Right Shift

Bitwise vs. Arithmetic

There are two types of Right Shifts, that can be performed on an integer. They are: Bitwise and Arithmetic. LSL currently only supports Arithmetic. The difference between the two modes is how it fills the bits revealed. In bitwise mode, the revealed bits are always zero; in Arithmetic mode, it duplicates the old top bit to all the new bits. If you take the expression value >> count where value is arithmetically shifted right count bits, then this is the same mathematically as doing value / (2count) or in LSL value / llPow(2.0, count).

How to do Bitwise Right Shifts in LSL

Since LSL does not have a bitwise right shift operator you have to do it yourself. There are two methods for doing this, each with it's advantages and disadvantages.

Method 1

<lsl>(value >> count) & ~((~value) >> count);</lsl> This method is good when count is dynamic. It works because all the bits that need to be turned on will be turned on both sides of the AND but on only one side will the arithmetic right shift cause the sign bit to be extended. The result of the and is a value that is always a bitwise right shift.

Method 2

<lsl>(value >> count) & mask;</lsl> This method can only be used when count is a constant value. You use a constant mask to remove the extended sign bits. Example: (value >> 5) & 0x07FFFFFF As you can see the top five bits have been turned off in the mask value, if you have trouble seeing that, you can just use the lookup table below.

Mask Lookup Table

Shift Mask 0 0xFFFFFFFF 1 0x7FFFFFFF 2 0x3FFFFFFF 3 0x1FFFFFFF 4 0x0FFFFFFF 5 0x07FFFFFF 6 0x03FFFFFF 7 0x01FFFFFF 8 0x00FFFFFF 9 0x007FFFFF 10 0x003FFFFF

Shift Mask 11 0x001FFFFF 12 0x000FFFFF 13 0x0007FFFF 14 0x0003FFFF 15 0x0001FFFF 16 0x0000FFFF 17 0x00007FFF 18 0x00003FFF 19 0x00001FFF 20 0x00000FFF 21 0x000007FF

Shift Mask 22 0x000003FF 23 0x000001FF 24 0x000000FF 25 0x0000007F 26 0x0000003F 27 0x0000001F 28 0x0000000F 29 0x00000007 30 0x00000003 31 0x00000001 32 0x00000000

Example Bitwise Right Shift Function

Here's a function to pass a signed 32 bit integer along with a value indicating how far to shift the bits. This method is primarily needed to shift bits for negative integers, as the regular bit shift operators work just fine for positive values.

<lsl>// the lsl right shift is an arithmetic right shift, // this means it more closely resembles dividing by a // positive power of two then a bitwise right shift. // To perform a bitwise right shift you need to be clever integer rightShift(integer value, integer count) {

   return (value >> count) & ~((~value) >> count);
   //This works because only once side of the '&' operation
   //Has the sign bit extended by the arithmetic right shift
   //The purpose of flipping of the bits on the right side 
   //ensures this, the subsequent '&' removes the extended
   //sign bits.

}</lsl> Example Usage: <lsl>default {

   state_entry()
   {

       // output should be 268435449
       llSay(DEBUG_CHANNEL, (string)rightShift(-99, 4));

       // before: 1111 1111 1111 1111 1111 1111 1001 1101
       // after:  0000 1111 1111 1111 1111 1111 1111 1001
   }

}</lsl>