2 files changed, 73 insertions, 0 deletions
diff --git a/src/data/coarena.rs b/src/data/coarena.rs
new file mode 100644
index 0000000..78cbfa7
--- /dev/null
+++ b/src/data/coarena.rs
@@ -0,0 +1,71 @@
+use crate::data::arena::Index;
+
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug)]
+/// A container for data associated to item existing into another Arena.
+pub struct Coarena<T> {
+    data: Vec<(u64, T)>,
+}
+
+impl<T> Coarena<T> {
+    /// A coarena with no element.
+    pub fn new() -> Self {
+        Self { data: Vec::new() }
+    }
+
+    /// Gets a specific element from the coarena, if it exists.
+    pub fn get(&self, index: Index) -> Option<&T> {
+        let (i, g) = index.into_raw_parts();
+        self.data
+            .get(i)
+            .and_then(|(gg, t)| if g == *gg { Some(t) } else { None })
+    }
+
+    /// Gets a mutable reference to a specific element from the coarena, if it exists.
+    pub fn get_mut(&mut self, index: Index) -> Option<&mut T> {
+        let (i, g) = index.into_raw_parts();
+        self.data
+            .get_mut(i)
+            .and_then(|(gg, t)| if g == *gg { Some(t) } else { None })
+    }
+
+    /// Ensure that elements at the two given indices exist in this coarena, and return their reference.
+    ///
+    /// Missing elements are created automatically and initialized with the `default` value.
+    pub fn ensure_pair_exists(&mut self, a: Index, b: Index, default: T) -> (&mut T, &mut T)
+    where
+        T: Clone,
+    {
+        let (i1, g1) = a.into_raw_parts();
+        let (i2, g2) = b.into_raw_parts();
+
+        assert_ne!(i1, i2, "Cannot index the same object twice.");
+
+        let (elt1, elt2) = if i1 > i2 {
+            if self.data.len() <= i1 {
+                self.data.resize(i1 + 1, (u32::MAX as u64, default.clone()));
+            }
+
+            let (left, right) = self.data.split_at_mut(i1);
+            (&mut right[0], &mut left[i2])
+        } else {
+            // i2 > i1
+            if self.data.len() <= i2 {
+                self.data.resize(i2 + 1, (u32::MAX as u64, default.clone()));
+            }
+
+            let (left, right) = self.data.split_at_mut(i2);
+            (&mut left[i1], &mut right[0])
+        };
+
+        if elt1.0 != g1 {
+            *elt1 = (g1, default.clone());
+        }
+
+        if elt2.0 != g2 {
+            *elt2 = (g2, default);
+        }
+
+        (&mut elt1.1, &mut elt2.1)
+    }
+}
diff --git a/src/data/mod.rs b/src/data/mod.rs
index 0e0459e..672bf94 100644
--- a/src/data/mod.rs
+++ b/src/data/mod.rs
@@ -1,8 +1,10 @@
 //! Data structures modified with guaranteed deterministic behavior after deserialization.
 
+pub use self::coarena::Coarena;
 pub use self::maybe_serializable_data::MaybeSerializableData;
 
 pub mod arena;
+mod coarena;
 pub(crate) mod graph;
 pub(crate) mod hashmap;
 mod maybe_serializable_data;