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

/**
 * @title AfterCoin (AFC)
 * @notice ERC-20 token for the Afterlife Project game preservation platform.
 *         1 AFC = 1 diamond. Zero decimals — integer-only balances.
 * @dev Self-contained implementation (no OpenZeppelin). Owner-gated mint,
 *      burn-from, and bridge-transfer helpers for the off-chain bridge service.
 */
contract AfterCoin {

    // ──────────────────────────── ERC-20 metadata ────────────────────────────

    string private constant _name   = "AfterCoin";
    string private constant _symbol = "AFC";
    uint8  private constant _decimals = 0;   // 1 token = 1 diamond

    // ──────────────────────────── State ──────────────────────────────────────

    uint256 private _totalSupply;

    mapping(address => uint256)                     private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

    address public owner;

    // ──────────────────────────── Events (ERC-20) ────────────────────────────

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

    // ──────────────────────────── Errors ─────────────────────────────────────

    error NotOwner();
    error ZeroAddress();
    error InsufficientBalance(address account, uint256 required, uint256 available);
    error InsufficientAllowance(address spender, uint256 required, uint256 available);

    // ──────────────────────────── Modifier ───────────────────────────────────

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

    // ──────────────────────────── Constructor ────────────────────────────────

    constructor() {
        owner = msg.sender;
        // Initial supply is 0 — tokens are minted on demand by the bridge.
    }

    // ──────────────────────────── ERC-20 view functions ──────────────────────

    function name() external pure returns (string memory) {
        return _name;
    }

    function symbol() external pure returns (string memory) {
        return _symbol;
    }

    function decimals() external pure returns (uint8) {
        return _decimals;
    }

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

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

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

    // ──────────────────────────── ERC-20 mutative 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) {
        _approve(msg.sender, spender, amount);
        return true;
    }

    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) external returns (bool) {
        uint256 currentAllowance = _allowances[from][msg.sender];
        if (currentAllowance != type(uint256).max) {
            if (currentAllowance < amount) {
                revert InsufficientAllowance(msg.sender, amount, currentAllowance);
            }
            unchecked {
                _approve(from, msg.sender, currentAllowance - amount);
            }
        }
        _transfer(from, to, amount);
        return true;
    }

    // ──────────────────────────── Owner-only functions ───────────────────────

    /**
     * @notice Mint new tokens to `to`. Only callable by the contract owner.
     * @param to   Recipient address.
     * @param amount Number of tokens to create.
     */
    function mint(address to, uint256 amount) external onlyOwner {
        if (to == address(0)) revert ZeroAddress();
        _totalSupply += amount;
        _balances[to] += amount;
        emit Transfer(address(0), to, amount);
    }

    /**
     * @notice Burn tokens from `from`. Only callable by the contract owner.
     *         Does NOT require an allowance — the owner is the bridge operator.
     * @param from   Address whose tokens are burned.
     * @param amount Number of tokens to destroy.
     */
    function burnFrom(address from, uint256 amount) external onlyOwner {
        if (from == address(0)) revert ZeroAddress();
        uint256 bal = _balances[from];
        if (bal < amount) {
            revert InsufficientBalance(from, amount, bal);
        }
        unchecked {
            _balances[from] = bal - amount;
        }
        _totalSupply -= amount;
        emit Transfer(from, address(0), amount);
    }

    /**
     * @notice Transfer tokens between two addresses on behalf of the bridge.
     *         Only callable by the contract owner.
     * @param from   Source address.
     * @param to     Destination address.
     * @param amount Number of tokens to move.
     */
    function bridgeTransfer(
        address from,
        address to,
        uint256 amount
    ) external onlyOwner {
        _transfer(from, to, amount);
    }

    // ──────────────────────────── Internal helpers ───────────────────────────

    function _transfer(address from, address to, uint256 amount) internal {
        if (from == address(0)) revert ZeroAddress();
        if (to == address(0)) revert ZeroAddress();

        uint256 fromBal = _balances[from];
        if (fromBal < amount) {
            revert InsufficientBalance(from, amount, fromBal);
        }
        unchecked {
            _balances[from] = fromBal - amount;
        }
        _balances[to] += amount;

        emit Transfer(from, to, amount);
    }

    function _approve(address tokenOwner, address spender, uint256 amount) internal {
        if (tokenOwner == address(0)) revert ZeroAddress();
        if (spender == address(0)) revert ZeroAddress();

        _allowances[tokenOwner][spender] = amount;
        emit Approval(tokenOwner, spender, amount);
    }
}