singly labelled graphs

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.

1 files changed, 343 insertions, 0 deletions

diff --git a/graph/src/labelled/single.rs b/graph/src/labelled/single.rs
new file mode 100644
index 0000000..bed65c1
--- /dev/null
+++ b/graph/src/labelled/single.rs
@@ -0,0 +1,343 @@
+//! This file implements a labelled graph that can index vertices by
+//! labels.
+//!
+//! Since we do not have to index edges by labels, the labels can be
+//! stored simply by adjacency maps.  This may save memory usage and
+//! improve performance, potentially.
+
+use super::*;
+
+use std::collections::{hash_map::Keys, HashMap as Map};
+
+use crate::builder::BuilderMut;
+
+/// The type of a node.
+#[derive(Debug, Clone, Default)]
+struct SLNode<T: GraphLabel> {
+    /// The edges are stored as an association from the target node
+    /// index to the label of the corresponding edge.
+    ///
+    /// An implication is that an edge can only have one label.
+    children: Map<usize, T>,
+    /// The label of this node.
+    label: T,
+}
+
+impl<T: GraphLabel> SLNode<T> {
+    fn new(children: Map<usize, T>, label: T) -> Self {
+        Self { children, label }
+    }
+
+    /// This function just adds an edge, blindly.
+    ///
+    /// # Safety
+    ///
+    /// Only use this after making sure that `target` refers to a
+    /// valid node, and there was no edge from this node to the node
+    /// pointed to by `target` previously.
+    unsafe fn add_edge(&mut self, target: usize, label: T) {
+        self.children.insert(target, label);
+    }
+}
+
+/// The type of labelled graphs implemented using adjacency maps.
+#[derive(Debug, Clone)]
+pub struct SLGraph<T: GraphLabel> {
+    nodes: Vec<SLNode<T>>,
+    label_index_map: Map<T, usize>,
+}
+
+impl<T: GraphLabel> Default for SLGraph<T> {
+    fn default() -> Self {
+        let nodes = Vec::new();
+        let label_index_map = Default::default();
+        Self {
+            nodes,
+            label_index_map,
+        }
+    }
+}
+
+impl<T: GraphLabel> Graph for SLGraph<T> {
+    type Iter<'a> = std::iter::Copied<Keys<'a, usize, T>>
+    where
+        Self: 'a;
+
+    #[inline]
+    fn is_empty(&self) -> bool {
+        self.nodes.is_empty()
+    }
+
+    #[inline]
+    fn nodes_len(&self) -> usize {
+        self.nodes.len()
+    }
+
+    #[inline]
+    fn children_of(&self, node_id: usize) -> Result<Self::Iter<'_>, Error> {
+        if let Some(node) = self.nodes.get(node_id) {
+            Ok(node.children.keys().copied())
+        } else {
+            Err(Error::IndexOutOfBounds(node_id, self.nodes.len()))
+        }
+    }
+
+    #[inline]
+    fn degree(&self, node_id: usize) -> Result<usize, Error> {
+        if let Some(node) = self.nodes.get(node_id) {
+            Ok(node.children.len())
+        } else {
+            Err(Error::IndexOutOfBounds(node_id, self.nodes.len()))
+        }
+    }
+
+    #[inline]
+    fn is_empty_node(&self, node_id: usize) -> Result<bool, Error> {
+        if let Some(node) = self.nodes.get(node_id) {
+            Ok(node.children.is_empty())
+        } else {
+            Err(Error::IndexOutOfBounds(node_id, self.nodes.len()))
+        }
+    }
+
+    #[inline]
+    fn has_edge(&self, source: usize, target: usize) -> Result<bool, Error> {
+        let nodes_len = self.nodes.len();
+
+        let node = if let Some(node) = self.nodes.get(source) {
+            node
+        } else {
+            return Err(Error::IndexOutOfBounds(source, nodes_len));
+        };
+
+        if !self.has_node(target) {
+            return Err(Error::IndexOutOfBounds(target, nodes_len));
+        }
+
+        Ok(node.children.contains_key(&target))
+    }
+
+    fn replace_by_builder(&mut self, _builder: impl Builder<Result = Self>) {
+        // In case this is not clear enough, I deliberately avoid
+        // implementing this.
+        unimplemented!()
+    }
+}
+
+/// An iterator of edge labels.
+///
+/// This is used to avoid a box allocation.
+#[derive(Copy, Clone, Debug)]
+pub struct EdgeLabelIter<T: GraphLabel>(Option<T>);
+
+impl<T: GraphLabel> Iterator for EdgeLabelIter<T> {
+    type Item = T;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.take()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.0.is_some() as usize;
+
+        (len, Some(len))
+    }
+}
+
+impl<T: GraphLabel> DoubleEndedIterator for EdgeLabelIter<T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.0.take()
+    }
+}
+
+impl<T: GraphLabel> ExactSizeIterator for EdgeLabelIter<T> {
+    #[inline]
+    fn len(&self) -> usize {
+        // Thanks to Clippy for teaching me about coercing a boolean
+        // value to an integer.
+        self.0.is_some() as usize
+    }
+}
+
+impl<T: GraphLabel> LabelGraph<T> for SLGraph<T> {
+    // The following two types are not used, as we do not need to
+    // index edges by labels.
+    type Iter<'a> = std::iter::Empty<usize>
+    where
+        Self: 'a;
+
+    type LabelIter<'a> = std::iter::Empty<(&'a T, <Self as LabelGraph<T>>::Iter<'a>)>
