// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

// This is modified from "@openzeppelin/contracts/token/ERC20/IERC20.sol"
// Modifications were made to make the tokenName, tokenSymbol, and
// tokenDecimals fields internal instead of private. Getters for them were
// removed to silence solidity inheritance issues

import "@openzeppelin/contracts/GSN/Context.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/math/SafeMath.sol";

/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;

    mapping(address => uint256) private balances;

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

    uint256 private supply;

    struct Token {
        string name;
        string symbol;
        uint8 decimals;
    }

    Token internal token;

    /**
     * @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(_msgSender(), _recipient, _amount);
        return true;
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `_spender` cannot be the zero address.
     */
    function approve(address _spender, uint256 _amount)
        public
        virtual
        override
        returns (bool)
    {
        _approve(_msgSender(), _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,
            _msgSender(),
            allowances[_sender][_msgSender()].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(
            _msgSender(),
            _spender,
            allowances[_msgSender()][_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(
            _msgSender(),
            _spender,
            allowances[_msgSender()][_spender].sub(
                _subtractedValue,
                "ERC20: decreased allowance below zero"
            )
        );
        return true;
    }

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

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

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

    /**
     * @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);

        supply = supply.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"
        );
        supply = supply.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 {
        token.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 {}
}