// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

/**
 * YAQUT BITCOIN (YQB)
 * Ett BEP20-mynt för BNB Smart Chain.
 *
 * - Fast maxutbud: 21 000 000 YQB (18 decimaler), allt mintas till ägaren vid utläggning.
 * - Ingen ytterligare mint är möjlig: mint-funktionen finns inte.
 * - Ägaren kan bränna sina egna mynt och överlåta ägarskapet.
 * - Standard BEP20/ERC20: transfer, approve, transferFrom, allowance.
 *
 * Kontraktet är avsiktligt litet och utan externa beroenden så att det kan
 * klistras in direkt i Remix (https://remix.ethereum.org) och läggas ut.
 */
contract YaqutBitcoin {
    string public constant name = "YAQUT BITCOIN";
    string public constant symbol = "YQB";
    uint8 public constant decimals = 18;

    uint256 public totalSupply;
    address public owner;

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

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

    constructor() {
        owner = msg.sender;
        totalSupply = 21_000_000 * 10 ** uint256(decimals);
        balanceOf[msg.sender] = totalSupply;
        emit Transfer(address(0), msg.sender, totalSupply);
        emit OwnershipTransferred(address(0), msg.sender);
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "YQB: endast agaren");
        _;
    }

    function transfer(address to, uint256 value) external returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

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

    function transferFrom(address from, address to, uint256 value) external returns (bool) {
        uint256 allowed = allowance[from][msg.sender];
        require(allowed >= value, "YQB: for lag allowance");
        if (allowed != type(uint256).max) {
            allowance[from][msg.sender] = allowed - value;
        }
        _transfer(from, to, value);
        return true;
    }

    function burn(uint256 value) external {
        require(balanceOf[msg.sender] >= value, "YQB: for lagt saldo");
        balanceOf[msg.sender] -= value;
        totalSupply -= value;
        emit Transfer(msg.sender, address(0), value);
    }

    function transferOwnership(address newOwner) external onlyOwner {
        require(newOwner != address(0), "YQB: ogiltig adress");
        emit OwnershipTransferred(owner, newOwner);
        owner = newOwner;
    }

    function _transfer(address from, address to, uint256 value) internal {
        require(to != address(0), "YQB: ogiltig mottagare");
        require(balanceOf[from] >= value, "YQB: for lagt saldo");
        balanceOf[from] -= value;
        balanceOf[to] += value;
        emit Transfer(from, to, value);
    }
}