forest: clone correctly

Now the forest can detect if a node is packed or cloned, and correctly
clones a node in those circumstances.  But it still needs to be
tested.

10 files changed, 323 insertions, 99 deletions

diff --git a/ChangeLog b/ChangeLog
index ac1dfdd..44e241a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2023-01-22  Jean Sévère Durand  <durand@jsdurand.xyz>
+
+	* forest: Correctly clone nodes.
+	Now the forest can detect if a node is packed or cloned, and
+	correctly clones a node in those circumstances.  But it still
+	needs to be tested.
+
 2023-01-20  Jean Sévère Durand  <durand@jsdurand.xyz>
 
 	* chain: A prototype is added, and passes some tests.  But I am
diff --git a/chain/src/atom/default.rs b/chain/src/atom/default.rs
index 14b7a9f..e88cfc9 100644
--- a/chain/src/atom/default.rs
+++ b/chain/src/atom/default.rs
@@ -429,12 +429,6 @@ impl DefaultAtom {
                                     GrammarError::IndexOutOfBounds(child, accepting_vec.len()),
                                 )?;
 
-                        if nt == 3
-                            && nfa.degree(child).map_err(index_out_of_bounds_conversion)? == 0
-                        {
-                            println!("accepting = {accepting}");
-                        }
-
                         if let Some((_, old_accepting)) = terminals_map.get_mut(&t) {
                             *old_accepting = *old_accepting || accepting;
                         } else {
@@ -455,40 +449,13 @@ impl DefaultAtom {
                 }
             }
 
-            if nt == 3 {
-                println!("map = {terminals_map:?}");
-            }
-
             for (t, (set, accepting)) in terminals_map.into_iter() {
-                let new_index = nfa
-                    .extend(set.into_iter())
-                    .map_err(index_out_of_bounds_conversion)?;
+                let new_index = nfa.extend(set).map_err(index_out_of_bounds_conversion)?;
 
                 if accepting_vec.get(new_index).is_none() {
                     #[cfg(debug_assertions)]
                     assert_eq!(new_index, accepting_vec.len());
 
-                    // let mut updated = false;
-                    // let nfa_len = nfa.nodes_len();
-
-                    // 'label_loop: for (label, target_iter) in nfa
-                    //     .labels_of(new_index)
-                    //     .map_err(|_| GrammarError::IndexOutOfBounds(new_index, nfa_len))?
-                    // {
-                    //     if label_is_nullable(*label) {
-                    //         for target in target_iter {
-                    //             if *accepting_vec
-                    //                 .get(target)
-                    //                 .ok_or(GrammarError::IndexOutOfBounds(target, nfa_len))?
-                    //             {
-                    //                 updated = true;
-
-                    //                 break 'label_loop;
-                    //             }
-                    //         }
-                    //     }
-                    // }
-
                     accepting_vec.push(accepting);
                 }
 
diff --git a/chain/src/default.rs b/chain/src/default.rs
index b9d7fe6..22befff 100644
--- a/chain/src/default.rs
+++ b/chain/src/default.rs
@@ -10,10 +10,7 @@ use crate::atom::{Atom, DefaultAtom};
 use core::fmt::Display;
 use forest::{default::DefaultForest, Forest};
 use grammar::{Error as GrammarError, GrammarLabel, GrammarLabelType, TNT};
-#[allow(unused_imports)]
-use graph::{
-    labelled::DLGBuilder, Builder, DLGraph, Graph, LabelExtGraph, LabelGraph, ParentsGraph,
-};
+use graph::{labelled::DLGBuilder, Builder, DLGraph, Graph, LabelExtGraph, LabelGraph};
 
 use std::collections::{HashMap as Map, TryReserveError};
 
@@ -156,11 +153,13 @@ impl Iterator for DerIter {
                         None
                     }
                 } else {
-                    self.index = DerIterIndex::Single(0);
+                    // If the zero-th element is present, we will
+                    // advance the index to one; if it is not present,
+                    // we will stop iteration.  In each case we can
+                    // safely set the index to one.
+                    self.index = DerIterIndex::Single(1);
 
                     if let Some((edge, to)) = self.singles.first() {
-                        self.index = DerIterIndex::Single(1);
-
                         Some(TwoLayers::One(*edge, *to))
                     } else {
                         None
@@ -776,15 +775,7 @@ mod test_chain {
 
         println!("repeating {repeat_times} times");
 
