Question

I’m testing a photo application for Facebook. I’m getting object IDs from the Facebook API, but I received some incorrect ones, which doesn’t make sense – why would Facebook send wrong IDs? I investigated a bit and found out that numbers with 17 and more digits are automatically turning into even numbers!

For example, let’s say the ID I’m supposed to receive from Facebook is 12345678912345679. In the debugger, I’ve noticed that Flash Player automatically turns it into 12345678912345678. And I even tried to manually set it back to an odd number, but it keeps changing back to even.

Is there any way to stop Flash Player from rounding the numbers? BTW the variable is defined as Object, I receive it like that from Facebook.

Answer 1

This is related to the implementation of data types:

• int is a 32-bit number, with an even distribution of positive and
  negative values, including 0. So the maximum value is

  (2^32 / 2 ) - 1 == 2,147,483,647.

• uint is also a 32-bit number, but it doesn’t have negative values. So the
  maximum value is

  2^32 - 1 == 4,294,967,295.

When you use a numerical value greater than the maximum value of int or uint, it is automatically cast to Number. From the Adobe Doc:

The Number data type is useful when you need to use floating-point
values. Flash runtimes handle int and uint data types more efficiently
than Number, but Number is useful in situations where the range of
values required exceeds the valid range of the int and uint data
types. The Number class can be used to represent integer values well
beyond the valid range of the int and uint data types. The Number data
type can use up to 53 bits to represent integer values, compared to
the 32 bits available to int and uint.

53 bits have a maximum value of:

2^53 - 1 == 9,007,199,254,740,989 => 16 digits

So when you use any value greater than that, the inner workings of floating point numbers apply.

You can read about floating point numbers here, but in short, for any floating point value, the first couple of bits are used to specify a multiplication factor, which determines the location of the point. This allows for a greater range of values than are actually possible to represent with the number of bits available – at the cost of reduced precision.

When you have a value greater than the maximum possible integer value a Number could have, the least significant bit (the one representing 0 and 1) is cut off to allow for a more significant bit (the one representing 2^54) to exist => hence, you lose the odd numbers.

There is a simple way to get around this: Keep all your IDs as Strings – they can have as many digits as your system has available bytes 😉 It’s unlikely you’re going to do any calculations with them, anyway.

By the way, if you had a value greater than 2^54-(1+2), your numbers would be rounded down to the next multiple of 4; if you had a value greater than 2^55-(1+2+4), they would be rounded down to the next multiple of 8, etc.