Question

I have little unusual problem to solve. I need some hint or links to get started with. I have queue with 10 data slots. Once the queue is full I need to send it to a server. However, along with that data, I also send start and end sequence number. Now, this numbers must be unique and in increment order. So, for the first send, start = 1, and end = 10. On second send, it would be start = 11, end = 20, and so on. Once the data from the queue is send, new entries will be recorded from the index 0 in the queue.

How do I solve this efficiently ?

Answer 1

(There is a lot of context missing from your question, so this is mostly a shot in the dark…)

Since any 16-bit number can fit in a Java int primitive, you can:

• Convert a Java int to a 16-bit number by ANDing the number with a suitable bitwise mask:

  i16 = i32 & 0xffff
  

  WARNING: This conversion is lossy and not easily reversible.

• Convert a 16-bit number to a 32-bit int by keeping a separate epoch int that is incremented by one on each roll-over:

  if (previous16 > current16)
      epoch += 1;
  
  current32 = (epoch << 16) | current16;
  

  I do not think that it can get much more efficient than that in Java. Not to mention that any CPU that can run Java would normally run circles around any 16-bit processor, except perhaps for some DSPs…

A couple of related concerns:

• Beware of signed/unsigned conversions: Java does not have unsigned types, which may complicate things, depending on what exactly your are doing.

• Please note that, according to the JLS, the byte, char and short primitive types are implicitly converted to int for all operations. Whether they are actually narrower than 32-bits when stored in memory is implementation specific. And yes, that makes the short type pretty much useless…