pragma solidity ^0.4.25;

contract ERC20Interface{ 
    
    function totalSupply() public view returns (uint);
    function balanceOf(address who) public view returns (uint);
    function transfer(address to, uint value) public returns (bool);
    event Transfer(address indexed from, address indexed to, uint value);

}

contract ERC20 is ERC20Interface{

    function allowance(address owner, address spender) public view returns (uint);
    function transferFrom(address from, address to, uint value) public returns (bool);
    function approve (address spender, uint value) public returns (bool);
    event Approval (address indexed owner, address indexed spender, uint value);

}

/**
 * @title SafeMath
 * @dev Math operations with safety checks that revert on error
 */
library SafeMath {

  /**
  * @dev Multiplies two numbers, reverts on overflow.
  */
  function mul(uint256 _a, uint256 _b) internal pure returns (uint256) {
    // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (_a == 0) {
      return 0;
    }

    uint256 c = _a * _b;
    require(c / _a == _b);

    return c;
  }

  /**
  * @dev Integer division of two numbers truncating the quotient, reverts on division by zero.
  */
  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
    require(_b > 0); // Solidity only automatically asserts when dividing by 0
    uint256 c = _a / _b;
    // assert(_a == _b * c + _a % _b); // There is no case in which this doesn't hold
    return c;
  }

  /**
  * @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
    require(_b <= _a);
    uint256 c = _a - _b;

    return c;
  }

  /**
  * @dev Adds two numbers, reverts on overflow.
  */
  function add(uint256 _a, uint256 _b) internal pure returns (uint256) {
    uint256 c = _a + _b;
    require(c >= _a);

    return c;
  }

  /**
  * @dev Divides two numbers and returns the remainder (unsigned integer modulo),
  * reverts when dividing by zero.
  */
  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b != 0);
    return a % b;
  }
}

//해당 컨트랙트는 인터페이스에서 선언한 함수들의 기능을 구현해준다.
contract BasicToken is ERC20Interface{
    using SafeMath for uint256;
    //using A for B : B 자료형에 A 라이브러리 함수를 붙여라.
    //dot(.)으로 호출 할수 있게됨.
    //ex) using SafeMath for uint256 이면 uint256자료형에 SafeMath 라이브러리 함수를 .을 이용해 사용가능하다는 뜻 => a.add(1) ,b.sub(2)를 사용가능하게 함.

    mapping (address => uint256) balances;


    uint totalSupply_;

// 토큰의 총 발행량을 구하는 함수.
  function totalSupply() public view returns (uint){
    return totalSupply_;
  }

  function transfer(address _to, uint _value) public returns (bool){
    require (_to != address(0));
    // address(0)은 값이 없다는 것.
    // require란 참이면 실행하는 것.
    require (_value <= balances[msg.sender]);
    // 함수를 호출한 '나'의 토큰 잔고가 보내는 토큰의 개수보다 크거나 같을때 실행.

    balances[msg.sender] = balances[msg.sender].sub(_value);
    //sub는 뺄셈. , 보낸 토큰개수만큼 뺀다.
    balances[_to] = balances[_to].add(_value);
    //add는 덧셈. , 받은 토큰개수 만큼 더한다.

    emit Transfer(msg.sender,_to,_value);
    // Transfer라는 이벤트를 실행하여 이더리움 블록체인상에 거래내역을 기록한다. 물론, 등록됬으므로 검색 가능.
    return true; //모든것이 실행되면 참을 출력.

  }

  function balanceOf(address _owner) public view returns(uint balance){
    return balances[_owner];
  }

}

contract StandardToken is ERC20, BasicToken{
  //ERC20에 선언된 인터페이스를 구현하는 컨트랙트.

  mapping (address => mapping (address => uint)) internal allowed;
  // allowed 매핑은 '누가','누구에게','얼마의' 인출권한을 줄지를 저장하는 것. ex) allowed[누가][누구에게] = 얼마;

  function transferFrom(address _from, address _to, uint _value) public returns (bool){
    require(_to != address(0));
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);
    //보내려는 토큰개수가 계좌주인 _from이 돈을 빼려는 msg.sender에게 허용한 개수보다 작거나 같으면 참.
    //_fromr에게 인출권한을 받은 msg.sender가 가스비를 소모함.

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    emit Transfer(_from,_to,_value);
    return true;

  }

  function approve(address _spender, uint _value) public returns (bool){
    allowed[msg.sender][_spender] = _value;
    //msg.sender의 계좌에서 _value 만큼 인출해 갈 수 있는 권리를 _spender 에게 부여한다.
    emit Approval(msg.sender,_spender,_value);
    return true;
  }

  function allowance(address _owner, address _spender) public view returns (uint){
    return allowed[_owner][_spender];
  }

}

contract JHToken is StandardToken{

  string public constant name = "JHToken";
  string public constant symbol = "JHTK";
  uint8 public constant decimals = 18;

  uint256 public constant INITIAL_SUPPLY =            4000000000 * (10**uint256(decimals));

  constructor() public{
    totalSupply_ = INITIAL_SUPPLY;
    balances[msg.sender] = INITIAL_SUPPLY;
    emit Transfer(0x0,msg.sender,INITIAL_SUPPLY);
  }
}