ERC20

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

interface IERC20 {
    function totalSupply() external view returns (uint256);
    function balanceOf(address who) external view returns (uint256);
    function allowance(address owner, address spender) external view returns (uint256);
    function transfer(address to, uint256 value) external returns (bool);
    function approve(address spender, uint256 value) external returns (bool);
    function transferFrom(address from, address to, uint256 value) external returns (bool);
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract ERC20 is IERC20 {
    mapping (address => uint256) private _balances;
    mapping (address => mapping (address => uint256)) private _allowed;

    uint256 private _totalSupply;
    string private _name;
    string private _symbol;
    uint8 private _decimals;

    constructor(
        string memory name_,
        string memory symbol_,
        uint8 decimals_,
        uint256 totalSupply_
    ) {
        _name = name_;
        _symbol = symbol_;
        _decimals = decimals_;
        _totalSupply = totalSupply_;
        _balances[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }

    function name() public view returns(string memory) { return _name; }
    function symbol() public view returns(string memory) { return _symbol; }
    function decimals() public view returns(uint8) { return _decimals; }
    function totalSupply() public view override returns (uint256) { return _totalSupply; }
    function balanceOf(address owner) public view override returns (uint256) { return _balances[owner]; }
    function allowance(address owner, address spender) public view override returns (uint256) { return _allowed[owner][spender]; }

    function transfer(address to, uint256 value) public override returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    function approve(address spender, uint256 value) public override returns (bool) {
        require(spender != address(0), "ERC20: approve to zero");
        _allowed[msg.sender][spender] = value;
        emit Approval(msg.sender, spender, value);
        return true;
    }

    function transferFrom(address from, address to, uint256 value) public override returns (bool) {
        require(value <= _allowed[from][msg.sender], "ERC20: allowance exceeded");
        _allowed[from][msg.sender] -= value;
        _transfer(from, to, value);
        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        require(spender != address(0), "ERC20: approve to zero");
        _allowed[msg.sender][spender] += addedValue;
        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        require(spender != address(0), "ERC20: approve to zero");
        _allowed[msg.sender][spender] -= subtractedValue;
        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
        return true;
    }

    function _transfer(address from, address to, uint256 value) internal {
        require(to != address(0), "ERC20: transfer to zero");
        require(value <= _balances[from], "ERC20: insufficient balance");
        _balances[from] -= value;
        _balances[to] += value;
        emit Transfer(from, to, value);
    }
}