如何为文档中的每个对象提供唯一的ID？

Question

I'm making a bitmap editor where a document consists of several layers where each layer represents a bitmap. Each layer must have a unique ID compared to all other layers that currently exist in the document. I also need to take into account that I need to save and load documents along with the layer IDs.

I'm using the command pattern to store actions that are performed on the document and the unique IDs are used to keep track of which layer an action should be performed on.

At the moment, I just keep a counter called X, when a new layer is created its ID is set to X then X is incremented. When loading, I need to make sure X is set to an appropriate number so that new layers are given unique IDs ie I could save the value of X and restore that, or set X based on the biggest layer ID loaded.

Given X is a 32-bit number, the user will need to create 4,294,967,296 layers working on the same file before IDs start to be reused which will cause weird behaviour. Should I implement a better unique ID system or is this generally good enough?

I'm in Java so I could use the UUID library which creates 128 bit unique IDs according to a standard algorithm. This seems overkill though.

Is there some general approach to this kind of problem?

Answer 1

This is perfectly good enough. At a rate of ten new layers per second 24/365, which is silly, it'll run fine for roughly three years.

Answer 2

If you think you might manipulate layers programmatically, and thus have some possibility of having 2^32 layers in the lifetime of the image, then throw all the layer IDs into a HashSet when you read the file, and update that set when you add/remove layers. (You don't need to explicitly store the set; the ID associated with each mask is enough.) Then instead of taking "the next number", take "the next number not in the set". Having a counter is still useful, but whether you set it to 0 or (max+1) when you read a file is immaterial; it won't take a significant amount of time to find an empty space unless you envision bafflingly large numbers of layers present simultaneously.

But since you're using Java, you could just use a long instead of an int, and then you wouldn't ever (practically speaking) be able to overflow the number even if all you did was create a one-pixel mask and destroy it over and over again.