When we originally stubbed ApplyMoves we didn't know yet how exactly we'd
be using the result, so we made it a double-indexed map allowing looking
up moves in both directions.
However, in practice we only actually need to look up old addresses by new
addresses, and so this commit first removes the double indexing so that
each move is only represented by one element in the map.
We also need to describe situations where a move was blocked, because in
a future commit we'll generate some warnings in those cases. Therefore
ApplyMoves now returns a MoveResults object which contains both a map of
changes and a map of blocks. The map of blocks isn't used yet as of this
commit, but we'll use it in a later commit to produce warnings within
the "terraform" package.
Per our rule that the content of the state can never make a move statement
invalid, our behavior for two objects trying to occupy the same address
will be to just ignore that and let the object already at the address
take priority.
For the moment this is silent from an end-user perspective and appears
only in our internal logs. However, I'm hoping that our future planned
adjustment to the interface of this function will include some way to
allow reporting these collisions in some end-user-visible way, either as
a separate warning per collision or as a single warning that collects
together all of the collisions into a single message somehow.
This situation can arise both because the previous run state already
contained an object at the target address of a move and because more than
one move ends up trying to target the same location. In the latter case,
which one "wins" is decided by our depth-first traversal order, which is
in turn derived from our chaining and nesting rules and is therefore
arbitrary but deterministic.
This is a whole lot of nothing right now, just stubbing out some control
flow that ultimately just leads to TODOs that cause it to do nothing at
all.
My intent here is to get this cross-cutting skeleton in place and thus
make it easier for us to collaborate on adding the meat to it, so that
it's more likely we can work on different parts separately and still get
a result that tessellates.
Martin Atkins