README.md

@@ -7,7 +7,7 @@ This crate provides data structures to model graphs and to perform algorithms on
 Currently supported are:
 
 * Undirected graph types `Graph`, `AppendGraph`, and `FrozenGraph`, built on a flat, index-based adjacency list,
-* Maps to freely associate custom data with vertices and edges, 
+* `ElementMap` to freely associate custom data with vertices and edges, 
 * Connectivity and pathing algorithms: Dijkstra's algorithm, DFS, and BFS in different variants,
 * Exposed traits to implement custom graph data structures or algorithms.
 
@@ -22,14 +22,14 @@ use grapherity::models::Graph;
 use grapherity::algorithms;
 
 let mut graph = Graph::new();
-// Creates a map for edge weights with a default weight.
-let mut weights = graph.edge_map(1);
 // Adds three vertices and two edges.
 let v1 = graph.add_vertex();
 let v2 = graph.add_vertex();
 let v3 = graph.add_vertex();
 graph.add_edge(v1, v2);
 let e = graph.add_edge(v2, v3);
+// Creates a map for edge weights with a default weight, after creating the edge.
+let mut weights = graph.edge_map(1);
 // Sets a non-default weight for the second edge.
 weights[e] = 5;
 

src/lib.rs

@@ -23,14 +23,14 @@
 //! use grapherity::algorithms;
 //!
 //! let mut graph = Graph::new();
-//! // Creates a map for edge weights with a default weight.
-//! let mut weights = graph.edge_map(1);
 //! // Adds three vertices and two edges.
 //! let v1 = graph.add_vertex();
 //! let v2 = graph.add_vertex();
 //! let v3 = graph.add_vertex();
 //! graph.add_edge(v1, v2);
 //! let e = graph.add_edge(v2, v3);
+//! // Creates a map for edge weights with a default weight, after creating the edges.
+//! let mut weights = graph.edge_map(1);
 //! // Sets a non-default weight for the second edge.
 //! weights[e] = 5;
 //!
@@ -97,7 +97,8 @@
 //! graph.degree(v);
 //! ```
 //!
-//! [`ElementMap`] is not required to panic if provided with an invalid vertex or edge.
+//! [`ElementMap`] does not check validity of vertices or edges and might or might not panic in case
+//! an invalid handle is provided.
 //!
 //! Vertices and edges are provided by the graph and not intended to be generated by users.
 //!

src/maps.rs

@@ -1,7 +1,7 @@
 //! Provides maps to associate custom data to graph vertices and edges.
 //!
-//! [`ElementMap`] provides a copy-on-write, [`Vec`]-backed map to associate data to either all
+//! [`ElementMap`] is a dense, [`Vec`]-backed map to associate data to either all vertices or all
-//! vertices or all edges in a graph.
+//! edges in a graph.
 
 use std::ops::{Index, IndexMut};
 
@@ -12,15 +12,19 @@ use std::ops::{Index, IndexMut};
 /// which means that the provided index conversion function should map elements to contiguous
 /// indices, or indices with relatively few gaps.
 ///
-/// Data allocation happens as copy-on-write, i.e. the backing [`Vec`] is only resized if a value is
-/// written beyond current capacity, but the `default` value can transparently be read. No bound or
-/// validity checks are performed by the map on the provided element handles.
-///
 /// Use [`GraphTopology::vertex_map`] or [`GraphTopology::edge_map`] to obtain an `ElementMap` for
 /// vertices or edges, respectively.
 ///
+/// The map does not grow on its own when the associated graph grows. Use the graph's
+/// [`GraphTopologyAddition::sync_vertex_map`] or [`GraphTopologyAddition::sync_edge_map`] to extend
+/// the map explicitly after adding vertices or edges to it.
+///
+/// No bound or validity checks are performed by the map on the provided element handles.
+///
 /// [`GraphTopology::edge_map`]: crate::traits::GraphTopology::edge_map
 /// [`GraphTopology::vertex_map`]: crate::traits::GraphTopology::vertex_map
+/// [`GraphTopologyAddition::sync_vertex_map`]: crate::traits::GraphTopologyAddition::sync_vertex_map
+/// [`GraphTopologyAddition::sync_edge_map`]: crate::traits::GraphTopologyAddition::sync_edge_map
 pub struct ElementMap<E: Copy, T: Clone> {
     data: Vec<T>,
     default: T,
@@ -29,47 +33,59 @@ pub struct ElementMap<E: Copy, T: Clone> {
 
