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Now I have a new type of labelled graphs, which can index vertices by
labels, but not index edges by labels. The biggest difference is that
I do not have to keep a hashmap of edge targets by labels, and I do
not have to guard against the duplication of nodes with the same set
of edges. I guard against nodes with the same label, though.
Also, in this graph, both vertices and edges have one label at a time,
whereas in the previous labelled graph there can be a multitude of
edges between the same source and target nodes, but with different
labels.
Now it remains to test this type of graphs, and to think through how
we attach forest fragments to nondeterministic finite automata edges,
and how to join forest fragments together while skipping nullable
edges, in order to finish the "compilation" part.
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I put functionalities that are not strictly core to separate crates,
so that the whole package becomes more modular, and makes it easier to
try other parsing algorithms in the future.
Also I have to figure the forests out before finishing the core
chain-rule algorithm, as the part about forests affects the labels of
the grammars directly. From my experiences in writing the previous
version, it is asking for trouble to change the labels type
dramatically at a later point: too many places need to be changed.
Thus I decide to figure the rough part of forests out.
Actually I only have to figure out how to attach forests fragments to
edges of the underlying atomic languages, and the more complex parts
of putting forests together can be left to the recorders, which is my
vision of assembling semi-ring values during the chain-rule machine.
It should be relatively easy to produce forests fragments from
grammars since we are just trying to extract some information from the
grammar, not to manipulate those information in some complicated way.
We have to do some manipulations in the process, though, in order to
make sure that the nulling and epsilon-removal processes do not
invalidate these fragments.
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Some changes:
- The core crate is renamed to "chain".
- The crate "viz" is added, which will provide layered graph drawing
algorithms.
- A function is added to convert from a grammar to the regular
language of its left-linear closures.
- A function is added to convert from a nondeterministic finite
automaton to its "null" closure. A null closure is the same
automaton with edges added, as if some edges are "null". Whether an
edge is null is determined by a function.
Combined with the previous change, we can convert a grammar to the
regular language of the null closure of its left-linear closures.
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Now it remains to test more grammars and add an Atom trait, before
finishing the part about compilations.
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Basic GNU standard files are added, and we now stop worrying about
monadic anamorphisms.
The current focus is on testing the correctness of the algorithm, so I
need convenient support for manipulating, interpreting, examining, and
per chance animating nondeterministic automata.
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