-        let input = {
-            let mut result = Vec::with_capacity(input_template.len() * repeat_times);
-
-            for _ in 0..repeat_times {
-                result.extend(input_template.iter().copied());
-            }
-
-            result
-        };
+        let input = input_template.repeat(repeat_times);
 
         let start = std::time::Instant::now();
 
diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index 91c37f7..a3d420b 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -131,6 +131,7 @@ where
                         Ok(new) => new,
                         Err(error) => {
                             // Prevent further iterations.
+                            self.stop = true;
 
                             return Some(Err(error.into()));
                         }
@@ -157,8 +158,7 @@ pub trait Chain: LabelExtGraph<Edge> {
 
     /// 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.
+    /// language.
     fn unit(atom: Self::Atom) -> Result<Self, Self::Error>;
 
     /// Return true if and only if the language contains the empty
@@ -171,7 +171,7 @@ pub trait Chain: LabelExtGraph<Edge> {
     /// 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`.
+    /// Take the derivative by a terminal `t` at position `pos`.
     fn derive(&mut self, t: usize, pos: usize) -> Result<Self::DerIter, Self::Error>;
 
     /// Take the union of all derivatives.
@@ -187,7 +187,7 @@ pub trait Chain: LabelExtGraph<Edge> {
 
         let edges = self.union(der_iter)?;
 
-        let new_index = self.extend(edges.into_iter())?;
+        let new_index = self.extend(edges)?;
 
         self.update_history(new_index);
 
diff --git a/forest/src/default.rs b/forest/src/default.rs
index 9295f1a..b96439f 100644
--- a/forest/src/default.rs
+++ b/forest/src/default.rs
@@ -24,6 +24,8 @@ pub enum Error {
     /// Encounter an invalid error in converting from an error of
     /// graphs.
     InvalidGraphError(GError),
+    /// Encounter an error when converting forest labels.
+    LabelConversion(ForestLabelError),
 }
 
 impl Display for Error {
@@ -35,6 +37,7 @@ impl Display for Error {
             Error::DuplicatedNode(n) => write!(f, "node {n} is duplicated"),
             Error::InvalidGraphError(ge) => write!(f, "invalid error: {ge}"),
             Error::NodeNoLabel(n) => write!(f, "node {n} has no labels, but it should have one"),
+            Error::LabelConversion(le) => write!(f, "fail to convert labels: {le}"),
         }
     }
 }
@@ -51,6 +54,12 @@ impl From<GError> for Error {
     }
 }
 
+impl From<ForestLabelError> for Error {
+    fn from(le: ForestLabelError) -> Self {
+        Self::LabelConversion(le)
+    }
+}
+
 /// A default implementation of forest.
 #[derive(Debug, Clone)]
 pub struct DefaultForest<T: GraphLabel> {
@@ -193,7 +202,7 @@ impl<T: GraphLabel> LabelGraph<T> for DefaultForest<T> {
     }
 }
 
-impl<T: GraphLabel> Forest<T> for DefaultForest<T> {
+impl<T: GraphLabel> Forest<T> for DefaultForest<ForestLabel<T>> {
     type Error = Error;
 
     fn root(&self) -> Option<usize> {
@@ -205,7 +214,7 @@ impl<T: GraphLabel> Forest<T> for DefaultForest<T> {
 
         let mut builder = PLGBuilderMut::from_graph_mut(&mut graph);
 
-        let root = Some(builder.add_vertex(label));
+        let root = Some(builder.add_vertex(ForestLabel::from(label)));
 
         Self { graph, root }
     }
@@ -299,12 +308,12 @@ impl<T: GraphLabel> Forest<T> for DefaultForest<T> {
 
         let nodes_len = fragment.nodes_len();
 
-        // If the fragment root has a duplicate label, the forest will
-        // not grow, so we use the label to find the adjoined node
-        // index.
-
-        // the nodes hava already been added to the forest, so it is
-        // safe to call unwrap.
+        /// If the fragment root has a duplicate label, the forest
+        /// will not grow, so we use the label to find the adjoined
+        /// node index.
+        ///
+        /// The nodes hava already been added to the forest, so it is
+        /// safe to call unwrap.
         macro_rules! conversion (
             ($node:expr) => {
                 {
@@ -360,20 +369,37 @@ impl<T: GraphLabel> Forest<T> for DefaultForest<T> {
         Ok(())
     }
 