 impl<E: Copy, T: Clone> ElementMap<E, T> {
     /// Creates a new map with the given `default` value, an index conversion function `to_index`,
-    /// and an initial `capacity`.
+    /// and an initial `len`, i.e. the number of slots that can be read from and written to.
-    pub fn new(default: T, to_index: fn(E) -> usize, capacity: usize) -> Self {
+    ///
+    /// Since `to_index` is usually an implementation detail of the graph model, this is not meant
+    /// to be called directly. Use [`GraphTopology::vertex_map`] or [`GraphTopology::edge_map`]
+    /// instead.
+    ///
+    /// [`GraphTopology::edge_map`]: crate::traits::GraphTopology::edge_map
+    /// [`GraphTopology::vertex_map`]: crate::traits::GraphTopology::vertex_map
+    pub fn new(default: T, to_index: fn(E) -> usize, len: usize) -> Self {
         Self {
-            data: vec![default.clone(); capacity],
+            data: vec![default.clone(); len],
             default,
             to_index,
         }
     }
 
-    /// Returns the total number of data entries the map can write without reallocating. Reads
+    /// Grows the map so that it contains at least `new_len` slots for elements, initialising any
-    /// beyond `capacity` are valid and return the default value.
+    /// newly added slots to the `default` value provided to [`new`], without modifying existing
-    pub fn capacity(&self) -> usize {
+    /// data. Does nothing if the map already has at least `new_len` slots.
-        self.data.capacity()
-    }
-
-    /// Reserves capacity for at least `additional` more data entries. Does nothing if capacity is
-    /// already sufficient.
     ///
-    /// Use this before writing data for new graph elements to avoid incremental growth on the first
+    /// Since the number of elements the map can actually store depend on the index conversion
-    /// write to each new element.
+    /// function, which is usually an implementation detail of the graph model, this is not meant to
-    pub fn reserve(&mut self, additional: usize) {
+    /// be called directly. Use [`GraphTopologyAddition::sync_vertex_map`] or
-        self.data.reserve(additional);
+    /// [`GraphTopologyAddition::sync_edge_map`] instead.
+    ///
+    /// [`new`]: ElementMap::new
+    /// [`GraphTopologyAddition::sync_vertex_map`]: crate::traits::GraphTopologyAddition::sync_vertex_map
+    /// [`GraphTopologyAddition::sync_edge_map`]: crate::traits::GraphTopologyAddition::sync_edge_map
+    pub fn grow(&mut self, new_len: usize) {
+        if new_len > self.data.len() {
+            self.data.resize(new_len, self.default.clone());
+        }
     }
 
     /// Returns a new map over the same elements with values transformed by `f`.
     ///
-    /// The default value of the new map is `f` applied to the original default, so reads beyond the
+    /// The default value of the new map is `f` applied to the original default, so slots later added
-    /// current capacity of the new map behave consistently with the transformation.
+    /// to the new map by [`GraphTopologyAddition::sync_vertex_map`] or
+    /// [`GraphTopologyAddition::sync_edge_map`] are filled consistently with the transformation.
+    ///
+    /// [`GraphTopologyAddition::sync_vertex_map`]: crate::traits::GraphTopologyAddition::sync_vertex_map
+    /// [`GraphTopologyAddition::sync_edge_map`]: crate::traits::GraphTopologyAddition::sync_edge_map
     ///
     /// # Examples
     ///
     /// ```
     /// # use grapherity::maps::ElementMap;
-    /// let mut map: ElementMap<usize, Option<i32>> = ElementMap::new(None, |e| e, 0);
+    /// let mut map: ElementMap<usize, Option<i32>> = ElementMap::new(None, |e| e, 2);
     /// map[1] = Some(42);
     ///
     /// let bools = map.map(|v| v.is_some());
     /// assert_eq!(bools[0], false);
     /// assert_eq!(bools[1], true);
-    /// // Returns mapped default for reads beyond capacity.
-    /// assert_eq!(bools[9], false);
     /// ```
     pub fn map<U: Clone>(&self, f: impl Fn(&T) -> U) -> ElementMap<E, U> {
         ElementMap {
@@ -84,22 +100,13 @@ impl<E: Copy, T: Clone> Index<E> for ElementMap<E, T> {
     type Output = T;
 
