Implement some basic cache program generation
A bit ago, @jessfraz added the ability to control reexecution from the
executor. When we did this, it used the digest to determine if there
was a code change rather than a non-code change (whitespace, comment,
etc).
This allows the creation of a new program to be run without clearing the
scene. This, in conjunction with being able to delete Engine objects by
ID will allow us to do some clever stuff when incrementally executing
a program.
I'm still working on something a bit more advanced, but a good first
step to derisk some of the caching behavior here fully is to implement a
basic "changed program" stub.
This process the ast programs (old and new) if it doesn't exactly match.
This would have been a complete refresh before this commit.
1) Check all overlapping top-level statements of the body of the new and
old AST and ensure they all match.
- If this is true, this means that one of the two AST programs has more
elements then the other, and they all otherwise match (addition or
deletion of the end of the program). We continue to #2 in this
case.
- If this is false, we have a meaingful difference in a section of
overlapping code. This will result in a cache miss and rebuild
the scene. We short-cut here and the scene is rebuilt.
2) Check the lengths of the two bodies.
- If they're the same, we shouldn't have even been called. We will
short-cut with a noop cache return (no clear, no program).
- if the old ast is longer, we've removed instructions from the
program. We can't delete things now, so this will result in a cache
miss and rebuild the scene. We short-cut here and the scene is
rebuilt.
- If the new ast is longer, we have an insertion of code at the end.
3) construct a new program using only the new elements from the new
ast, and return a `CacheResult` that *does not clear the scene*.
This means nothing will be rebuilt, and only a new object will polp
onto the scene. This is the first case where we diverge with
existing behavior.
Signed-off-by: Paul R. Tagliamonte <paul@zoo.dev>
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