The Digital Dollar (USD)

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

/// @title The Digital Dollar (USD) - ERC20 compliant token for DEX use
/// @notice Standard ERC20 with admin mint/burn and address freeze for compliance
contract TheDigitalDollar {
    string public constant name = "The Digital Dollar";
    string public constant symbol = "USD";
    uint8 public constant decimals = 18;

    uint256 public totalSupply;

    address public owner;
    address public compliance;

    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;
    mapping(address => bool) public frozen;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed ownerAddr, address indexed spender, uint256 value);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    event ComplianceTransferred(address indexed previousCompliance, address indexed newCompliance);
    event AddressFrozen(address indexed account, bool frozen);
    event Mint(address indexed to, uint256 amount);
    event Burn(address indexed from, uint256 amount);

    error NotOwner();
    error NotCompliance();
    error FrozenAddress();
    error InsufficientBalance();
    error AllowanceExceeded();
    error ZeroAddress();

    uint256 public constant INITIAL_SUPPLY = 100_000_000_000_000 * 1e18;

    constructor() {
        owner = msg.sender;
        compliance = msg.sender;
        totalSupply = INITIAL_SUPPLY;
        balanceOf[msg.sender] = INITIAL_SUPPLY;
        emit Transfer(address(0), msg.sender, INITIAL_SUPPLY);
    }

    modifier onlyOwner() {
        if (msg.sender != owner) revert NotOwner();
        _;
    }

    modifier onlyCompliance() {
        if (msg.sender != compliance) revert NotCompliance();
        _;
    }

    modifier notFrozen(address account) {
        if (frozen[account]) revert FrozenAddress();
        _;
    }

    function transfer(address to, uint256 value)
        external
        notFrozen(msg.sender)
        notFrozen(to)
        returns (bool)
    {
        uint256 senderBalance = balanceOf[msg.sender];
        if (senderBalance < value) revert InsufficientBalance();
        unchecked {
            balanceOf[msg.sender] = senderBalance - value;
            balanceOf[to] += value;
        }
        emit Transfer(msg.sender, to, value);
        return true;
    }

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

    function transferFrom(address from, address to, uint256 value)
        external
        notFrozen(msg.sender)
        notFrozen(from)
        notFrozen(to)
        returns (bool)
    {
        uint256 fromBalance = balanceOf[from];
        if (fromBalance < value) revert InsufficientBalance();
        uint256 allowed = allowance[from][msg.sender];
        if (allowed < value) revert AllowanceExceeded();
        if (allowed != type(uint256).max) {
            unchecked {
                allowance[from][msg.sender] = allowed - value;
            }
            emit Approval(from, msg.sender, allowance[from][msg.sender]);
        }
        unchecked {
            balanceOf[from] = fromBalance - value;
            balanceOf[to] += value;
        }
        emit Transfer(from, to, value);
        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue)
        external
        notFrozen(msg.sender)
        notFrozen(spender)
        returns (bool)
    {
        uint256 newAllowance = allowance[msg.sender][spender] + addedValue;
        allowance[msg.sender][spender] = newAllowance;
        emit Approval(msg.sender, spender, newAllowance);
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue)
        external
        notFrozen(msg.sender)
        notFrozen(spender)
        returns (bool)
    {
        uint256 current = allowance[msg.sender][spender];
        uint256 newAllowance = (subtractedValue >= current) ? 0 : current - subtractedValue;
        allowance[msg.sender][spender] = newAllowance;
        emit Approval(msg.sender, spender, newAllowance);
        return true;
    }

    function mint(address to, uint256 amount) external onlyOwner {
        if (to == address(0)) revert ZeroAddress();
        totalSupply += amount;
        balanceOf[to] += amount;
        emit Mint(to, amount);
        emit Transfer(address(0), to, amount);
    }

    function burn(address from, uint256 amount) external onlyOwner {
        if (from == address(0)) revert ZeroAddress();
        uint256 bal = balanceOf[from];
        if (bal < amount) revert InsufficientBalance();
        unchecked {
            balanceOf[from] = bal - amount;
            totalSupply -= amount;
        }
        emit Burn(from, amount);
        emit Transfer(from, address(0), amount);
    }

    function burnFrom(address from, uint256 amount)
        external
        notFrozen(msg.sender)
        notFrozen(from)
    {
        uint256 allowed = allowance[from][msg.sender];
        if (allowed < amount) revert AllowanceExceeded();
        if (allowed != type(uint256).max) {
            unchecked {
                allowance[from][msg.sender] = allowed - amount;
            }
            emit Approval(from, msg.sender, allowance[from][msg.sender]);
        }
        uint256 bal = balanceOf[from];
        if (bal < amount) revert InsufficientBalance();
        unchecked {
            balanceOf[from] = bal - amount;
            totalSupply -= amount;
        }
        emit Burn(from, amount);
        emit Transfer(from, address(0), amount);
    }

    function setCompliance(address newCompliance) external onlyOwner {
        if (newCompliance == address(0)) revert ZeroAddress();
        emit ComplianceTransferred(compliance, newCompliance);
        compliance = newCompliance;
    }

    function transferOwnership(address newOwner) external onlyOwner {
        if (newOwner == address(0)) revert ZeroAddress();
        emit OwnershipTransferred(owner, newOwner);
        owner = newOwner;
    }

    function setFrozen(address account, bool isFrozen) external onlyCompliance {
        frozen[account] = isFrozen;
        emit AddressFrozen(account, isFrozen);
    }
}