SimpleToken.sol

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

contract SimpleERC20 {
    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

    uint256 private _totalSupply;
    string private _name;
    string private _symbol;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

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

    function name() public view returns (string memory) {
        return _name;
    }

    function symbol() public view returns (string memory) {
        return _symbol;
    }

    function decimals() public pure returns (uint8) {
        return 18;
    }

    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

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

    function _transfer(address from, address to, uint256 value) internal {
        require(from != address(0), "ERC20: transfer from the zero address");
        require(to != address(0), "ERC20: transfer to the zero address");
        require(_balances[from] >= value, "ERC20: transfer amount exceeds balance");

        _balances[from] -= value;
        _balances[to] += value;

        emit Transfer(from, to, value);
    }

    function _mint(address account, uint256 value) internal {
        require(account != address(0), "ERC20: mint to the zero address");
        _totalSupply += value;
        _balances[account] += value;
        emit Transfer(address(0), account, value);
    }

    function _burn(address account, uint256 value) internal {
        require(account != address(0), "ERC20: burn from the zero address");
        require(_balances[account] >= value, "ERC20: burn amount exceeds balance");

        _balances[account] -= value;
        _totalSupply -= value;
        emit Transfer(account, address(0), value);
    }

    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];
    }

    function approve(address spender, uint256 value) public returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    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");
        _allowances[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        address spender = msg.sender;

        uint256 currentAllowance = _allowances[from][spender];
        require(currentAllowance >= value, "ERC20: transfer amount exceeds allowance");

        _approve(from, spender, currentAllowance - value);

        _transfer(from, to, value);
        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        _approve(msg.sender, spender, allowance(msg.sender, spender) + addedValue);
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        uint256 currentAllowance = allowance(msg.sender, spender);
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
        unchecked {
            _approve(msg.sender, spender, currentAllowance - subtractedValue);
        }
        return true;
    }
}