Sample ERC20 Contract

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.28;

// OpenZeppelin Ownable module for owner-based access control
import "@openzeppelin/contracts/access/Ownable.sol";

/**
 * @title Sample20Token
 * @dev A custom ERC20 token contract implementing essential ERC20 functionality.
 *      - Implements standard functions: transfer, approve, transferFrom
 *      - Only the owner can call mint and burn
 *      - Uses OpenZeppelin's Ownable for access control (deployer as owner)
 */
contract Sample20Token is Ownable {
    // ----------------------
    // ░░░ Custom Errors ░░░
    // ----------------------

    error ERC20InsufficientBalance(address sender, uint256 balance, uint256 needed); // Insufficient balance
    error ERC20InvalidSender(address sender); // Invalid sender address
    error ERC20InvalidReceiver(address receiver); // Invalid receiver address
    error ERC20InsufficientAllowance(address spender, uint256 allowance, uint256 needed); // Allowance too low
    error ERC20InvalidApprover(address approver); // Invalid approver address
    error ERC20InvalidSpender(address spender); // Invalid spender address

    // ----------------------
    // ░░░ Events ░░░
    // ----------------------

    event Transfer(address indexed from, address indexed to, uint256 value); // Emitted on token transfer
    event Approval(address indexed owner, address indexed spender, uint256 value); // Emitted on approval

    // ----------------------
    // ░░░ State Variables ░░░
    // ----------------------

    mapping(address => uint256) private _balances; // Token balance by address
    mapping(address => mapping(address => uint256)) private _allowances; // owner → spender → allowance

    uint256 public totalSupply;      // Total token supply
    string public name;              // Token name
    string public symbol;            // Token symbol
    uint8 public immutable decimals; // Number of decimals

    // ----------------------
    // ░░░ Constructor ░░░
    // ----------------------

    /**
     * @param name_ Token name
     * @param symbol_ Token symbol
     * @param decimals_ Number of decimals (commonly 18)
     */
    constructor(string memory name_, string memory symbol_, uint8 decimals_) Ownable(msg.sender) {
        name = name_;
        symbol = symbol_;
        decimals = decimals_;
    }

    // ----------------------
    // ░░░ Owner-Only Functions ░░░
    // ----------------------

    /// @notice Mints new tokens to a specified address.
    /// @dev Transfers from address(0) to `to`, emits Transfer event.
    /// @param to Recipient address
    /// @param value Amount to mint
    function mint(address to, uint256 value) external onlyOwner returns (bool) {
        if (to == address(0)) {
            revert ERC20InvalidReceiver(address(0));
        }
        _update(address(0), to, value);
        return true;
    }

    /// @notice Burns tokens from a specified address.
    /// @dev Transfers from `from` to address(0), emits Transfer event.
    /// @param from Address to burn from
    /// @param value Amount to burn
    function burn(address from, uint256 value) external onlyOwner returns (bool) {
        if (from == address(0)) {
            revert ERC20InvalidSender(address(0));
        }
        _update(from, address(0), value);
        return true;
    }

    // ----------------------
    // ░░░ ERC20 Standard Functions ░░░
    // ----------------------

    /// @notice Returns the token balance of a specific account.
    /// @param account Address to query
    /// @return Token balance
    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    /// @notice Transfers tokens from the caller to a recipient.
    /// @param to Recipient address
    /// @param value Amount to transfer
    /// @return Success flag
    function transfer(address to, uint256 value) public virtual returns (bool) {
        address owner = _msgSender();
        _transfer(owner, to, value);
        return true;
    }

    /// @notice Transfers tokens from `from` to `to` using allowance.
    /// @param from Token owner
    /// @param to Recipient
    /// @param value Amount to transfer
    /// @return Success flag
    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        address spender = _msgSender();
        _spendAllowance(from, spender, value);
        _transfer(from, to, value);
        return true;
    }

    /// @notice Returns the remaining allowance from owner to spender.
    /// @param owner Token owner
    /// @param spender Spender address
    /// @return Remaining allowance
    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];
    }

    /// @notice Approves a spender to use tokens on behalf of the caller.
    /// @param spender Address to approve
    /// @param value Allowance amount
    /// @return Success flag
    function approve(address spender, uint256 value) public returns (bool) {
        address owner = _msgSender();
        _approve(owner, spender, value, true);
        return true;
    }

    // ----------------------
    // ░░░ Internal Logic ░░░
    // ----------------------

    /// @dev Internal approval logic.
    /// @param owner Token owner
    /// @param spender Spender address
    /// @param value Allowance amount
    /// @param emitEvent Whether to emit Approval event
    function _approve(address owner, address spender, uint256 value, bool emitEvent) internal virtual {
        if (owner == address(0)) {
            revert ERC20InvalidApprover(address(0));
        }
        if (spender == address(0)) {
            revert ERC20InvalidSpender(address(0));
        }

        _allowances[owner][spender] = value;

        if (emitEvent) {
            emit Approval(owner, spender, value);
        }
    }

    /// @dev Deducts allowance from the spender.
    /// @param owner Token owner
    /// @param spender Caller (spender)
    /// @param value Amount to spend
    function _spendAllowance(address owner, address spender, uint256 value) internal virtual {
        uint256 currentAllowance = allowance(owner, spender);

        if (currentAllowance < type(uint256).max) {
            if (currentAllowance < value) {
                revert ERC20InsufficientAllowance(spender, currentAllowance, value);
            }
            unchecked {
                _approve(owner, spender, currentAllowance - value, false);
            }
        }
    }

    /// @dev Internal transfer logic.
    /// @param from Sender address
    /// @param to Recipient address
    /// @param value Amount to transfer
    function _transfer(address from, address to, uint256 value) internal {
        if (from == address(0)) {
            revert ERC20InvalidSender(address(0));
        }
        if (to == address(0)) {
            revert ERC20InvalidReceiver(address(0));
        }
        _update(from, to, value);
    }

    /// @dev Unified logic for mint, burn, and transfer.
    /// @param from Sender address (mint: 0x0)
    /// @param to Recipient address (burn: 0x0)
    /// @param value Token amount to process
    function _update(address from, address to, uint256 value) internal virtual {
        if (from == address(0)) {
            // Mint: increase total supply
            totalSupply += value;
        } else {
            // Decrease sender balance
            uint256 fromBalance = _balances[from];
            if (fromBalance < value) {
                revert ERC20InsufficientBalance(from, fromBalance, value);
            }
            unchecked {
                _balances[from] = fromBalance - value;
            }
        }

        if (to == address(0)) {
            // Burn: decrease total supply
            unchecked {
                totalSupply -= value;
            }
        } else {
            // Increase recipient balance
            unchecked {
                _balances[to] += value;
            }
        }

        emit Transfer(from, to, value);
    }
}