     fn index(&self, e: E) -> &T {
-        let i = (self.to_index)(e);
+        &self.data[(self.to_index)(e)]
-        if i < self.data.len() {
-            &self.data[i]
-        } else {
-            &self.default
-        }
     }
 }
 
 impl<E: Copy, T: Clone> IndexMut<E> for ElementMap<E, T> {
     fn index_mut(&mut self, e: E) -> &mut T {
-        let i = (self.to_index)(e);
+        &mut self.data[(self.to_index)(e)]
-        if i >= self.data.len() {
-            self.data.resize(i + 1, self.default.clone());
-        }
-        &mut self.data[i]
     }
 }
 
@@ -109,67 +116,97 @@ mod tests {
 
     #[test]
     fn initial_values_are_default() {
-        let map = ElementMap::new(42, |e| e, 0);
+        let map = ElementMap::new(42, |e| e, 6);
         assert_eq!(map[0], 42, "initial read at 0 should be default");
         assert_eq!(map[5], 42, "initial read at 5 should be default");
     }
 
     #[test]
-    fn reserve_increases_capacity_without_initializing() {
+    #[should_panic]
-        let mut map = ElementMap::new(0, |e| e, 0);
+    fn read_out_of_bounds() {
-        let capacity_before = map.capacity();
+        let map = ElementMap::new(42, |e| e, 3);
-        map.reserve(capacity_before + 10);
+        let _ = map[3];
-        assert!(
-            map.capacity() > capacity_before,
-            "expected sufficient capacity increase after reserve"
-        );
-        assert_eq!(map[5], 0, "expected read beyond length to return default");
     }
 
     #[test]
-    fn reserve_prevents_reallocation_on_write() {
+    #[should_panic]
-        let mut map = ElementMap::new(0, |e| e, 0);
+    fn write_out_of_bounds() {
-        map.reserve(10);
+        let mut map = ElementMap::new(0, |e| e, 3);
-        let capacity_before = map.capacity();
+        map[3] = 1;
-        for i in 0..10 {
-            map[i] = i as i32;
-        }
-        assert_eq!(
-            map.capacity(),
-            capacity_before,
-            "writes within reserved capacity should not reallocate"
-        );
-    }
-
-    #[test]
-    fn reserve_does_not_overwrite_existing_values() {
-        let mut map = ElementMap::new(0, |e| e, 0);
-        map[0] = 5;
-        let capacity_before = map.capacity();
-        map.reserve(capacity_before + 10);
-        assert_eq!(map[0], 5, "reserve should not change existing data");
     }
 
     #[test]
     fn write_and_read() {
-        let mut map = ElementMap::new(0, |e| e, 0);
+        let mut map = ElementMap::new(0, |e| e, 2);
         map[1] = 7;
         assert_eq!(map[1], 7, "unexpected read value after write");
         assert_eq!(map[0], 0, "initial read should be default");
     }
 
