[spawarotti]

One generalization of this concept I see is: Instead of having a sequence of successive states, you only need the initial state and a function telling you how to compute the next state from previous one.

You can also see a connection to a version control system like Git. Instead of keeping snapshots of all the contents of the repository after each commit, one can keep only the initial repository state and changes in each commit. Then to get to N-th state you say "Apply first N commits to the initial state".

In the bouncing DVD logo example the "function to compute next state" or "commit contents" is just easy and regular, to the point of being expressible via simple math functions.

This is also known as Event Sourcing.

[ekimekim]

> You can also see a connection to a version control system like Git. Instead of keeping snapshots of all the contents of the repository after each commit, one can keep only the initial repository state and changes in each commit.

Not to invalidate your point, but this is a common misconception with git. Yes, many git commands will treat a commit as a diff compared to its parent(s), but the actual commit object is storing the full state of the tree. This still works out to be fairly efficient because if a file is identical between two commits, it is only stored once, and both commits point at the same stored file.

[account42]

This is a common misconception with git. Yes, conceptually git will treat each commit as complete tree, but the actual pack files are storing deltas because anything else is way too inefficient. Loose objects are only the initial format stored before there is enough data for pack files to make sense.

[ghusbands]

The concept is what's important, here, since they were correcting the idea that git works mainly on diffs, which it doesn't. The diffs are merely a storage optimisation.

[spawarotti]

Thx, nice explanation. I think possibly Mercurial actually stores only the diffs, but I am not sure.

[lysecret]

Yes. Also storing each individual state can be seen as caching there is a place for both of course.

Each derived state is a pain and has to be maintained but might be essential.