Add ElementMap::reserve to replace ElementMap::expand, update tests

ElementMap tests are now self contained unit tests, without any graph context. The only exception is the repurposed test "reused_slot_returns_old_value", which is not suited for GraphTopologyDeletion, but actually specific for Graph.

src/maps.rs

@@ -41,15 +41,13 @@ impl<E: Copy, T: Clone> ElementMap<E, T> {
         self.data.capacity()
     }
 
-    /// Expands the internal data storage capacity of the map to `capacity`, Does nothing if
-    /// capacity is already sufficient.
+    /// Reserves capacity for at least `additional` more data entries. Does nothing if capacity is
+    /// already sufficient.
     ///
-    /// Use this before writing data for new graph entities to avoid incremental growth on the first
-    /// write to each new entity.
-    pub fn expand(&mut self, capacity: usize) {
-        if capacity > self.data.len() {
-            self.data.resize(capacity, self.default.clone());
-        }
+    /// Use this before writing data for new graph elements to avoid incremental growth on the first
+    /// write to each new element.
+    pub fn reserve(&mut self, additional: usize) {
+        self.data.reserve(additional);
     }
 }
 
@@ -75,3 +73,59 @@ impl<E: Copy, T: Clone> IndexMut<E> for ElementMap<E, T> {
         &mut self.data[i]
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn initial_values_are_default() {
+        let map = ElementMap::new(42, |e| e, 0);
+        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() {
+        let mut map = ElementMap::new(0, |e| e, 0);
+        let capacity_before = map.capacity();
+        map.reserve(capacity_before + 10);
+        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() {
+        let mut map = ElementMap::new(0, |e| e, 0);
+        map.reserve(10);
+        let capacity_before = map.capacity();
+        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);
+        map[1] = 7;
+        assert_eq!(map[1], 7, "unexpected read value after write");
+        assert_eq!(map[0], 0, "initial read should be default");
+    }
+}