+    #[test]
+    fn grow() {
+        let mut map = ElementMap::new(0, |e| e, 2);
+        assert_eq!(
+            map.data.len(),
+            2,
+            "precondition: unexpected data length before grow"
+        );
+        map.grow(10);
+        assert_eq!(map.data.len(), 10, "unexpected data length after grow");
+    }
+
+    #[test]
+    fn grow_extends_with_default() {
+        let mut map = ElementMap::new(17, |e| e, 3);
+        map.grow(6);
+        assert_eq!(map[5], 17, "read of newly grown slot should be default");
+        map[5] = 9;
+        assert_eq!(
+            map[5], 9,
+            "unexpected value after write in newly grown slot"
+        );
+    }
+
+    #[test]
+    fn grow_preserves_existing_values() {
+        let mut map = ElementMap::new(22, |e| e, 3);
+        map[0] = 5;
+        map.grow(10);
+        assert_eq!(map[0], 5, "unexpected data after grow");
+    }
+
+    #[test]
+    fn grow_to_smaller_len() {
+        let mut map = ElementMap::new(23, |e| e, 6);
+        map[5] = 9;
+        map.grow(3);
+        assert_eq!(
+            map[5], 9,
+            "grow to a smaller length should not discard data"
+        );
+        assert_eq!(map.data.len(), 6, "unexpected data length");
+    }
+
     #[test]
     fn map() {
-        let mut source: ElementMap<usize, Option<i32>> = ElementMap::new(None, |e| e, 0);
+        let mut source: ElementMap<usize, Option<i32>> = ElementMap::new(None, |e| e, 3);
         source[1] = Some(42);
         source[2] = None;
         let mapped = source.map(|v| v.is_some());
-        assert_eq!(mapped[0], false, "unset entry should map to false");
+        assert!(!mapped[0], "unset entry should map to false");
-        assert_eq!(mapped[1], true, "Some entry should map to true");
+        assert!(mapped[1], "Some entry should map to true");
-        assert_eq!(mapped[2], false, "explicit None entry should map to false");
+        assert!(!mapped[2], "explicit None entry should map to false");
-        assert_eq!(
+    }
-            mapped[9], false,
+
-            "read beyond capacity should return mapped default"
+    #[test]
+    fn map_grow() {
+        let mut source: ElementMap<usize, Option<i32>> = ElementMap::new(None, |e| e, 2);
+        source[1] = Some(42);
+        let mut mapped = source.map(|v| v.is_some());
+        mapped.grow(4);
+        assert!(
+            !mapped[3],
+            "slot added by grow should hold the mapped default"
         );
     }
 }

src/models/append_graph.rs

@@ -170,7 +170,6 @@ impl GraphTopology for AppendGraph {
         self.vertices.len()
     }
 
-    // TODO: Create with capacity 0, maybe offer pre-allocated alternative?
     fn vertex_map<T: Clone>(&self, default: T) -> ElementMap<Self::Vertex, T> {
         ElementMap::new(default, |v| v.0, self.vertex_capacity())
     }
@@ -179,7 +178,6 @@ impl GraphTopology for AppendGraph {
         self.incidences.len() / 2
     }
 
-    // TODO: Create with capacity 0, maybe offer pre-allocated alternative?
     fn edge_map<T: Clone>(&self, default: T) -> ElementMap<Self::Edge, T> {
         ElementMap::new(default, |e| e.index() / 2, self.edge_capacity())
     }

src/models/frozen_graph.rs

@@ -144,7 +144,8 @@ impl GraphTopology for FrozenGraph {
 
     // TODO: Find a way to reduce the needed capacity for the edge ElementMap. This could be improved by finding a small vertex cover and store those vertices first. See https://en.wikipedia.org/wiki/Vertex_cover#Approximate_evaluation
     fn edge_map<T: Clone>(&self, default: T) -> ElementMap<Self::Edge, T> {
-        ElementMap::new(default, |e| e.0, self.incidences.len() - 1)
+        // "incidences.len() - 1" would underflow for an edge-free graph.
+        ElementMap::new(default, |e| e.0, self.incidences.len().saturating_sub(1))
     }
 
     fn degree(&self, v: Self::Vertex) -> usize {

src/models/graph.rs

@@ -262,7 +262,6 @@ impl GraphTopology for Graph {
         self.vertices.len()
     }
 
-    // TODO: Create with capacity 0, maybe offer pre-allocated alternative?
     fn vertex_map<T: Clone>(&self, default: T) -> ElementMap<Self::Vertex, T> {
         ElementMap::new(default, |v| v.arr_idx(), self.vertex_capacity())
     }
@@ -271,7 +270,6 @@ impl GraphTopology for Graph {
         self.incidences.len() / 2
     }
 
