Add no_item parameter.

* chain/src/default.rs:
* chain/src/lib.rs: Add a parameter that controls whether or not the
chain-rule machine computes the item derivation forest as well.
Sometimes we only need to recognize whether an input belongs to the
grammar, but do not care about the derivations.  This parameter can
speed up the machine in that case.

1 files changed, 14 insertions, 6 deletions

diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index d7fc519..9de1df7 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -258,11 +258,9 @@ pub trait Chain: LabelExtGraph<Edge> {
     /// An iterator that iterates all layers that need to be merged.
     type DerIter: Iterator<Item = TwoLayers>;
 
-    // FIXME: Add a parameter to control whether to manipulate the
-    // forests or not.
-
     /// Take the derivative by a terminal `t` at position `pos`.
-    fn derive(&mut self, t: usize, pos: usize) -> Result<Self::DerIter, Self::Error>;
+    fn derive(&mut self, t: usize, pos: usize, no_item: bool)
+        -> Result<Self::DerIter, Self::Error>;
 
     /// Take the union of all derivatives.
     fn union(&mut self, der_iter: Self::DerIter) -> Result<Vec<(Roi, usize)>, Self::Error> {
@@ -279,8 +277,18 @@ pub trait Chain: LabelExtGraph<Edge> {
 
     /// 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)?;
+    ///
+    /// # Arguments
+    ///
+    /// The argument `t` is the terminal we computet the derivative
+    /// with.
+    ///
+    /// The argument `pos` is the position within the input.
+    ///
+    /// The argument `no_item` determines whether we want the item
+    /// derivation forest as well.
+    fn chain(&mut self, t: usize, pos: usize, no_item: bool) -> Result<(), Self::Error> {
+        let der_iter = self.derive(t, pos, no_item)?;
 
         let edges = self.union(der_iter)?;