Create ERC20.sol

ERC20.sol

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+pragma solidity ^0.5.7;
+
+import "./IERC20.sol";
+import "./SafeMath.sol";
+
+/**
+ * @title Standard ERC20 token
+ *
+ * @dev Implementation of the basic standard token.
+ * https://eips.ethereum.org/EIPS/eip-20
+ * Originally based on code by FirstBlood:
+ * https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
+ *
+ * This implementation emits additional Approval events, allowing applications to reconstruct the allowance status for
+ * all accounts just by listening to said events. Note that this isn't required by the specification, and other
+ * compliant implementations may not do it.
+ */
+contract ERC20 is IERC20 {
+    using SafeMath for uint256;
+
+    mapping (address => uint256) private _balances;
+
+    mapping (address => mapping (address => uint256)) private _allowed;
+
+    uint256 private _totalSupply;
+
+    /**
+     * @dev Total number of tokens in existence.
+     */
+    function totalSupply() public view returns (uint256) {
+        return _totalSupply;
+    }
+
+    /**
+     * @dev Gets the balance of the specified address.
+     * @param owner The address to query the balance of.
+     * @return A uint256 representing the amount owned by the passed address.
+     */
+    function balanceOf(address owner) public view returns (uint256) {
+        return _balances[owner];
+    }
+
+    /**
+     * @dev Function to check the amount of tokens that an owner allowed to a spender.
+     * @param owner address The address which owns the funds.
+     * @param spender address The address which will spend the funds.
+     * @return A uint256 specifying the amount of tokens still available for the spender.
+     */
+    function allowance(address owner, address spender) public view returns (uint256) {
+        return _allowed[owner][spender];
+    }
+
+    /**
+     * @dev Transfer token to a specified address.
+     * @param to The address to transfer to.
+     * @param value The amount to be transferred.
+     */
+    function transfer(address to, uint256 value) public returns (bool) {
+        _transfer(msg.sender, to, value);
+        return true;
+    }
+
+    /**
+     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
+     * Beware that changing an allowance with this method brings the risk that someone may use both the old
+     * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
+     * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
+     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
+     * @param spender The address which will spend the funds.
+     * @param value The amount of tokens to be spent.
+     */
+    function approve(address spender, uint256 value) public returns (bool) {
+        require(value== 0 || _allowed[msg.sender][spender] == 0);
+        _approve(msg.sender, spender, value);
+        return true;
+    }
+
+    /**
+     * @dev Transfer tokens from one address to another.
+     * Note that while this function emits an Approval event, this is not required as per the specification,
+     * and other compliant implementations may not emit the event.
+     * @param from address The address which you want to send tokens from
+     * @param to address The address which you want to transfer to
+     * @param value uint256 the amount of tokens to be transferred
+     */
+    function transferFrom(address from, address to, uint256 value) public returns (bool) {
+        _transfer(from, to, value);
+        _approve(from, msg.sender, _allowed[from][msg.sender].sub(value));
+        return true;
+    }
+
+    /**
+     * @dev Increase the amount of tokens that an owner allowed to a spender.
+     * approve should be called when _allowed[msg.sender][spender] == 0. To increment
+     * allowed value is better to use this function to avoid 2 calls (and wait until
+     * the first transaction is mined)
+     * From MonolithDAO Token.sol
+     * Emits an Approval event.
+     * @param spender The address which will spend the funds.
+     * @param addedValue The amount of tokens to increase the allowance by.
+     */
+    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
+        _approve(msg.sender, spender, _allowed[msg.sender][spender].add(addedValue));
+        return true;
+    }
+
+    /**
+     * @dev Decrease the amount of tokens that an owner allowed to a spender.
+     * approve should be called when _allowed[msg.sender][spender] == 0. To decrement
+     * allowed value is better to use this function to avoid 2 calls (and wait until
+     * the first transaction is mined)
+     * From MonolithDAO Token.sol
+     * Emits an Approval event.
+     * @param spender The address which will spend the funds.
+     * @param subtractedValue The amount of tokens to decrease the allowance by.
+     */
+    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
+        _approve(msg.sender, spender, _allowed[msg.sender][spender].sub(subtractedValue));
+        return true;
+    }
+
+    /**
+     * @dev Transfer token for a specified addresses.
+     * @param from The address to transfer from.
+     * @param to The address to transfer to.
+     * @param value The amount to be transferred.
+     */
+    function _transfer(address from, address to, uint256 value) internal {
+        require(to != address(0), "ERC20: transfer to the zero address");
+
+        _balances[from] = _balances[from].sub(value);
+        _balances[to] = _balances[to].add(value);
+        emit Transfer(from, to, value);
+    }
+
+    /**
+     * @dev Internal function that mints an amount of the token and assigns it to
+     * an account. This encapsulates the modification of balances such that the
+     * proper events are emitted.
+     * @param account The account that will receive the created tokens.
+     * @param value The amount that will be created.
+     */
+    function _mint(address account, uint256 value) internal {
+        require(account != address(0), "ERC20: mint to the zero address");
+
+        _totalSupply = _totalSupply.add(value);
+        _balances[account] = _balances[account].add(value);
+        emit Transfer(address(0), account, value);
+    }
+
+    /**
+     * @dev Internal function that burns an amount of the token of a given
+     * account.
+     * @param account The account whose tokens will be burnt.
+     * @param value The amount that will be burnt.
+     */
+    function _burn(address account, uint256 value) internal {
+        require(account != address(0), "ERC20: burn from the zero address");
+
+        _totalSupply = _totalSupply.sub(value);
+        _balances[account] = _balances[account].sub(value);
+        emit Transfer(account, address(0), value);
+    }
+
+    /**
+     * @dev Approve an address to spend another addresses' tokens.
+     * @param owner The address that owns the tokens.
+     * @param spender The address that will spend the tokens.
+     * @param value The number of tokens that can be spent.
+     */
+    function _approve(address owner, address spender, uint256 value) internal {
+        require(owner != address(0), "ERC20: approve from the zero address");
+        require(spender != address(0), "ERC20: approve to the zero address");
+
+        _allowed[owner][spender] = value;
+        emit Approval(owner, spender, value);
+    }
+
+    /**
+     * @dev Internal function that burns an amount of the token of a given
+     * account, deducting from the sender's allowance for said account. Uses the
+     * internal burn function.
+     * Emits an Approval event (reflecting the reduced allowance).
+     * @param account The account whose tokens will be burnt.
+     * @param value The amount that will be burnt.
+     */
+    function _burnFrom(address account, uint256 value) internal {
+        _burn(account, value);
+        _approve(account, msg.sender, _allowed[account][msg.sender].sub(value));
+    }
+}