spender,
            nonce,
            expiry,
            allowed
          )
        )
      )
    );

    require(holder != address(0), "Dai/invalid-address-0");
    require(holder == ecrecover(digest, v, r, s), "Dai/invalid-permit");
    require(expiry == 0 || now <= expiry, "Dai/permit-expired");
    require(nonce == nonces[holder]++, "Dai/invalid-nonce");
    uint wad = allowed ? uint(-1) : 0;
    _allowances[holder][spender] = wad;
    emit Approval(holder, spender, wad);
  }

  function mint(address to, uint256 amount) external {
    _mint(to, amount);
  }
}