NEXBIT (NXB)

/**
 * @title NEXBIT Token Contract
 * @dev Implementation of the NEXBIT ERC20 token
 * 
 * ███╗   ██╗███████╗██╗  ██╗██████╗ ██╗████████╗
 * ████╗  ██║██╔════╝╚██╗██╔╝██╔══██╗██║╚══██╔══╝
 * ██╔██╗ ██║█████╗   ╚███╔╝ ██████╔╝██║   ██║   
 * ██║╚██╗██║██╔══╝   ██╔██╗ ██╔══██╗██║   ██║   
 * ██║ ╚████║███████╗██╔╝ ██╗██████╔╝██║   ██║   
 * ╚═╝  ╚═══╝╚══════╝╚═╝  ╚═╝╚═════╝ ╚═╝   ╚═╝   
 * 
 * Token Name: NEXBIT
 * Symbol: NXB
 * Total Supply: 2,000,000,000 NXB
 * Decimals: 18
 */

// SPDX-License-Identifier: MIT

pragma solidity 0.8.6;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address recipient, uint256 amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @dev Interface for the optional metadata functions from the ERC20 standard.
 */
interface IERC20Metadata is IERC20 {
    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function decimals() external view returns (uint8);
}

/**
 * @dev Provides information about the current execution context.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
       return msg.data;
    }
}

/**
 * @dev Two-step ownership transfer mechanism for enhanced security.
 * Prevents accidental transfers to incorrect addresses.
 */
abstract contract Ownable2Step is Context {
    address private _owner;
    address private _pendingOwner;

    event OwnershipTransferStarted(address indexed previousOwner, address indexed newOwner);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    // Set deployer as initial owner
    constructor () {
        _owner = _msgSender();
        emit OwnershipTransferred(address(0), _owner);
    }

    function owner() public view virtual returns (address) {
        return _owner;
    }

    function pendingOwner() public view virtual returns (address) {
        return _pendingOwner;
    }

    modifier onlyOwner() {
        require(owner() == _msgSender(), "Ownable2Step: caller is not the owner");
        _;
    }

    modifier onlyPendingOwner() {
        require(_msgSender() == _pendingOwner, "Ownable2Step: caller is not the pending owner");
        _;
    }

    /**
     * @dev Step 1: Current owner initiates transfer
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable2Step: new owner is the zero address");
        _pendingOwner = newOwner;
        emit OwnershipTransferStarted(_owner, newOwner);
    }

    /**
     * @dev Step 2: New owner accepts transfer
     */
    function acceptOwnership() public virtual onlyPendingOwner {
        emit OwnershipTransferred(_owner, _pendingOwner);
        _owner = _pendingOwner;
        _pendingOwner = address(0);
    }
}

/**
 * @title NEXBIT Token
 * @dev Main token contract with ERC20 implementation
 */
contract NEXBIT is Context, IERC20, IERC20Metadata, Ownable2Step {
    // State variables
    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

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

    /**
     * @dev Constructor mints initial supply to deployer
     */
    constructor() {
        _name = "NEXBIT";
        _symbol = "NXB";
        // Mint 2 billion tokens with 18 decimals
        _mint(msg.sender, 2000000000 * 10**18); 
    }
    
    // Token metadata functions
    function name() public view virtual override returns (string memory) {
        return _name;
    }

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

    function decimals() public view virtual override returns (uint8) {
        return 18;
    }

    // Token supply and balance queries
    function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }

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

    /**
     * @dev Transfer tokens to a specified address
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

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

    /**
     * @dev Approve the passed address to spend the specified amount of tokens
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev Transfer tokens from one address to another using allowance
     */
    function transferFrom(address sender, address recipient, uint256 amount) 
        public 
        virtual 
        override 
        returns (bool) 
    {
        _transfer(sender, recipient, amount);

        // Update allowance
        uint256 currentAllowance = _allowances[sender][_msgSender()];
        require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
        _approve(sender, _msgSender(), currentAllowance - amount);

        return true;
    }

    /**
     * @dev Atomically increases the allowance (safer than approve)
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender] + addedValue);
        return true;
    }

    /**
     * @dev Atomically decreases the allowance (safer than approve)
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        uint256 currentAllowance = _allowances[_msgSender()][spender];
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
        _approve(_msgSender(), spender, currentAllowance - subtractedValue);
        return true;
    }

    /**
     * @dev Internal transfer function with balance checks
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        // Check balance and transfer
        uint256 senderBalance = _balances[sender];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
        _balances[sender] = senderBalance - amount;
        _balances[recipient] += amount;

        emit Transfer(sender, recipient, amount);
    }

    /**
     * @dev Creates new tokens (internal use only)
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply += amount;
        _balances[account] += amount;
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys tokens from account
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        uint256 accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
        _balances[account] = accountBalance - amount;
        _totalSupply -= amount;

        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Public burn function - only owner can burn their tokens
     */
    function burn(uint256 amount) public onlyOwner {
        _burn(msg.sender, amount);
    }
    
    /**
     * @dev Sets allowance amount for spender
     */
    function _approve(address owner, address spender, uint256 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);
    }

    // Hook for future extensions
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}