1// SPDX-License-Identifier: MIT
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1pragma solidity >=0.6.0 <0.8.0;
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1/**
2 * @dev Interface of the ERC20 standard as defined in EIP.
3 */
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1interface IERC20 {
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1    /**
2     * @dev Returns the amount of tokens in existence.
3     */
4    function totalSupply() external view returns (uint256);
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1    /**
2     * @dev Returns the amount of tokens owned by `account`.
3     */
4    function balanceOf(address account) external view returns (uint256);
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1    /**
2     * @dev Moves `amount` tokens from the caller's account to `recipient`.
3     *
4     * Returns a boolean value indicating whether the operation succeeded.
5     *
6     * Emits a {Transfer} event.
7     */
8    function transfer(address recipient, uint256 amount) external returns (bool);
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1    /**
2     * @dev Returns the remaining number of tokens that `spender` will be
3     * allowed to spend on behalf of `owner` through {transferFrom}. This is
4     * zero by default.
5     *
6     * This value changes when {approve} or {transferFrom} are called.
7     */
8    function allowance(address owner, address spender) external view returns (uint256);
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1    /**
2     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
3     *
4     * Returns a boolean value indicating whether the operation succeeded.
5     *
6     * IMPORTANT: Beware that changing an allowance with this method brings the risk
7     * that someone may use both the old and the new allowance by unfortunate
8     * transaction ordering. One possible solution to mitigate this race
9     * condition is to first reduce the spender's allowance to 0 and set the
10     * desired value afterwards:
11     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
12     *
13     * Emits an {Approval} event.
14     */
15    function approve(address spender, uint256 amount) external returns (bool);
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1    /**
2     * @dev Moves `amount` tokens from `sender` to `recipient` using the
3     * allowance mechanism. `amount` is then deducted from the caller's
4     * allowance.
5     *
6     * Returns a boolean value indicating whether the operation succeeded.
7     *
8     * Emits a {Transfer} event.
9     */
10    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
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1
2    /**
3     * @dev Emitted when `value` tokens are moved from one account (`from`) to
4     * another (`to`).
5     *
6     * Note that `value` may be zero.
7     */
8    event Transfer(address indexed from, address indexed to, uint256 value);
9
10    /**
11     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
12     * a call to {approve}. `value` is the new allowance.
13     */
14    event Approval(address indexed owner, address indexed spender, uint256 value);
15}
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1// SPDX-License-Identifier: MIT
2pragma solidity >=0.6.0 <0.8.0;
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1
2import "../../GSN/Context.sol";
3import "./IERC20.sol";
4import "../../math/SafeMath.sol";
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1/**
2 * @dev Implementation of the {IERC20} interface.
3 *
4 * This implementation is agnostic to the way tokens are created. This means
5 * that a supply mechanism has to be added in a derived contract using {_mint}.
6 * For a generic mechanism see {ERC20PresetMinterPauser}.
7 *
8 * TIP: For a detailed writeup see our guide
9 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
10 * to implement supply mechanisms].
11 *
12 * We have followed general OpenZeppelin guidelines: functions revert instead
13 * of returning `false` on failure. This behavior is nonetheless conventional
14 * and does not conflict with the expectations of ERC20 applications.
15 *
16 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
17 * This allows applications to reconstruct the allowance for all accounts just
18 * by listening to said events. Other implementations of the EIP may not emit
19 * these events, as it isn't required by the specification.
20 *
21 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
22 * functions have been added to mitigate the well-known issues around setting
23 * allowances. See {IERC20-approve}.
24 */
25
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1contract ERC20 is Context, IERC20 {
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1
2    using SafeMath for uint256;
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1    mapping (address => uint256) private _balances;
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1    映射 (address => mapping (address => uint256)) private _allowances;
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1    uint256 private _totalSupply;
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1    string private _name;
2    string private _symbol;
3    uint8 private _decimals;
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1    /**
2     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
3     * a default value of 18.
4     *
5     * To select a different value for {decimals}, use {_setupDecimals}.
6     *
7     * All three of these values are immutable: they can only be set once during
8     * construction.
9     */
10    constructor (string memory name_, string memory symbol_) public {
11        _name = name_;
12        _symbol = symbol_;
13        _decimals = 18;
14    }
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1    /**
2     * @dev Returns the name of the token.
3     */
4    function name() public view returns (string memory) {
5        return _name;
6    }
7
8    /**
9     * @dev Returns the symbol of the token, usually a shorter version of the
10     * name.
11     */
12    function symbol() public view returns (string memory) {
13        return _symbol;
14    }
15
16    /**
17     * @dev Returns the number of decimals used to get its user representation.
18     * For example, if `decimals` equals `2`, a balance of `505` tokens should
19     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
20     *
21     * Tokens usually opt for a value of 18, imitating the relationship between
22     * ether and wei. This is the value {ERC20} uses, unless {_setupDecimals} is
23     * called.
24     *
25     * NOTE: This information is only used for _display_ purposes: it in
26     * no way affects any of the arithmetic of the contract, including
27     * {IERC20-balanceOf} and {IERC20-transfer}.
28     */
29    function decimals() public view returns (uint8) {
30        return _decimals;
31    }
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1    /**
2     * @dev See {IERC20-totalSupply}.
3     */
4    function totalSupply() public view override returns (uint256) {
5        return _totalSupply;
6    }
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1    /**
2     * @dev See {IERC20-balanceOf}.
3     */
4    function balanceOf(address account) public view override returns (uint256) {
5        return _balances[account];
6    }
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1    /**
2     * @dev See {IERC20-transfer}.
3     *
4     * Requirements:
5     *
6     * - `recipient` cannot be the zero address.
7     * - the caller must have a balance of at least `amount`.