-    fn clone_node<F>(&mut self, node_id: usize, clone_transform: F) -> Result<usize, Self::Error>
-    where
-        F: Fn(T) -> T,
-    {
-        let mut builder = PLGBuilderMut::from_graph_mut(&mut self.graph);
+    fn clone_node(&mut self, node_id: usize) -> Result<usize, Self::Error> {
+        let builder = PLGBuilderMut::from_graph_mut(&mut self.graph);
 
         let old_label = builder
             .vertex_label(node_id)?
             .ok_or(Error::NodeNoLabel(node_id))?;
 
-        let new_label = clone_transform(old_label);
+        if old_label.is_packed() {
+            return Err(ForestLabelError::ClonePack.into());
+        }
+
+        if old_label.is_cloned().is_some() {
+            let mut parents = self.parents_of(node_id)?;
+
+            // Make sure it only has one parent, which is the
+            // representative node.
+            assert_eq!(parents.len(), 1);
+
+            // We checked its length is 1, so it is safe to unwrap
+            // here.
+            return Ok(parents.next().unwrap().node());
+        }
+
+        let rep_label = old_label.pack()?;
+
+        let new_label = old_label.clone(self)?;
+
+        let mut builder = PLGBuilderMut::from_graph_mut(&mut self.graph);
 
         // Make a new node
-        let new_index = builder.add_vertex(new_label);
+        let new_index = builder.add_vertex(rep_label);
 
         // Re-direct parents to the new node.
         //
@@ -393,6 +419,10 @@ impl<T: GraphLabel> Forest<T> for DefaultForest<T> {
 
         builder.add_edge(new_index, node_id, old_label)?;
 
+        // Modify the label of the old node.
+
+        builder.set_label(node_id, new_label)?;
+
         Ok(new_index)
     }
 }
@@ -403,10 +433,10 @@ mod forest_test {
 
     macro_rules! leaf (
         ($label:expr, $type:tt) =>{
-            DefaultForest::<$type>::new_leaf($label)
+            DefaultForest::<ForestLabel<$type>>::new_leaf($label)
         };
         ($label:expr) => {
-            DefaultForest::<usize>::new_leaf($label)
+            DefaultForest::<ForestLabel<usize>>::new_leaf($label)
         }
     );
 
@@ -479,11 +509,9 @@ mod forest_test {
         // add a duplicate label
         forest.plant(3, leaf!(5), false)?;
 
-        let len = forest.nodes_len();
-
-        let clone_transform = |label| label + len;
+        let _len = forest.nodes_len();
 
-        forest.clone_node(5, clone_transform)?;
+        forest.clone_node(5)?;
 
         assert_eq!(forest.nodes_len(), 7);
 
diff --git a/forest/src/lib.rs b/forest/src/lib.rs
index 8c9b918..02c5fcd 100644
--- a/forest/src/lib.rs
+++ b/forest/src/lib.rs
@@ -13,13 +13,144 @@
 
 use graph::{error::Error as GError, GraphLabel, LabelGraph, ParentsGraph};
 
