SimpleERC20

// SPDX-License-Identifier: MIT
pragma solidity 0.8.30;


/// @title SimpleERC20 - fixed supply ERC20 token
/// @notice All tokens are minted to deployer on contract creation. Decimals = 18.
contract SimpleERC20 {
    string public name;
    string public symbol;
    uint8 public immutable decimals = 18;
    uint256 public totalSupply;

    mapping(address => uint256) private _balanceOf;
    mapping(address => mapping(address => uint256)) private _allowance;

    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,
        uint256 _initialSupply // supply in whole tokens (without decimals)
    ) {
        name = _name;
        symbol = _symbol;
        uint256 supplyWithDecimals = _initialSupply * (10 ** uint256(decimals));
        totalSupply = supplyWithDecimals;
        _balanceOf[msg.sender] = supplyWithDecimals;
        emit Transfer(address(0), msg.sender, supplyWithDecimals);
    }

    /* ===== ERC20 read functions ===== */
    function balanceOf(address account) external view returns (uint256) {
        return _balanceOf[account];
    }

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

    /* ===== ERC20 core functions ===== */
    function transfer(address to, uint256 amount) external returns (bool) {
        _transfer(msg.sender, to, amount);
        return true;
    }

    function approve(address spender, uint256 amount) external returns (bool) {
        _allowance[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(address from, address to, uint256 amount) external returns (bool) {
        uint256 currentAllowance = _allowance[from][msg.sender];
        require(currentAllowance >= amount, "ERC20: exceeds allowance");
        _allowance[from][msg.sender] = currentAllowance - amount;
        emit Approval(from, msg.sender, _allowance[from][msg.sender]);
        _transfer(from, to, amount);
        return true;
    }

    /* ===== internal helper ===== */
    function _transfer(address from, address to, uint256 amount) internal {
        require(to != address(0), "ERC20: transfer to zero");
        uint256 fromBal = _balanceOf[from];
        require(fromBal >= amount, "ERC20: exceeds balance");
        _balanceOf[from] = fromBal - amount;
        _balanceOf[to] += amount;
        emit Transfer(from, to, amount);
    }
}