1 files changed, 62 insertions, 0 deletions

diff --git a/crates/arena/src/map.rs b/crates/arena/src/map.rs
new file mode 100644
index 000000000..0f33907c0
--- /dev/null
+++ b/crates/arena/src/map.rs
@@ -0,0 +1,62 @@
+//! A map from arena IDs to some other type. Space requirement is O(highest ID).
+
+use std::marker::PhantomData;
+
+use crate::Idx;
+
+/// A map from arena IDs to some other type. Space requirement is O(highest ID).
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ArenaMap<ID, V> {
+    v: Vec<Option<V>>,
+    _ty: PhantomData<ID>,
+}
+
+impl<T, V> ArenaMap<Idx<T>, V> {
+    pub fn insert(&mut self, id: Idx<T>, t: V) {
+        let idx = Self::to_idx(id);
+
+        self.v.resize_with((idx + 1).max(self.v.len()), || None);
+        self.v[idx] = Some(t);
+    }
+
+    pub fn get(&self, id: Idx<T>) -> Option<&V> {
+        self.v.get(Self::to_idx(id)).and_then(|it| it.as_ref())
+    }
+
+    pub fn get_mut(&mut self, id: Idx<T>) -> Option<&mut V> {
+        self.v.get_mut(Self::to_idx(id)).and_then(|it| it.as_mut())
+    }
+
+    pub fn values(&self) -> impl Iterator<Item = &V> {
+        self.v.iter().filter_map(|o| o.as_ref())
+    }
+
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &mut V> {
+        self.v.iter_mut().filter_map(|o| o.as_mut())
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (Idx<T>, &V)> {
+        self.v.iter().enumerate().filter_map(|(idx, o)| Some((Self::from_idx(idx), o.as_ref()?)))
+    }
+
+    fn to_idx(id: Idx<T>) -> usize {
+        u32::from(id.into_raw()) as usize
+    }
+
+    fn from_idx(idx: usize) -> Idx<T> {
+        Idx::from_raw((idx as u32).into())
+    }
+}
+
+impl<T, V> std::ops::Index<Idx<V>> for ArenaMap<Idx<V>, T> {
+    type Output = T;
+    fn index(&self, id: Idx<V>) -> &T {
+        self.v[Self::to_idx(id)].as_ref().unwrap()
+    }
+}
+
+impl<T, V> Default for ArenaMap<Idx<V>, T> {
+    fn default() -> Self {
+        ArenaMap { v: Vec::new(), _ty: PhantomData }
+    }
+}