+/// An internal type that indicates the status of a node as either a
+/// packed, cloned, or plain node.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Default)]
+enum ForestLabelType {
+    Packed,
+    #[default]
+    Plain,
+    Cloned(usize),
+}
+
+impl core::fmt::Display for ForestLabelType {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Packed => write!(f, "packed"),
+            Self::Plain => write!(f, "plain"),
+            Self::Cloned(index) => write!(f, "the {index}-th clone"),
+        }
+    }
+}
+
+/// A type that encodes the properties demanded by a forest.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+pub struct ForestLabel<T: GraphLabel> {
+    label: T,
+    status: ForestLabelType,
+}
+
+impl<T: GraphLabel> core::fmt::Display for ForestLabel<T> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        writeln!(f, "label: {}, status: {}", self.label, self.status)
+    }
+}
+
+/// The type of erros for converting forest labels.
+#[non_exhaustive]
+#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
+pub enum ForestLabelError {
+    /// Try to pack a cloned node.
+    PackClone,
+    /// Try to clone a packed node.
+    ClonePack,
+}
+
+impl core::fmt::Display for ForestLabelError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::PackClone => write!(f, "cannot pack a cloned node"),
+            Self::ClonePack => write!(f, "cannot clone a packed node"),
+        }
+    }
+}
+
+impl std::error::Error for ForestLabelError {}
+
+impl<T: GraphLabel> ForestLabel<T> {
+    /// Retrieve the label.
+    pub fn label(&self) -> T {
+        self.label
+    }
+
+    /// Return true if and only if this node is packed.
+    pub fn is_packed(&self) -> bool {
+        matches!(self.status, ForestLabelType::Packed)
+    }
+
+    /// Retrieve the optional clone index.
+    pub fn is_cloned(&self) -> Option<usize> {
+        match self.status {
+            ForestLabelType::Cloned(index) => Some(index),
+            _ => None,
+        }
+    }
+
+    /// Try to clone a node.
+    pub fn clone<F>(self, forest: &F) -> Result<Self, ForestLabelError>
+    where
+        F: Forest<T>,
+    {
+        if self.is_packed() {
+            Err(ForestLabelError::ClonePack)
+        } else {
+            let clone_index = if let Some(old_index) = self.is_cloned() {
+                let mut new_index: usize = old_index + 1;
+
+                let mut new_label = Self {
+                    status: ForestLabelType::Cloned(new_index),
+                    ..self
+                };
+
+                while forest.query_label(new_label).is_some() {
+                    new_index += 1;
+                    new_label = Self {
+                        status: ForestLabelType::Cloned(new_index),
+                        ..self
+                    };
+                }
+
+                new_index
+            } else {
+                0
+            };
+
+            Ok(Self {
+                status: ForestLabelType::Cloned(clone_index),
+                ..self
+            })
+        }
+    }
+
+    /// Try to pack a node.
+    pub fn pack(self) -> Result<Self, ForestLabelError> {
+        if self.is_cloned().is_some() {
+            Err(ForestLabelError::PackClone)
+        } else {
+            let new_label = Self {
+                status: ForestLabelType::Packed,
+                ..self
+            };
+
+            Ok(new_label)
+        }
+    }
+}
+
+impl<T: GraphLabel> From<T> for ForestLabel<T> {
+    fn from(label: T) -> Self {
+        let status = ForestLabelType::default();
+        Self { label, status }
+    }
+}
+
 /// The expected behaviours of a forest.
 ///
 /// Note that it requires only a subset of the functionalities of
 /// labelled graphs.
-pub trait Forest<T: GraphLabel>: ParentsGraph + LabelGraph<T> {
+pub trait Forest<T: GraphLabel>: ParentsGraph + LabelGraph<ForestLabel<T>> {
     /// The type of errors for operations on the forest.
-    type Error: std::error::Error + From<GError>;
+    type Error: std::error::Error + From<GError> + From<ForestLabelError>;
 
     /// Return the root of the forest.
     ///
@@ -44,13 +175,13 @@ pub trait Forest<T: GraphLabel>: ParentsGraph + LabelGraph<T> {
     /// Clone a node by making a new node and making all the nodes
     /// that previously pointed to the old node now point to the new
     /// node, and the new node points to the old node.  Return the
-    /// index of the new node.
+    /// index of the new node.  In addition, if the old node had
+    /// already been cloned, just return the index of its
+    /// representitave.
     ///
-    /// The label of the new node will be given by the label
-    /// transformer.
-    fn clone_node<F>(&mut self, node_id: usize, clone_transform: F) -> Result<usize, Self::Error>
-    where
-        F: Fn(T) -> T;
+    /// The labels of the representing node and of the cloned node
+    /// will be handled automatically.
+    fn clone_node(&mut self, node_id: usize) -> Result<usize, Self::Error>;
 }
 
