openzeppelin-contracts/contracts/drafts/SignedSafeMath.sol

pragma solidity ^0.5.7;

/**
 * @title SignedSafeMath
 * @dev Signed math operations with safety checks that revert on error.
 */
library SignedSafeMath {
    int256 constant private INT256_MIN = -2**255;

    /**
     * @dev Multiplies two signed integers, reverts on overflow.
     */
    function mul(int256 a, int256 b) internal pure returns (int256) {
        // 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;
        }

        require(!(a == -1 && b == INT256_MIN)); // This is the only case of overflow not detected by the check below

        int256 c = a * b;
        require(c / a == b);

        return c;
    }

    /**
     * @dev Integer division of two signed integers truncating the quotient, reverts on division by zero.
     */
    function div(int256 a, int256 b) internal pure returns (int256) {
        require(b != 0); // Solidity only automatically asserts when dividing by 0
        require(!(b == -1 && a == INT256_MIN)); // This is the only case of overflow

        int256 c = a / b;

        return c;
    }

    /**
     * @dev Subtracts two signed integers, reverts on overflow.
     */
    function sub(int256 a, int256 b) internal pure returns (int256) {
        int256 c = a - b;
        require((b >= 0 && c <= a) || (b < 0 && c > a));

        return c;
    }

    /**
     * @dev Adds two signed integers, reverts on overflow.
     */
    function add(int256 a, int256 b) internal pure returns (int256) {
        int256 c = a + b;
        require((b >= 0 && c >= a) || (b < 0 && c < a));

        return c;
    }
}