Update EIP20.sol

contracts/eip20/EIP20.sol

@@ -1,72 +1,71 @@
-/*
-Implements EIP20 token standard: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
-.*/
-
-
-pragma solidity ^0.4.21;
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
 
+import "@openzeppelin/contracts/utils/math/SafeMath.sol";
 import "./EIP20Interface.sol";
 
-
 contract EIP20 is EIP20Interface {
+    using SafeMath for uint256;
+
+    mapping (address => uint256) private balances;
+    mapping (address => mapping (address => uint256)) private allowed;
 
-    uint256 constant private MAX_UINT256 = 2**256 - 1;
-    mapping (address => uint256) public balances;
-    mapping (address => mapping (address => uint256)) public allowed;
-    /*
-    NOTE:
-    The following variables are OPTIONAL vanities. One does not have to include them.
-    They allow one to customise the token contract & in no way influences the core functionality.
-    Some wallets/interfaces might not even bother to look at this information.
-    */
-    string public name;                   //fancy name: eg Simon Bucks
-    uint8 public decimals;                //How many decimals to show.
-    string public symbol;                 //An identifier: eg SBX
+    string public name;        // Название токена
+    uint8 public decimals;     // Количество знаков после запятой
+    string public symbol;      // Символ токена
+    uint256 public override totalSupply;
 
-    function EIP20(
+    event Transfer(address indexed _from, address indexed _to, uint256 _value);
+    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
+
+    constructor(
         uint256 _initialAmount,
-        string _tokenName,
+        string memory _tokenName,
         uint8 _decimalUnits,
-        string _tokenSymbol
-    ) public {
-        balances[msg.sender] = _initialAmount;               // Give the creator all initial tokens
-        totalSupply = _initialAmount;                        // Update total supply
-        name = _tokenName;                                   // Set the name for display purposes
-        decimals = _decimalUnits;                            // Amount of decimals for display purposes
-        symbol = _tokenSymbol;                               // Set the symbol for display purposes
+        string memory _tokenSymbol
+    ) {
+        balances[msg.sender] = _initialAmount;
+        totalSupply = _initialAmount;
+        name = _tokenName;
+        decimals = _decimalUnits;
+        symbol = _tokenSymbol;
     }
 
-    function transfer(address _to, uint256 _value) public returns (bool success) {
-        require(balances[msg.sender] >= _value);
-        balances[msg.sender] -= _value;
-        balances[_to] += _value;
-        emit Transfer(msg.sender, _to, _value); //solhint-disable-line indent, no-unused-vars
+    function transfer(address _to, uint256 _value) public override returns (bool success) {
+        require(_to != address(0), "Invalid address");
+        require(balances[msg.sender] >= _value, "Insufficient balance");
+
+        balances[msg.sender] = balances[msg.sender].sub(_value);
+        balances[_to] = balances[_to].add(_value);
+        emit Transfer(msg.sender, _to, _value);
         return true;
     }
 
-    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
-        uint256 allowance = allowed[_from][msg.sender];
-        require(balances[_from] >= _value && allowance >= _value);
-        balances[_to] += _value;
-        balances[_from] -= _value;
-        if (allowance < MAX_UINT256) {
-            allowed[_from][msg.sender] -= _value;
-        }
-        emit Transfer(_from, _to, _value); //solhint-disable-line indent, no-unused-vars
+    function transferFrom(address _from, address _to, uint256 _value) public override returns (bool success) {
+        require(_to != address(0), "Invalid address");
+        require(balances[_from] >= _value, "Insufficient balance");
+        require(allowed[_from][msg.sender] >= _value, "Allowance exceeded");
+
+        balances[_from] = balances[_from].sub(_value);
+        balances[_to] = balances[_to].add(_value);
+        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
+        emit Transfer(_from, _to, _value);
         return true;
     }
 
-    function balanceOf(address _owner) public view returns (uint256 balance) {
+    function balanceOf(address _owner) public view override returns (uint256 balance) {
         return balances[_owner];
     }
 
-    function approve(address _spender, uint256 _value) public returns (bool success) {
+    function approve(address _spender, uint256 _value) public override returns (bool success) {
+        // Безопасный паттерн: сначала установить allowance в 0 или требовать, чтобы текущее значение было 0
+        require(_value == 0 || allowed[msg.sender][_spender] == 0, "Must reset allowance to zero before changing it");
         allowed[msg.sender][_spender] = _value;
-        emit Approval(msg.sender, _spender, _value); //solhint-disable-line indent, no-unused-vars
+        emit Approval(msg.sender, _spender, _value);
         return true;
     }
 
-    function allowance(address _owner, address _spender) public view returns (uint256 remaining) {
+    function allowance(address _owner, address _spender) public view override returns (uint256 remaining) {
         return allowed[_owner][_spender];
     }
 }