pragma solidity ^0.5.6;

/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error.
 */
library SafeMath {
  /**
   * @dev Multiplies two unsigned integers, 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 unsigned integers truncating the quotient, reverts on division by zero.
   */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // Solidity only automatically asserts when dividing by 0
    require(b > 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 unsigned integers, 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 unsigned integers, 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 unsigned integers 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;
  }
}

/**
 * @title ERC20 interface
 * @dev see https://eips.ethereum.org/EIPS/eip-20
 */
interface IERC20 {
  function transfer(address to, uint256 value) external returns (bool);

  function approve(address spender, uint256 value) external returns (bool);

  function transferFrom(address from, address to, uint256 value)
    external
    returns (bool);

  function totalSupply() external view returns (uint256);

  function balanceOf(address who) external view returns (uint256);

  function allowance(address owner, address spender)
    external
    view
    returns (uint256);

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

  event Approval(address indexed owner, address indexed spender, uint256 value);
}

/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * https://eips.ethereum.org/EIPS/eip-20
 * Originally based on OpenZeppelin implementation
 * https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/token/ERC20/ERC20.sol
 *
 * This implementation is meant to be a migration of the original AURA token. On deployment
 * the total supply is set to equal that of AURA and locked in the new contract. New tokens
 * can only be obtained by calling the swap function
 */
contract IDEX is IERC20 {
  using SafeMath for uint256;

  mapping(address => uint256) private _balances;
  mapping(address => mapping(address => uint256)) private _allowed;
  uint256 private _totalSupply;
  string private _name;
  string private _symbol;
  uint8 private _decimals;
  IERC20 _oldToken;

  event Swap(address indexed owner, uint256 value);

  constructor(
    string memory name,
    string memory symbol,
    uint8 decimals,
    IERC20 oldToken
  ) public {
    _name = name;
    _symbol = symbol;
    _decimals = decimals;
    _totalSupply = oldToken.totalSupply();
    _balances[address(this)] = _totalSupply;
    _oldToken = oldToken;

    emit Transfer(address(0), address(this), _totalSupply);
  }

  function swap(uint256 value) external returns (bool) {
    require(
      _oldToken.transferFrom(msg.sender, address(this), value),
      "AURA transfer failed"
    );
    require(this.transfer(msg.sender, value), "IDEX transfer failed");

    emit Swap(msg.sender, value);

    return true;
  }

  /**
   * @return the name of the token.
   */
  function name() public view returns (string memory) {
    return _name;
  }

  /**
   * @return the symbol of the token.
   */
  function symbol() public view returns (string memory) {
    return _symbol;
  }

  /**
   * @return the number of decimals of the token.
   */
  function decimals() public view returns (uint8) {
    return _decimals;
  }

  /**
   * @dev Total number of tokens in existence.
   */
  function totalSupply() public view returns (uint256) {
    return _totalSupply;
  }

  /**
   * @dev Gets the balance of the specified address.
   * @param owner The address to query the balance of.
   * @return A uint256 representing the amount owned by the passed address.
   */
  function balanceOf(address owner) public view returns (uint256) {
    return _balances[owner];
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param owner address The address which owns the funds.
   * @param spender address The address which will spend the funds.
   * @return A uint256 specifying the amount of tokens still available for the spender.
   */
  function allowance(address owner, address spender)
    public
    view
    returns (uint256)
  {
    return _allowed[owner][spender];
  }

  /**
   * @dev Transfer token to a specified address.
   * @param to The address to transfer to.
   * @param value The amount to be transferred.
   */
  function transfer(address to, uint256 value) public returns (bool) {
    _transfer(msg.sender, to, value);
    return true;
  }

  /**
   * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   * Beware that changing an allowance with this method brings the risk that someone may use both the old
   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   * @param spender The address which will spend the funds.
   * @param value The amount of tokens to be spent.
   */
  function approve(address spender, uint256 value) public returns (bool) {
    _approve(msg.sender, spender, value);
    return true;
  }

  /**
   * @dev Transfer tokens from one address to another.
   * Note that while this function emits an Approval event, this is not required as per the specification,
   * and other compliant implementations may not emit the event.
   * @param from address The address which you want to send tokens from
   * @param to address The address which you want to transfer to
   * @param value uint256 the amount of tokens to be transferred
   */
  function transferFrom(address from, address to, uint256 value)
    public
    returns (bool)
  {
    _transfer(from, to, value);
    _approve(from, msg.sender, _allowed[from][msg.sender].sub(value));
    return true;
  }

  /**
   * @dev Increase the amount of tokens that an owner allowed to a spender.
   * approve should be called when _allowed[msg.sender][spender] == 0. To increment
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   * Emits an Approval event.
   * @param spender The address which will spend the funds.
   * @param addedValue The amount of tokens to increase the allowance by.
   */
  function increaseAllowance(address spender, uint256 addedValue)
    public
    returns (bool)
  {
    _approve(
      msg.sender,
      spender,
      _allowed[msg.sender][spender].add(addedValue)
    );
    return true;
  }

  /**
   * @dev Decrease the amount of tokens that an owner allowed to a spender.
   * approve should be called when _allowed[msg.sender][spender] == 0. To decrement
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   * Emits an Approval event.
   * @param spender The address which will spend the funds.
   * @param subtractedValue The amount of tokens to decrease the allowance by.
   */
  function decreaseAllowance(address spender, uint256 subtractedValue)
    public
    returns (bool)
  {
    _approve(
      msg.sender,
      spender,
      _allowed[msg.sender][spender].sub(subtractedValue)
    );
    return true;
  }

  /**
   * @dev Transfer token for a specified addresses.
   * @param from The address to transfer from.
   * @param to The address to transfer to.
   * @param value The amount to be transferred.
   */
  function _transfer(address from, address to, uint256 value) internal {
    require(from != address(0), "Invalid from");
    require(to != address(0), "Invalid to");

    _balances[from] = _balances[from].sub(value);
    _balances[to] = _balances[to].add(value);
    emit Transfer(from, to, value);
  }

  /**
   * @dev Approve an address to spend another addresses' tokens.
   * @param owner The address that owns the tokens.
   * @param spender The address that will spend the tokens.
   * @param value The number of tokens that can be spent.
   */
  function _approve(address owner, address spender, uint256 value) internal {
    require(spender != address(0), "Invalid spender");

    _allowed[owner][spender] = value;
    emit Approval(owner, spender, value);
  }
}