Question

I am implementing decoding of BER-compressed integers and recently I’ve found a weird JavaScript behavior related to bitwise operations with big integers.

E.g.:

var a = 17516032;          // has 25 bits
alert(a << 7)              // outputs -2052915200
alert(a * 128)             // outputs  2242052096
alert(2242052096 >> 16)    // outputs -31325
alert(2242052096 / 65536)  // outputs  34211

While the first workaround (multiplication instead of left shift) is acceptable, the second isn’t.

Why it happens? How to bear with it?

Answer 1

17516032 in binary is 00000001000010110100011000000000. Shifting to the left by 7 gives you 10000101101000110000000000000000. This is equal to -2052915200 in two’s complement (which is how almost all computers represent negative numbers).

>> is a signed right shift. That means that the leftmost bit (which determines the sign of a number) will be shifted into the left side.

e.g.

1100 >> 2 == 1111
0111 >> 2 == 0001

If you want to do an unsigned shift (which ignores the sign bit), use >>> which will zero-fill the left end of the bitstring.