Openzeppelin库第二期：Math库详解

• SafeMath.sol:   安全运算
• Math.sol:   uint256与uint64的最值获取

math.sol：

pragma solidity ^0.4.24;


/**
 * @title Math
 * @dev Assorted math operations
 */
library Math {
  // 取最大值
  function max64(uint64 _a, uint64 _b) internal pure returns (uint64) {
    return _a >= _b ? _a : _b;
  }
  // 取最小值
  function min64(uint64 _a, uint64 _b) internal pure returns (uint64) {
    return _a < _b ? _a : _b; // 三目运算符
  }

  function max256(uint256 _a, uint256 _b) internal pure returns (uint256) {
    return _a >= _b ? _a : _b;
  }

  function min256(uint256 _a, uint256 _b) internal pure returns (uint256) {
    return _a < _b ? _a : _b;
  }
}

SafeMath.sol：

pragma solidity ^0.4.24;


/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
 // 安全的数学运算库
library