Converting binary to signed decimal in Actionscript

Wednesday, April 8th, 2009

If you’re like me, you may have been living these past few years taking advantage of the luxury of new sleek programming languages that rarely require you to do low level operations. One of the old skills I seem to always let get rusty enough is bitwise operation. Most code written with the help of frameworks doesn’t require this type of code and most programmers probably don’t even bother to learn it anymore.

I came into an instance yesterday where I needed to do some bitwise work in actionscript. The code was reading a bytestream from a binary file and converting to an array of numbers. Since this project was ported from Java (by someone else), a simple operation was overlooked. It was taking the binary number and converting it to a signed short. Since actionscript doesn’t have the same casting mechanisms conversion isn’t as straight forward in java.

In java the operation looked like this:

//  binary: 1111 1111 1111 0001
// value = -15 

Since short in java is always stored with a signed bit, the cast appropriately set the value at -15.

The way this was translated into Actionscript looked liked this:

//  binary: 1111 1111 1111 0001
// value = 65521  // Wrong! 

Actionscript has no concept of casting for primitives so the int(hexValue) doesn’t do anything at all.

Here is the workaround I came up with, I suspect there is a shorter way to get the same result but I can’t think of one right now.

//  binary: 1111 1111 1111 0001
// get everything except the signed bit "111 1111 1111 0001"
// unsignedValue now equals 32,753
// if the signed flag is set, flip the value
// 0x800 =  32,768 (maximum 15 bit number)
}
 
// signedValue = -15