ERC20.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract ERC20 {
    event Transfer(address indexed from, address indexed to, uint amount);
    event Approval(address indexed from, address indexed to, uint amount);

    address owner = msg.sender;
    address spender = msg.sender;

    mapping(address => uint) public _balances;
    mapping(address => mapping(address => uint)) public _allowances;

    uint public _totalSupply;
    string public _name;
    string public _symbol;

    constructor(string memory name_, string memory symbol_){
        _name = name_;
        _symbol = symbol_;
    }

    function name() public view virtual returns(string memory){
        return _name;
    }
    function symbol() public view virtual returns(string memory){
        return _symbol;
    }
    function decimals() public view virtual returns(uint8){
        return 18;
    }
    function totalSupply() public view virtual returns(uint){
        return _totalSupply;
    }
    function balanceOf(address account) public view virtual returns(uint){
        return _balances[account];
    }

    function transfer(address to, uint amount) public virtual returns(bool){
        _transfer(owner, to, amount);
        return true;
    }
    function allowance(address owner, address spender) public view virtual returns(uint){
        return _allowances[owner][spender];
    }
    function approve(address spender, uint amount) public virtual returns(bool){
        _approve(owner, spender, amount);
        return true;
    }
    function transferFrom(address from, address to, uint amount) public virtual returns(bool){
        _spendAllowance(from, spender, amount);
        _transfer(from, to, amount);
        return true;
    }

    function increaseAlowance(address spender, uint addedValue) public virtual returns(bool){
        _approve(owner, spender, allowance(owner, spender) + addedValue);
        return true;
    }
    function decreaseAllowance(address spender, uint subtractedValue) public virtual returns(bool){
        uint currentAllowance = allowance(owner, spender);
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
        unchecked {
            _approve(owner, spender, currentAllowance - subtractedValue);
        }
        return true;
    }
    function _transfer(address from, address to, uint amount) internal virtual {
        require(from != address(0), "ERC20: transfer from the zero address");
        require(to != address(0), "ERC20: transfer to the zero address");
        _beforeTokenTransfer(from, to, amount);
        uint fromBalance = _balances[from];
        require(fromBalance >= amount, "ERC20: transfer amount exceeds balance");
        unchecked {
            _balances[from] = fromBalance - amount;
            _balances[to] += amount;
        }
        emit Transfer(from, to, amount);
        _afterTokenTransfer(from, to, amount);
    }
    function _burn(address account, uint amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");
        _beforeTokenTransfer(account, address(0), amount);
        uint accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
        unchecked {
            _balances[account] = accountBalance - amount;
            _totalSupply -= amount;
        }
        emit Transfer(account, address(0), amount);
        _afterTokenTransfer(account, address(0), amount);
    }
    function _approve(address owner, address spender, uint 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);
    }
    function _spendAllowance(address from, address to, uint amount) internal virtual {
        uint currentAllowance = allowance(owner, spender);
        if (currentAllowance != type(uint).max){
            require(currentAllowance >= amount, "ERC20: insufficient allowance");
            unchecked {
                _approve(owner, spender, currentAllowance - amount);
            }
        }
    }
    function _beforeTokenTransfer(address from, address to, uint amount) internal virtual {}
    function _afterTokenTransfer(address from, address to, uint amount) internal virtual {}
}