1 files changed, 171 insertions, 23 deletions

diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index 4e21e1d..91c37f7 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -10,41 +10,189 @@
 
 pub mod atom;
 
-// TODO: Define errors.
+use graph::{error::Error as GError, LabelExtGraph, Parent};
+
+use forest::Error as ForestError;
+
+/// An edge in the Chain-Rule machine.
+#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
+pub struct Edge {
+    /// The position in the atomic languages.
+    label: usize,
+    /// The source of the associated forest edge.
+    forest_source: Parent,
+    /// Whether or not this edge is "accepting".
+    accepting: bool,
+}
+
+impl Edge {
+    /// Construct a new edge.
+    pub fn new(label: usize, forest_source: Parent, accepting: bool) -> Self {
+        Self {
+            label,
+            forest_source,
+            accepting,
+        }
+    }
+
+    /// Return the label of the edge.
+    pub fn label(&self) -> usize {
+        self.label
+    }
+
+    /// Tell whether or not the edge is accepting.
+    pub fn is_accepting(&self) -> bool {
+        self.accepting
+    }
+
+    /// Return the associated forest edge of the edge.
+    pub fn forest_source(&self) -> Parent {
+        self.forest_source
+    }
+}
+
+impl core::fmt::Display for Edge {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        let label = self.label();
+        let forest_source = self.forest_source();
+
+        write!(
+            f,
+            "edge label {label} with forest source {} and edge index {}",
+            forest_source.node(),
+            forest_source.edge()
+        )
+    }
+}
+
+/// Each derivation is a concatenation of two items, so there are two
+/// layers.  But some items have no children and are accepting, in
+/// which case we just skip that item completely, for performance
+/// reasons, and hence there could be only one layer as well.
+///
+/// It might even happen that both layers have no children, in which
+/// case we shall just put all previous edges here.
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub enum TwoLayers {
+    /// One layer has no children.
+    One(Edge, usize),
+    // REVIEW: Maybe we do not need to store an edge in the forest: we
+    // only need the source of the edge?
+    /// Both layers have children.
+    ///
+    /// The first element is the label of the second layer, the second
+    /// the source of the associated forest edge of the second layer,
+    /// and the third is a list of edges, which are the common first
+    /// layers.
+    Two(usize, Parent, bool, Vec<(Edge, usize)>),
+}
+
+/// The type of a collapsed iterator.
+pub struct Collapsed<'a, I, C>
+where
+    I: Iterator<Item = TwoLayers>,
+    C: Chain<DerIter = I>,
+{
+    iter: I,
+    chain: &'a mut C,
+    stop: bool,
+}
+
+impl<'a, I, C> Collapsed<'a, I, C>
+where
+    I: Iterator<Item = TwoLayers>,
+    C: Chain<DerIter = I>,
+{
+    /// Collapse an iterator.
+    pub fn collapse(iter: I, chain: &'a mut C) -> Self {
+        let stop = false;
+
+        Self { iter, chain, stop }
+    }
+}
+
+impl<'a, I, C> Iterator for Collapsed<'a, I, C>
+where
+    I: Iterator<Item = TwoLayers>,
+    C: Chain<DerIter = I>,
+{
+    type Item = Result<(Edge, usize), <C as Chain>::Error>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.stop {
+            return None;
+        }
+
+        if let Some(two_layer) = self.iter.next() {
+            match two_layer {
+                TwoLayers::One(edge, to) => Some(Ok((edge, to))),
+                TwoLayers::Two(label, forest_source, accepting, edges) => {
+                    let new_index = match self.chain.extend(edges.into_iter()) {
+                        Ok(new) => new,
+                        Err(error) => {
+                            // Prevent further iterations.
+
+                            return Some(Err(error.into()));
+                        }
+                    };
+
+                    Some(Ok((Edge::new(label, forest_source, accepting), new_index)))
+                }
+            }
+        } else {
+            None
+        }
+    }
+}
 
 /// The expected behaviours of a language which can take derivatives
 /// by chain rule.
-pub trait Chain: Default {
+pub trait Chain: LabelExtGraph<Edge> {
     /// The implementations should choose a type to represent errors.
-    type Error: std::error::Error;
+    type Error: std::error::Error + From<GError> + From<ForestError>;
+
+    /// A type of atomic languages that is chosen for this chain rule
+    /// machine.
+    type Atom: atom::Atom;
 
     /// Represents the language that is present after we parse the
     /// empty string, that is the initial configuration of the
     /// language.  This may or may not be different from what
     /// `Default::default` gives.
-    fn unit() -> Self;
-
-    /// Take the derivative by a terminal symbol.
-    ///
-    /// This takes care of the union and the prepending operations.
-    ///
-    /// # A little remark about the design
-    ///
-    /// I have thought to separate different operations (like the
-    /// union, the prepending, and single derivatives) and then define
-    /// a function to put everything together.  But I think that
-    /// design is not convenient to use.  Also, I really do not need
-    /// those operations other than to provide this derivative
-    /// operation, so why define them?  And putting all things
-    /// together may reduce the number of bugs caused by wrong uses of
-    /// those component functions, and can reduce the amount of
-    /// documentation strings a library user needs to read, in order
-    /// to make use of this trait.  So I ended up with this design.
-    fn chain(&mut self, t: usize);
+    fn unit(atom: Self::Atom) -> Result<Self, Self::Error>;
 
     /// Return true if and only if the language contains the empty
     /// string.
-    fn epsilon(&self) -> bool;
+    fn epsilon(&self) -> Result<bool, Self::Error>;
+
+    /// Update the history
+    fn update_history(&mut self, new: usize);
+
+    /// An iterator that iterates all layers that need to be merged.
+    type DerIter: Iterator<Item = TwoLayers>;
+
+    /// Take the derivative by a terminal symbol at position `POS`.
+    fn derive(&mut self, t: usize, pos: usize) -> Result<Self::DerIter, Self::Error>;
+
+    /// Take the union of all derivatives.
+    fn union(&mut self, der_iter: Self::DerIter) -> Result<Vec<(Edge, usize)>, Self::Error> {
+        // REVIEW: Find a way to avoid allocations.
+        Collapsed::<_, Self>::collapse(der_iter, self).collect()
+    }
+
+    /// Use chain rule to compute the derivative with respect to a
+    /// terminal.
+    fn chain(&mut self, t: usize, pos: usize) -> Result<(), Self::Error> {
+        let der_iter = self.derive(t, pos)?;
+
+        let edges = self.union(der_iter)?;
+
+        let new_index = self.extend(edges.into_iter())?;
+
+        self.update_history(new_index);
+
+        Ok(())
+    }
 }
 
 pub mod default;