//SPDX-License-Identifier: MIT

pragma solidity 0.8.0;

interface IERC20 {
    function balanceOf(address who) external view returns (uint256);
    function transfer(address to, uint256 value) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function transferFrom(address from, address to, uint256 value) external returns (bool);
    function approve(address spender, uint256 value) external returns (bool);
}

contract Ownable {

  address private owner;

  event NewOwner(address oldOwner, address newOwner);

  constructor() {
    owner = msg.sender;
  }

  modifier onlyOwner() {
    require(isOwner(), "Ownable: caller is not the owner");
    _;
  }

  function contractOwner() external view returns (address) {
    return owner;
  }

  function isOwner() public view returns (bool) {
    return msg.sender == owner;
  }

  function transferOwnership(address _newOwner) external onlyOwner {
    require(_newOwner != address(0), 'Ownable: address is not valid');
    owner = _newOwner;
    emit NewOwner(msg.sender, _newOwner);
  } 
}

contract InitialD is IERC20, Ownable {

    string public name;
    string public symbol;
    uint8 public decimals;
    uint256 public totalSupply;

    mapping (address => uint256) internal _balances;
    mapping (address => mapping (address => uint256)) internal _allowed;

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    constructor () {
        symbol = "STT";
        name = "Smart Trade Token";
        decimals = 4;
        totalSupply = 10000000000000;
        _balances[msg.sender] = 1000000000000;
    }

    function transfer(
        address _to, 
        uint256 _value
    ) external override returns (bool) {
        require(_to != address(0), 'ERC20: to address is not valid');
        require(_value <= _balances[msg.sender], 'ERC20: insufficient balance');

        _balances[msg.sender] = _balances[msg.sender] - _value;
        _balances[_to] = _balances[_to] + _value;
        
        emit Transfer(msg.sender, _to, _value);
        
        return true;
    }

   function balanceOf(
       address _owner
    ) external override view returns (uint256 balance) {
        return _balances[_owner];
    }

    function approve(
       address _spender, 
       uint256 _value
    ) external override returns (bool) {
        _allowed[msg.sender][_spender] = _value;
        
        emit Approval(msg.sender, _spender, _value);
        
        return true;
   }

   function transferFrom(
        address _from, 
        address _to, 
        uint256 _value
    ) external override returns (bool) {
        require(_from != address(0), 'ERC20: from address is not valid');
        require(_to != address(0), 'ERC20: to address is not valid');
        require(_value <= _balances[_from], 'ERC20: insufficient balance');
        require(_value <= _allowed[_from][msg.sender], 'ERC20: transfer from value not allowed');

        _allowed[_from][msg.sender] = _allowed[_from][msg.sender] - _value;
        _balances[_from] = _balances[_from] - _value;
        _balances[_to] = _balances[_to] + _value;
        
        emit Transfer(_from, _to, _value);
        
        return true;
   }

    function allowance(
        address _owner, 
        address _spender
    ) external override view returns (uint256) {
        return _allowed[_owner][_spender];
    }

    function increaseApproval(
        address _spender, 
        uint256 _addedValue
    ) external returns (bool) {
        _allowed[msg.sender][_spender] = _allowed[msg.sender][_spender] + _addedValue;

        emit Approval(msg.sender, _spender, _allowed[msg.sender][_spender]);
        
        return true;
    }

    function decreaseApproval(
        address _spender, 
        uint256 _subtractedValue
    ) external returns (bool) {
        uint256 oldValue = _allowed[msg.sender][_spender];
        
        if (_subtractedValue > oldValue) {
            _allowed[msg.sender][_spender] = 0;
        } else {
            _allowed[msg.sender][_spender] = oldValue - _subtractedValue;
        }
        
        emit Approval(msg.sender, _spender, _allowed[msg.sender][_spender]);
        
        return true;
   }

    function mintTo(
        address _to,
        uint256 _amount
    ) external onlyOwner returns (bool) {
        require(_to != address(0), 'ERC20: to address is not valid');

        _balances[_to] = _balances[_to] + _amount;
        totalSupply = totalSupply + _amount;

        emit Transfer(address(0), _to, _amount);

        return true;
    }

    function burn(
        uint256 _amount
    ) external returns (bool) {
        require(_balances[msg.sender] >= _amount, 'ERC20: insufficient balance');

        _balances[msg.sender] = _balances[msg.sender] - _amount;
        totalSupply = totalSupply - _amount;

        emit Transfer(msg.sender, address(0), _amount);

        return true;
    }

    function burnFrom(
        address _from,
        uint256 _amount
    ) external returns (bool) {
        require(_from != address(0), 'ERC20: from address is not valid');
        require(_balances[_from] >= _amount, 'ERC20: insufficient balance');
        require(_amount <= _allowed[_from][msg.sender], 'ERC20: burn from value not allowed');
        
        _allowed[_from][msg.sender] = _allowed[_from][msg.sender] - _amount;
        _balances[_from] = _balances[_from] - _amount;
        totalSupply = totalSupply - _amount;

        emit Transfer(_from, address(0), _amount);

        return true;
    }

}