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 SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  
  function mul(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting '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) { //优化gas消耗，节省gas
      return 0;
    }

    c = _a * _b;
    assert(c / _a == _b); // 安全检查
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
    // assert(_b > 0); // Solidity automatically throws 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 _a / _b;
  }

  /**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
    assert(_b <= _a); // 判断 a 不能小于 b
    return _a - _b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    c = _a + _b;
    assert(c >= _a);
    return c;
  }
}