8     */
9    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
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1        _transfer(_msgSender(), recipient, amount);
2        return true;
3    }
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1    /**
2     * @dev See {IERC20-allowance}.
3     */
4    function allowance(address owner, address spender) public view virtual override returns (uint256) {
5        return _allowances[owner][spender];
6    }
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1    /**
2     * @dev See {IERC20-approve}.
3     *
4     * Requirements:
5     *
6     * - `spender` cannot be the zero address.
7     */
8    function approve(address spender, uint256 amount) public virtual override returns (bool) {
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1        _approve(_msgSender(), spender, amount);
2        return true;
3    }
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1    /**
2     * @dev See {IERC20-transferFrom}.
3     *
4     * Emits an {Approval} event indicating the updated allowance. This is not
5     * required by the EIP. See the note at the beginning of {ERC20}.
6     *
7     * Requirements:
8     *
9     * - `sender` and `recipient` cannot be the zero address.
10     * - `sender` must have a balance of at least `amount`.
11     * - the caller must have allowance for ``sender``'s tokens of at least
12     * `amount`.
13     */
14    function transferFrom(address sender, address recipient, uint256 amount) public virtual
15                                                override returns (bool) {
16        _transfer(sender, recipient, amount);
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1        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount,
2             "ERC20: transfer amount exceeds allowance"));
3        return true;
4    }
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1    /**
2     * @dev Atomically increases the allowance granted to `spender` by the caller.
3     *
4     * This is an alternative to {approve} that can be used as a mitigation for
5     * problems described in {IERC20-approve}.
6     *
7     * Emits an {Approval} event indicating the updated allowance.
8     *
9     * Requirements:
10     *
11     * - `spender` cannot be the zero address.
12     */
13    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
14        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
15        return true;
16    }
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1
2    /**
3     * @dev Atomically decreases the allowance granted to `spender` by the caller.
4     *
5     * This is an alternative to {approve} that can be used as a mitigation for
6     * problems described in {IERC20-approve}.
7     *
8     * Emits an {Approval} event indicating the updated allowance.
9     *
10     * Requirements:
11     *
12     * - `spender` cannot be the zero address.
13     * - `spender` must have allowance for the caller of at least
14     * `subtractedValue`.
15     */
16    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
17        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue,
18                "ERC20: decreased allowance below zero"));
19        return true;
20    }
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1    /**
2     * @dev Moves tokens `amount` from `sender` to `recipient`.
3     *
4     * This is internal function is equivalent to {transfer}, and can be used to
5     * e.g. implement automatic token fees, slashing mechanisms, etc.
6     *
7     * Emits a {Transfer} event.
8     *
9     * Requirements:
10     *
11     * - `sender` cannot be the zero address.
12     * - `recipient` cannot be the zero address.
13     * - `sender` must have a balance of at least `amount`.
14     */
15    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
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1        require(sender != address(0), "ERC20: transfer from the zero address");
2        require(recipient != address(0), "ERC20: transfer to the zero address");
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1        _beforeTokenTransfer(sender, recipient, amount);
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1        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
2        _balances[recipient] = _balances[recipient].add(amount);
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1        emit Transfer(sender, recipient, amount);
2    }
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1    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
2     * the total supply.
3     *
4     * Emits a {Transfer} event with `from` set to the zero address.
5     *
6     * Requirements:
7     *
8     * - `to` cannot be the zero address.
9     */
10    function _mint(address account, uint256 amount) internal virtual {
11        require(account != address(0), "ERC20: mint to the zero address");
12        _beforeTokenTransfer(address(0), account, amount);
13        _totalSupply = _totalSupply.add(amount);
14        _balances[account] = _balances[account].add(amount);
15        emit Transfer(address(0), account, amount);
16    }
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1    /**
2     * @dev Destroys `amount` tokens from `account`, reducing the
3     * total supply.
4     *
5     * Emits a {Transfer} event with `to` set to the zero address.
6     *
7     * Requirements:
8     *
9     * - `account` cannot be the zero address.
10     * - `account` must have at least `amount` tokens.
11     */
12    function _burn(address account, uint256 amount) internal virtual {
13        require(account != address(0), "ERC20: burn from the zero address");
14
15        _beforeTokenTransfer(account, address(0), amount);
16
17        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
18        _totalSupply = _totalSupply.sub(amount);
19        emit Transfer(account, address(0), amount);
20    }
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1    /**
2     * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
3     *
4     * This internal function is equivalent to `approve`, and can be used to
5     * e.g. set automatic allowances for certain subsystems, etc.
6     *
7     * Emits an {Approval} event.
8     *
9     * Requirements:
10     *
11     * - `owner` cannot be the zero address.
12     * - `spender` cannot be the zero address.
13     */
14    function _approve(address owner, address spender, uint256 amount) internal virtual {
15        require(owner != address(0), "ERC20: approve from the zero address");
16        require(spender != address(0), "ERC20: approve to the zero address");
17
18        _allowances[owner][spender] = amount;
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1        emit Approval(owner, spender, amount);
2    }
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1
2
3    /**
4     * @dev Sets {decimals} to a value other than the default one of 18.
5     *
6     * WARNING: This function should only be called from the constructor. Most
7     * applications that interact with token contracts will not expect
8     * {decimals} to ever change, and may work incorrectly if it does.
9     */
10    function _setupDecimals(uint8 decimals_) internal {
11        _decimals = decimals_;
12    }
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1
2    /**
3     * @dev Hook that is called before any transfer of tokens. This includes
4     * minting and burning.
5     *
6     * Calling conditions:
7     *
8     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
9     * will be to transferred to `to`.
10     * - when `from` is zero, `amount` tokens will be minted for `to`.
11     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
12     * - `from` and `to` are never both zero.
13     *
14     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
15     */
16    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
17}
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