Add FromGraph trait, and add implementation for FrozenGraph

src/models/frozen_graph.rs

@@ -3,7 +3,7 @@
 use std::ops::IndexMut;
 
 use crate::maps::ElementMap;
-use crate::traits::{GraphTopology, Incidence, IncidenceCursor};
+use crate::traits::{FromGraph, GraphTopology, Incidence, IncidenceCursor};
 
 /// An opaque handle identifying a vertex in a [`FrozenGraph`].
 ///
@@ -103,67 +103,6 @@ pub struct FrozenGraph {
 }
 
 impl FrozenGraph {
-    // TODO: Move this to a new FromGraph trait and adapt the MakeTestGraph impl to use helper functions like "fn fixture_empty<G: FromGraph>() -> G { G::from_graph(&AppendGraph::empty()).0 }".
-    /// Creates a graph instance as a copy of another graph.
-    ///
-    /// Returns the copied graph instance and two mappings of the `source` graph vertices and edges
-    /// to the new ones. Note that the returned mappings cannot support additional growth, i.e. any
-    /// vertex or edge added to the `source` graph after calling `from_graph` will map to an invalid
-    /// vertex or edge in the new `FrozenGraph`.
-    pub fn from_graph<G: GraphTopology>(
-        source: &G,
-    ) -> (
-        Self,
-        ElementMap<G::Vertex, Vertex>,
-        ElementMap<G::Edge, Edge>,
-    ) {
-        // Builds vertex list with sentinel and vertex map.
-        let mut vertex_map = source.vertex_map(Vertex(usize::MAX));
-        let mut vertices = Vec::with_capacity(source.vertex_count() + 1);
-        let mut offset = 0;
-        for (i, v) in source.vertices().enumerate() {
-            vertex_map[v] = Vertex(i);
-            vertices.push(offset);
-            offset += source.degree(v);
-        }
-        vertices.push(offset);
-
-        // Builds incidence list and edge map. Uses "usize::MAX" as "unset" here: an edge can never
-        // be normalized to "usize::MAX", since it must be the smaller index of two incidences.
-        const UNSET: usize = usize::MAX;
-        let mut edge_map = source.edge_map(Edge(UNSET));
-        let mut incidences = Vec::with_capacity(source.edge_count() * 2);
-        for v in source.vertices() {
-            for Incidence { vertex: u, edge: e } in source.incidences(v) {
-                let em = edge_map.index_mut(e);
-                if em.0 == UNSET {
-                    // Adds the first incidence for the edge, but we don't know the opposite, yet.
-                    *em = Edge(incidences.len());
-                    incidences.push(IncidenceEntry {
-                        adjacent: vertex_map[u],
-                        opposite: 0,
-                    });
-                } else {
-                    // Adds the second incidence for the edge and fills the opposite for the first.
-                    incidences[em.0].opposite = incidences.len();
-                    incidences.push(IncidenceEntry {
-                        adjacent: vertex_map[u],
-                        opposite: em.0,
-                    });
-                }
-            }
-        }
-
-        (
-            Self {
-                vertices,
-                incidences,
-            },
-            vertex_map,
-            edge_map,
-        )
-    }
-
     fn raw_incidences(&self, v: Vertex) -> impl Iterator<Item = usize> {
         self.vertices[v.0]..self.vertices[v.0 + 1]
     }
@@ -186,12 +125,6 @@ impl FrozenGraph {
     }
 }
 
-impl<G: GraphTopology> From<&G> for FrozenGraph {
-    fn from(graph: &G) -> Self {
-        Self::from_graph(graph).0
-    }
-}
-
 impl GraphTopology for FrozenGraph {
     type Vertex = Vertex;
     type Edge = Edge;
@@ -257,6 +190,68 @@ impl GraphTopology for FrozenGraph {
     }
 }
 
+impl<G: GraphTopology> From<&G> for FrozenGraph {
+    fn from(graph: &G) -> Self {
+        Self::from_graph(graph).0
+    }
+}
+
+impl FromGraph for FrozenGraph {
+    fn from_graph<G: GraphTopology>(
+        source: &G,
+    ) -> (
+        Self,
+        ElementMap<G::Vertex, Vertex>,
+        ElementMap<G::Edge, Edge>,
+    ) {
+        // Builds vertex list with sentinel and vertex map.
+        let mut vertex_map = source.vertex_map(Vertex(usize::MAX));
+        let mut vertices = Vec::with_capacity(source.vertex_count() + 1);
+        let mut offset = 0;
+        for (i, v) in source.vertices().enumerate() {
+            vertex_map[v] = Vertex(i);
+            vertices.push(offset);
+            offset += source.degree(v);
+        }
+        vertices.push(offset);
+
+        // Builds incidence list and edge map. Uses "usize::MAX" as "unset" here: an edge can never
+        // be normalized to "usize::MAX", since it must be the smaller index of two incidences.
+        const UNSET: usize = usize::MAX;
+        let mut edge_map = source.edge_map(Edge(UNSET));
+        let mut incidences = Vec::with_capacity(source.edge_count() * 2);
+        for v in source.vertices() {
+            for Incidence { vertex: u, edge: e } in source.incidences(v) {
+                let em = edge_map.index_mut(e);
+                if em.0 == UNSET {
+                    // Adds the first incidence for the edge, but we don't know the opposite, yet.
+                    *em = Edge(incidences.len());
+                    incidences.push(IncidenceEntry {
+                        adjacent: vertex_map[u],
+                        opposite: 0,
+                    });
+                } else {
+                    // Adds the second incidence for the edge and fills the opposite for the first.
+                    incidences[em.0].opposite = incidences.len();
+                    incidences.push(IncidenceEntry {
+                        adjacent: vertex_map[u],
+                        opposite: em.0,
+                    });
+                }
+            }
+        }
+
+        (
+            Self {
+                vertices,
+                incidences,
+            },
+            vertex_map,
+            edge_map,
+        )
+    }
+}
+
 #[cfg(test)]
 mod trait_tests {
     use super::FrozenGraph;