 pub mod default;
diff --git a/grammar/src/label.rs b/grammar/src/label.rs
index 58eaddc..05b0b1e 100644
--- a/grammar/src/label.rs
+++ b/grammar/src/label.rs
@@ -93,10 +93,24 @@ impl GrammarLabel {
     /// Return a string description with the help of the associated
     /// grammar.
     pub fn to_string(&self, grammar: &Grammar) -> Result<String, Error> {
-        // REVIEW: It needs at least 34 bytes, so we just double it.
-        // Of course we can also calculate the length exactly, but
-        // this can be postponed till later.
-        let mut s = String::with_capacity(68);
+        // First calculate the length of the resulting string.
+
+        let mut num = 2 + 7 + 14 + 6;
+
+        num += self.label().name(grammar)?.len();
+
+        num += format!("{} ", self.start()).len();
+
+        if let Some(end) = self.end() {
+            num += format!("to {end}").len();
+        } else {
+            num += 7;
+        }
+
+        let num = num;
+
+        let mut s = String::with_capacity(num);
+
         s.push_str("a ");
 
         if self.is_packed() {
diff --git a/grammar/src/lib.rs b/grammar/src/lib.rs
index 50a2b9b..b29fc69 100644
--- a/grammar/src/lib.rs
+++ b/grammar/src/lib.rs
@@ -213,12 +213,22 @@ pub struct Grammar {
     // The following attribute is empty until we call `closure` on the
     // NFA with `transform_label_null_epsilon` as the transformer.
     /// A hash map that maps a tuple `(pos1, pos2)` of positions
-    /// `pos1` and `pos2` in the rules to a vector of rule positions.
+    /// `pos1` and `pos2` in the rules to a vector of non-terminals
+    /// and rule positions.
+    ///
     /// This vector means that in order to expand from `pos1` to
-    /// `pos`, it is necessary to expand according to the positions in
-    /// the vector, so we need to add all these expansions into the
-    /// parse forest later.
-    expansion_map: HashMap<(usize, usize), Vec<usize>>,
+    /// `pos`, it is necessary to expand according to the
+    /// non-terminals and positions in the vector, so we need to add
+    /// all these expansions into the parse forest later.
+    expansion_map: HashMap<(usize, usize), Vec<(usize, usize)>>,
+    /// A hash map that maps a tuple `(pos1, pos2)` of positions
+    /// `pos1` and `pos2` in the rules to a vector of non-terminals.
+    ///
+    /// This vector means that in order to expand from `pos1` to
+    /// `pos`, it is necessary to expand according to the
+    /// non-terminals, so we can use this information to find out
+    /// where to join a new node in the parse forest later.
+    reduction_map: HashMap<(usize, usize), Vec<usize>>,
     /// The state of the grammar, which tells us what information has
     /// been computed for this grammar.
     state: GrammarState,
@@ -269,6 +279,7 @@ impl Grammar {
         let state = Default::default();
 
         let expansion_map = Default::default();
+        let reduction_map = Default::default();
 
         // NOTE: We cannot calculate accumulators here, as we want the
         // accumulators of the regular expression of the left-closure,
@@ -283,6 +294,7 @@ impl Grammar {
             first_nodes,
             state,
             expansion_map,
+            reduction_map,
             accumulators,
         }
     }
@@ -350,7 +362,7 @@ impl Grammar {
 
     /// Query if a position is the starting position of a
     /// non-terminal.  If it is, return the non-terminal, else return
-    /// `None` .
+    /// `None`.
     #[inline]
     pub fn get_nt_start_in_nfa(&self, pos: usize) -> Option<usize> {
         for (index, accumulator) in self.accumulators.iter().copied().enumerate() {
@@ -370,6 +382,18 @@ impl Grammar {
         None
     }
 
+    /// Query if a position is the ending position of a
+    /// non-terminal.  If it is, return the non-terminal, else return
+    /// `None`.
+    #[inline]
+    pub fn get_nt_end_in_nfa(&self, pos: usize) -> Option<usize> {
+        if pos >= 1 {
+            self.get_nt_start_in_nfa(pos - 1)
+        } else {
+            None
+        }
+    }
+
     /// Return the number of terminals.
     #[inline]
     pub fn ter_num(&self) -> usize {
@@ -465,7 +489,11 @@ impl Grammar {
     /// Query the expansion information from the position `pos1` to
     /// the position `pos2` .
     #[inline]
-    pub fn query_expansion(&self, pos1: usize, pos2: usize) -> Result<Option<&[usize]>, Error> {
+    pub fn query_expansion(
+        &self,
+        pos1: usize,
+        pos2: usize,
+    ) -> Result<Option<&[(usize, usize)]>, Error> {
         if pos1 >= self.total() {
             return Err(Error::IndexOutOfBounds(pos1, self.total()));
         }
@@ -484,11 +512,36 @@ impl Grammar {
             }
         }
 