-    // TODO: Create with capacity 0, maybe offer pre-allocated alternative?
     fn edge_map<T: Clone>(&self, default: T) -> ElementMap<Self::Edge, T> {
         ElementMap::new(default, |e| e.arr_idx() / 2, self.edge_capacity())
     }

src/testing/graph_topology_addition_testing.rs

@@ -15,8 +15,10 @@ macro_rules! graph_topology_addition_tests {
             reserve_edges_does_not_affect_vertex_capacity,
             add_vertex,
             add_edge,
-            vertex_map_new_vertex,
+            sync_vertex_map_after_add,
-            edge_map_new_edge,
+            sync_edge_map_after_add,
+            sync_vertex_map_preserves_values,
+            sync_edge_map_preserves_values,
         );
     };
 
@@ -122,31 +124,68 @@ where
     assert_ne!(graph.add_edge(u, v), e, "unexpected duplicate edge");
 }
 
-pub fn vertex_map_new_vertex<G: GraphTopologyAddition>() {
+pub fn sync_vertex_map_after_add<G: GraphTopologyAddition>() {
     let mut graph = G::default();
     let mut map = graph.vertex_map(27);
+    let u = graph.add_vertex();
     let v = graph.add_vertex();
+    graph.sync_vertex_map(&mut map);
     assert_eq!(
-        map[v], 27,
+        map[u], 27,
         "unexpected value from default map read for new vertex"
     );
-    map[v] = 9;
+    map[u] = 9;
     assert_eq!(
-        map[v], 9,
+        map[u], 9,
         "unexpected value from map read after write for new vertex"
     );
+    assert_eq!(
+        map[v], 27,
+        "unexpected value from default map read for second new vertex"
+    );
 }
 
-pub fn edge_map_new_edge<G: GraphTopologyAddition>() {
+pub fn sync_edge_map_after_add<G: GraphTopologyAddition>() {
     let mut graph = G::default();
     let u = graph.add_vertex();
     let v = graph.add_vertex();
     let mut map = graph.edge_map(28);
     let e = graph.add_edge(u, v);
-    assert_eq!(map[e], 28);
+    let f = graph.add_edge(u, v);
+    graph.sync_edge_map(&mut map);
+    assert_eq!(
+        map[e], 28,
+        "unexpected value from default map read for new edge"
+    );
     map[e] = 8;
     assert_eq!(
         map[e], 8,
         "unexpected value from map read after write for new edge"
     );
+    assert_eq!(
+        map[f], 28,
+        "unexpected value from default map read for second new edge"
+    );
+}
+
+pub fn sync_vertex_map_preserves_values<G: GraphTopologyAddition>() {
+    let mut graph = G::default();
+    let v = graph.add_vertex();
+    let mut map = graph.vertex_map(0);
+    map[v] = 5;
+    graph.reserve_vertices(20);
+    graph.sync_vertex_map(&mut map);
+    assert_eq!(map[v], 5, "sync should not change existing map values");
+}
+
+pub fn sync_edge_map_preserves_values<G: GraphTopologyAddition>() {
+    let mut graph = G::default();
+    let u = graph.add_vertex();
+    let v = graph.add_vertex();
+    let e = graph.add_edge(u, v);
+    let mut map = graph.edge_map(0);
+    map[e] = 6;
+    graph.reserve_edges(20);
+    graph.sync_edge_map(&mut map);
+    assert_eq!(map[e], 6, "sync should not change existing map values");
 }

src/testing/graph_topology_testing.rs

@@ -72,12 +72,22 @@ pub fn vertex_count<G: MakeTestGraph>() {
     assert_eq!(graph.vertex_count(), 10, "unexpected vertex count");
 }
 