+    where
+        Self: 'a,
+        T: 'a;
+
+    type EdgeLabelIter<'a> = EdgeLabelIter<T>
+    where
+        Self: 'a,
+        T: 'a;
+
+    #[inline]
+    fn query_label(&self, label: T) -> Option<usize> {
+        self.label_index_map.get(&label).copied()
+    }
+
+    #[inline]
+    fn vertex_label(&self, node_id: usize) -> Result<Option<T>, Error> {
+        if let Some(node) = self.nodes.get(node_id) {
+            Ok(Some(node.label))
+        } else {
+            Err(Error::IndexOutOfBounds(node_id, self.nodes_len()))
+        }
+    }
+
+    #[inline]
+    fn edge_label(&self, source: usize, target: usize) -> Result<Self::EdgeLabelIter<'_>, Error> {
+        let nodes_len = self.nodes.len();
+
+        let node = if let Some(node) = self.nodes.get(source) {
+            node
+        } else {
+            return Err(Error::IndexOutOfBounds(source, nodes_len));
+        };
+
+        if !self.has_node(target) {
+            return Err(Error::IndexOutOfBounds(target, nodes_len));
+        }
+
+        Ok(EdgeLabelIter(node.children.get(&target).copied()))
+    }
+
+    fn find_children_with_label(
+        &self,
+        _node_id: usize,
+        _label: &T,
+    ) -> Result<<Self as LabelGraph<T>>::Iter<'_>, Error> {
+        unimplemented!()
+    }
+
+    fn labels_of(&self, _node_id: usize) -> Result<Self::LabelIter<'_>, Error> {
+        unimplemented!()
+    }
+
+    #[inline]
+    fn has_edge_label(&self, node_id: usize, label: &T, target: usize) -> Result<bool, Error> {
+        let nodes_len = self.nodes.len();
+
+        let node = if let Some(node) = self.nodes.get(node_id) {
+            node
+        } else {
+            return Err(Error::IndexOutOfBounds(node_id, nodes_len));
+        };
+
+        if !self.has_node(target) {
+            return Err(Error::IndexOutOfBounds(target, nodes_len));
+        }
+
+        Ok(node.children.get(&target) == Some(label))
+    }
+}
+
+/// The type of `builder_mut` builders for this type of graphs.
+#[derive(Debug)]
+pub struct SLGBuilderMut<'a, T: GraphLabel> {
+    graph: &'a mut SLGraph<T>,
+}
+
+impl<'a, T: GraphLabel> BuilderMut for SLGBuilderMut<'a, T> {
+    type Label = T;
+
+    type Graph = SLGraph<T>;
+
+    type ResultBuilder<'b> = SLGBuilderMut<'b, T>
+    where
+        T:'b;
+
+    fn from_graph_mut(graph: &mut Self::Graph) -> Self::ResultBuilder<'_> {
+        SLGBuilderMut { graph }
+    }
+
+    fn add_vertex(&mut self, label: T) -> Result<usize, Error> {
+        if let Some(old_node) = self.graph.label_index_map.get(&label) {
+            dbg!(label);
+            return Err(Error::DuplicatedNode(*old_node));
+        }
+
+        self.graph
+            .nodes
+            .push(SLNode::new(Default::default(), label));
+
+        let new = self.graph.nodes_len() - 1;
+
+        self.graph.label_index_map.insert(label, new);
+
+        Ok(new)
+    }
+
+    fn add_edge(&mut self, source: usize, target: usize, label: Self::Label) -> Result<(), Error> {
+        let graph = &mut self.graph;
+
+        let nodes_len = graph.nodes.len();
+
+        // NOTE: We check the validity of `target` first because we
+        // need to borrow graph mutably later, which would prevent us
+        // from borrowing graph immutably to check the validity of
+        // `target` then.
+        if !graph.has_node(target) {
+            return Err(Error::IndexOutOfBounds(target, nodes_len));
+        }
+
+        let node = if let Some(node) = graph.nodes.get_mut(source) {
+            node
+        } else {
+            return Err(Error::IndexOutOfBounds(source, nodes_len));
+        };
+
+        if node.children.get(&target).is_some() {
+            return Err(Error::DuplicatedEdge(source, target));
+        }
+
+        // We checked what we need to check, so this is safe.
+        unsafe {
+            node.add_edge(target, label);
+        }
+
+        Ok(())
+    }
+
+    fn remove_edge<F>(&mut self, source: usize, target: usize, predicate: F) -> Result<(), Error>
+    where
+        F: Fn(Self::Label) -> bool,
+    {
+        let graph = &mut self.graph;
+
+        let nodes_len = graph.nodes.len();
+
+        // NOTE: We check the validity of `target` first because we
+        // need to borrow graph mutably later, which would prevent us
+        // from borrowing graph immutably to check the validity of
+        // `target` then.
+        if !graph.has_node(target) {
+            return Err(Error::IndexOutOfBounds(target, nodes_len));
+        }
+
+        let node = if let Some(node) = graph.nodes.get_mut(source) {
+            node
+        } else {
+            return Err(Error::IndexOutOfBounds(source, nodes_len));
+        };
+
+        if let Some(child_label) = node.children.get(&target) {
+            // There is only one label, so we just check this label.
+            if predicate(*child_label) {
+                node.children.remove(&target);
+            }
+
+            Ok(())
+        } else {
+            Err(Error::DuplicatedEdge(source, target))
+        }
+    }
+}