Negation of Signed Integers

Solidity provides several types to work with signed integers. Like in most programming languages, in Solidity a signed integer with N bits can represent values from -2^(N-1) to 2^(N-1)-1. This means that there is no positive equivalent for the MIN_INT. Negation is implemented as finding the two's complement of a number, so the negation of the most negative number will result in the same number.

This is true for all signed integer types in Solidity (int8, int16, ..., int256).

contract Negation {
    function negate8(int8 _i) public pure returns(int8) {
        return -_i;
    }

    function negate16(int16 _i) public pure returns(int16) {
        return -_i;
    }

    int8 public a = negate8(-128); // -128
    int16 public b = negate16(-128); // 128
    int16 public c = negate16(-32768); // -32768
}

One way to handle this is to check the value of a variable before negation and throw if it's equal to the MIN_INT. Another option is to make sure that the most negative number will never be achieved by using a type with a higher capacity (e.g. int32 instead of int16).

A similar issue with int types occurs when MIN_INT is multiplied or divided by -1.

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