-pub fn vertex_map<G: MakeTestGraph>() {
+pub fn vertex_map<G: MakeTestGraph>()
-    let (graph, v) = G::single_vertex();
+where
-    let mut map = graph.vertex_map(27);
+    G::Vertex: Debug,
-    assert_eq!(map[v], 27, "unexpected value from default map read");
+{
-    map[v] = 9;
+    let (graph, vertices, _, _) = G::standard();
-    assert_eq!(map[v], 9, "unexpected value from map after write");
+    let mut map = graph.vertex_map(88);
+    for (i, &v) in vertices.iter().enumerate() {
+        assert_eq!(
+            map[v], 88,
+            "unexpected value from default map read for vertex {v:?}"
+        );
+        map[v] = i;
+    }
+    for (i, &v) in vertices.iter().enumerate() {
+        assert_eq!(map[v], i, "unexpected value after write for vertex {v:?}");
+    }
 }
 
 pub fn edge_count_empty<G: MakeTestGraph>() {
@@ -90,12 +100,22 @@ pub fn edge_count<G: MakeTestGraph>() {
     assert_eq!(graph.edge_count(), 18, "unexpected edge count");
 }
 
-pub fn edge_map<G: MakeTestGraph>() {
+pub fn edge_map<G: MakeTestGraph>()
-    let (graph, _, e) = G::single_edge();
+where
-    let mut map = graph.edge_map(28);
+    G::Edge: Debug,
-    assert_eq!(map[e], 28, "unexpected value from default map read");
+{
-    map[e] = 8;
+    let (graph, _, edges, _) = G::standard();
-    assert_eq!(map[e], 8, "unexpected value from map after write");
+    let mut map = graph.edge_map(99);
+    for (i, &(e, _, _)) in edges.iter().enumerate() {
+        assert_eq!(
+            map[e], 99,
+            "unexpected value from default map read for edge {e:?}"
+        );
+        map[e] = i;
+    }
+    for (i, &(e, _, _)) in edges.iter().enumerate() {
+        assert_eq!(map[e], i, "unexpected value after write for edge {e:?}");
+    }
 }
 
 pub fn degree_zero<G: MakeTestGraph>() {

src/traits.rs

@@ -47,6 +47,10 @@ pub trait GraphTopology {
     /// Creates and returns an [`ElementMap`] for the vertices with every slot initialised to
     /// `default`.
     ///
+    /// The returned map only covers the vertices that exist in the graph at the time of the call.
+    /// Use [`GraphTopologyAddition::sync_vertex_map`] to extend the map before accessing new
+    /// vertices.
+    ///
     /// # Examples
     ///
     /// ```
@@ -54,13 +58,11 @@ pub trait GraphTopology {
     /// # use grapherity::models::Graph;
     /// let mut graph = Graph::new();
     /// let v1 = graph.add_vertex();
+    /// let v2 = graph.add_vertex();
     /// let mut labels = graph.vertex_map("z");
     /// assert_eq!(labels[v1], "z");
     /// labels[v1] = "a";
     /// assert_eq!(labels[v1], "a");
-    ///
-    /// // A new vertex is immediately available for read and write.
-    /// let v2 = graph.add_vertex();
     /// assert_eq!(labels[v2], "z");
     /// labels[v2] = "b";
     /// assert_eq!(labels[v2], "b");
@@ -72,6 +74,9 @@ pub trait GraphTopology {
 
     /// Creates and returns an [`ElementMap`] for the edges with every slot initialised to `default`.
     ///
+    /// The returned map only covers the edges that exist in the graph at the time of the call. Use
+    /// [`GraphTopologyAddition::sync_edge_map`] to extend the map before accessing new edges.
+    ///
     /// # Examples
     ///
     /// ```
@@ -81,13 +86,11 @@ pub trait GraphTopology {
     /// let v1 = graph.add_vertex();
     /// let v2 = graph.add_vertex();
     /// let e1 = graph.add_edge(v1, v2);
+    /// let e2 = graph.add_edge(v1, v2);
     /// let mut weights = graph.edge_map(5);
     /// assert_eq!(weights[e1], 5);
     /// weights[e1] = 1;
     /// assert_eq!(weights[e1], 1);
-    ///
-    /// // A new edge is immediately available for read and write.
-    /// let e2 = graph.add_edge(v1, v2);
     /// assert_eq!(weights[e2], 5);
     /// weights[e2] = 2;
     /// assert_eq!(weights[e2], 2);
@@ -180,9 +183,21 @@ pub trait GraphTopology {
 /// undirected graph.
 pub trait GraphTopologyAddition: GraphTopology + Default {
     /// Returns the total number of vertices the graph can hold without reallocating.
+    ///
+    /// For every valid [`Vertex`] handle, the index produced by the conversion function used in
+    /// [`GraphTopology::vertex_map`] must be strictly less than this capacity. Implementors of
+    /// [`GraphTopology::vertex_map`] must size the returned vertex [`ElementMap`] to this capacity.
+    ///
+    /// [`Vertex`]: GraphTopology::Vertex
     fn vertex_capacity(&self) -> usize;
 
