Javascript补码

Question

The following python code seems to work very well to het the twos complement of a number:

def twos_comp(self, val, bits):
    if (val & (1 << (bits - 1))) != 0:
        val -= 1 << bits
    return val

It is used as num = self.twos_comp(int(binStr, 2), len(binStr)) where binStr is a string that contains an arbitrary length binary number.

I need to do the exact same thing in javascript (for node.js). I've been fighting it all day and am about to resign from the human race. Clearly binary / bitwise math is not my strong suit.

Could someone please assist so I can go on to more productive time wasting :-)

Answer 1

function toTwosComplement(integer, numberBytes, dontCheckRange) {   
    // @integer - > the integer to convert
    // @numberBytes -> the number of bytes representing the number (defaults to 1 if not specified)

    var numberBits = (numberBytes || 1) * 8;

    // make sure its in range given the number of bits
    if (!dontCheckRange && (integer < (-(1 << (numberBits - 1))) || integer > ((1 << (numberBits - 1)) - 1))) 
        throw "Integer out of range given " + numberBytes + " byte(s) to represent.";

    // if positive, return the positive value
    if (integer >= 0)
        return integer;

    // if negative, convert to twos complement representation
    return ~(((-integer) - 1) | ~((1 << numberBits) - 1));
}