1  pragma solidity ^0.5.2;
 2  
 3  
 4  library SafeMath {
 5      /**
 6          @dev returns the sum of _x and _y, reverts if the calculation overflows
 7          @param _x   value 1
 8          @param _y   value 2
 9          @return sum
10      */
11      function add(uint256 _x, uint256 _y) internal pure returns (uint256) {
12          uint256 z = _x + _y;
13          require(z >= _x, "SafeMath failed");
14          return z;
15      }
16  
17      /**
18          @dev returns the difference of _x minus _y, reverts if the calculation underflows
19          @param _x   minuend
20          @param _y   subtrahend
21          @return difference
22      */
23      function sub(uint256 _x, uint256 _y) internal pure returns (uint256) {
24          require(_x >= _y, "SafeMath failed");
25          return _x - _y;
26      }
27  
28      /**
29          @dev returns the product of multiplying _x by _y, reverts if the calculation overflows
30          @param _x   factor 1
31          @param _y   factor 2
32          @return product
33      */
34      function mul(uint256 _x, uint256 _y) internal pure returns (uint256) {
35          // gas optimization
36          if (_x == 0)
37              return 0;
38  
39          uint256 z = _x * _y;
40          require(z / _x == _y, "SafeMath failed");
41          return z;
42      }
43  
44        /**
45          @dev Integer division of two numbers truncating the quotient, reverts on division by zero.
46          @param _x   dividend
47          @param _y   divisor
48          @return quotient
49      */
50      function div(uint256 _x, uint256 _y) internal pure returns (uint256) {
51          require(_y > 0, "SafeMath failed");
52          uint256 c = _x / _y;
53  
54          return c;
55      }
56  }