structural change: separate crates out

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.

1 files changed, 41 insertions, 0 deletions

diff --git a/chain/src/atom/mod.rs b/chain/src/atom/mod.rs
new file mode 100644
index 0000000..084acca
--- /dev/null
+++ b/chain/src/atom/mod.rs
@@ -0,0 +1,41 @@
+//! This file defines the behaviour of the Atomic languages, and
+//! provides a default implementation.
+//!
+//! Here I do not to substitute external packages' implementations in
+//! the future, so why define a trait for the atomic languages?
+//! Because this way I can easily substitute other implementations if
+//! I have better ideas in the future.
+
+use grammar::{Error as GrammarError, TNT};
+use nfa::{DOption, Nfa};
+
+/// The expected behaviours of an atomic language.
+pub trait Atom: Nfa<DOption<TNT>> {
+    /// Return the index of a node representing the derivative of the
+    /// left-linear null closure of `nt` with respect to `t`.
+    fn atom(&self, nt: usize, t: usize) -> Result<Option<usize>, GrammarError>;
+
+    /// Return the index of the empty state.
+    fn empty(&self) -> usize;
+}
+
+pub mod default;
+
+pub use default::DefaultAtom;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use grammar::test_grammar_helper::*;
+
+    #[test]
+    fn atom() -> Result<(), Box<dyn std::error::Error>> {
+        let grammar = new_notes_grammar()?;
+
+        let atom = DefaultAtom::from_grammar(grammar)?;
+
+        println!("atom = {atom}");
+
+        Ok(())
+    }
+}