-        Ok(self.expansion_map.get(&(pos1, pos2)).map(|v| v.as_ref()))
+        Ok(self.expansion_map.get(&(pos1, pos2)).map(AsRef::as_ref))
     }
 
-    // REVIEW: Do we have a better way to record expansion information
-    // than to compute the transitive closure?
+    /// Query the reduction information from the position `pos1` to
+    /// the position `pos2` .
+    #[inline]
+    pub fn query_reduction(&self, pos1: usize, pos2: usize) -> Result<Option<&[usize]>, Error> {
+        if pos1 >= self.total() {
+            return Err(Error::IndexOutOfBounds(pos1, self.total()));
+        }
+
+        if pos2 >= self.total() {
+            return Err(Error::IndexOutOfBounds(pos2, self.total()));
+        }
+
+        match self.state {
+            GrammarState::AfterLeftClosure => {}
+            _ => {
+                return Err(Error::WrongState(
+                    self.state,
+                    GrammarState::AfterLeftClosure,
+                ));
+            }
+        }
+
+        Ok(self.reduction_map.get(&(pos1, pos2)).map(AsRef::as_ref))
+    }
+
+    // REVIEW: Do we have a better way to record expansion and
+    // reduction information than to compute the transitive closure?
 
     // REVIEW: We need a way to eliminate those left-linearly expanded
     // edges whose labels had already been considered, and we need to
@@ -550,14 +603,28 @@ impl Grammar {
                     (first_source, second_target),
                     if let Some(original_expansion) = original_expansion {
                         let mut result = original_expansion.clone();
-                        result.push(second_nt);
+                        result.push((second_nt, second_label.get_moved()));
 
                         result
                     } else {
-                        vec![second_nt]
+                        vec![(second_nt, second_label.get_moved())]
                     },
                 );
             }
+        } else if let Some(second_nt) = self.get_nt_end_in_nfa(second_source) {
+            let original_reduction = self.reduction_map.get(&(second_source, second_target));
+
+            self.reduction_map.insert(
+                (first_source, second_target),
+                if let Some(original_reduction) = original_reduction {
+                    let mut result = original_reduction.clone();
+                    result.push(second_nt);
+
+                    result
+                } else {
+                    vec![second_nt]
+                },
+            );
         }
 
         NfaLabel::new(second_label.get_value(), second_label.get_moved(), left_p)
diff --git a/graph/src/builder.rs b/graph/src/builder.rs
index b04e7f6..c5f9252 100644
--- a/graph/src/builder.rs
+++ b/graph/src/builder.rs
@@ -82,6 +82,9 @@ pub trait BuilderMut {
     /// Add an edge from the source to the target.
     fn add_edge(&mut self, source: usize, target: usize, label: Self::Label) -> Result<(), Error>;
 
+    /// Set the label of an existing node to a new label.
+    fn set_label(&mut self, node_id: usize, label: Self::Label) -> Result<(), Error>;
+
     /// Remove an edge from the source to the target.
     ///
     /// Since some graphs are labelled, the users are allowed to pass
diff --git a/graph/src/labelled/binary.rs b/graph/src/labelled/binary.rs
index e3e1943..201dda2 100644
--- a/graph/src/labelled/binary.rs
+++ b/graph/src/labelled/binary.rs
@@ -551,6 +551,22 @@ impl<'a, T: GraphLabel> BuilderMut for PLGBuilderMut<'a, T> {
 
         Ok(())
     }
+
+    #[inline]
+    fn set_label(&mut self, node_id: usize, label: Self::Label) -> Result<(), Error> {
+        if !self.graph.has_node(node_id) {
+            return Err(Error::IndexOutOfBounds(node_id, self.nodes_len()));
+        }
+
+        // node_id is now guaranteed to be valid.
+
+        self.graph.nodes.get_mut(node_id).unwrap().label = label;
+
+        self.graph.label_index_map.remove(&label);
+        self.graph.label_index_map.insert(label, node_id);
+
+        Ok(())
+    }
 }
 
 #[cfg(test)]