     /// Returns the total number of edges the graph can hold without reallocating.
+    ///
+    /// For every valid [`Edge`] handle, the index produced by the conversion function used in
+    /// [`GraphTopology::edge_map`] must be strictly less than this capacity. Implementors of
+    /// [`GraphTopology::edge_map`] must size the returned edge [`ElementMap`] to this capacity.
+    ///
+    /// [`Edge`]: GraphTopology::Edge
     fn edge_capacity(&self) -> usize;
 
     /// Reserves capacity for at least `additional` more vertices. Does nothing if capacity is
@@ -212,6 +227,65 @@ pub trait GraphTopologyAddition: GraphTopology + Default {
     ///
     /// Panics if `u` or `v` is not a valid vertex of this graph.
     fn add_edge(&mut self, u: Self::Vertex, v: Self::Vertex) -> Self::Edge;
+
+    /// Grows `map` so that it covers a number of vertices up to at least [`vertex_capacity`].
+    ///
+    /// The data for any newly covered vertices is initialised to the map's original default value.
+    /// Does nothing if `map` already covers enough vertices.
+    ///
+    /// An [`ElementMap`] does not grow on its own, so call this after [`reserve_vertices`] (or
+    /// after vertices have already been added) and before accessing `map` for any newly added
+    /// vertices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use grapherity::prelude::*;
+    /// # use grapherity::models::Graph;
+    /// let mut graph = Graph::new();
+    /// let mut labels = graph.vertex_map("z");
+    /// graph.reserve_vertices(1);
+    /// graph.sync_vertex_map(&mut labels);
+    /// let v = graph.add_vertex();
+    /// labels[v] = "a";
+    /// assert_eq!(labels[v], "a");
+    /// ```
+    ///
+    /// [`reserve_vertices`]: Self::reserve_vertices
+    /// [`vertex_capacity`]: Self::vertex_capacity
+    fn sync_vertex_map<T: Clone>(&self, map: &mut ElementMap<Self::Vertex, T>) {
+        map.grow(self.vertex_capacity());
+    }
+
+    /// Grows `map` so that it covers a number of edges up to at least [`edge_capacity`].
+    ///
+    /// The data for any newly covered edges is initialised to the map's original default value.
+    /// Does nothing if `map` already covers enough edges.
+    ///
+    /// An [`ElementMap`] does not grow on its own, so call this after [`reserve_edges`] (or after
+    /// edges have already been added) and before accessing `map` for any newly added edges.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use grapherity::prelude::*;
+    /// # use grapherity::models::Graph;
+    /// let mut graph = Graph::new();
+    /// let mut weights = graph.edge_map(1);
+    /// let v1 = graph.add_vertex();
+    /// let v2 = graph.add_vertex();
+    /// graph.reserve_edges(1);
+    /// graph.sync_edge_map(&mut weights);
+    /// let e = graph.add_edge(v1, v2);
+    /// weights[e] = 5;
+    /// assert_eq!(weights[e], 5);
+    /// ```
+    ///
+    /// [`reserve_edges`]: Self::reserve_edges
+    /// [`edge_capacity`]: Self::edge_capacity
+    fn sync_edge_map<T: Clone>(&self, map: &mut ElementMap<Self::Edge, T>) {
+        map.grow(self.edge_capacity());
+    }
 }
 
 /// A trait that adds deletion operations to an undirected graph topology.