2 files changed, 72 insertions, 0 deletions

diff --git a/crates/ra_arena/src/lib.rs b/crates/ra_arena/src/lib.rs
index a5eeb4118..040977dc4 100644
--- a/crates/ra_arena/src/lib.rs
+++ b/crates/ra_arena/src/lib.rs
@@ -6,6 +6,8 @@ use std::{
     ops::{Index, IndexMut},
 };
 
+pub mod map;
+
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct RawId(u32);
 
diff --git a/crates/ra_arena/src/map.rs b/crates/ra_arena/src/map.rs
new file mode 100644
index 000000000..2f09d677f
--- /dev/null
+++ b/crates/ra_arena/src/map.rs
@@ -0,0 +1,70 @@
+//! A map from arena IDs to some other type. Space requirement is O(highest ID).
+
+use std::marker::PhantomData;
+
+use super::ArenaId;
+
+/// 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, T> {
+    v: Vec<Option<T>>,
+    _ty: PhantomData<ID>,
+}
+
+impl<ID: ArenaId, T> ArenaMap<ID, T> {
+    pub fn insert(&mut self, id: ID, t: T) {
+        let idx = Self::to_idx(id);
+        if self.v.capacity() <= idx {
+            self.v.reserve(idx + 1 - self.v.capacity());
+        }
+        if self.v.len() <= idx {
+            while self.v.len() <= idx {
+                self.v.push(None);
+            }
+        }
+        self.v[idx] = Some(t);
+    }
+
+    pub fn get(&self, id: ID) -> Option<&T> {
+        self.v.get(Self::to_idx(id)).and_then(|it| it.as_ref())
+    }
+
+    pub fn values(&self) -> impl Iterator<Item = &T> {
+        self.v.iter().filter_map(|o| o.as_ref())
+    }
+
+    pub fn values_mut(&mut self) -> impl Iterator<Item = &mut T> {
+        self.v.iter_mut().filter_map(|o| o.as_mut())
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = (ID, &T)> {
+        self.v
+            .iter()
+            .enumerate()
+            .filter_map(|(idx, o)| Some((Self::from_idx(idx), o.as_ref()?)))
+    }
+
+    fn to_idx(id: ID) -> usize {
+        u32::from(id.into_raw()) as usize
+    }
+
+    fn from_idx(idx: usize) -> ID {
+        ID::from_raw((idx as u32).into())
+    }
+}
+
+impl<ID: ArenaId, T> std::ops::Index<ID> for ArenaMap<ID, T> {
+    type Output = T;
+    fn index(&self, id: ID) -> &T {
+        self.v[Self::to_idx(id)].as_ref().unwrap()
+    }
+}
+
+impl<ID, T> Default for ArenaMap<ID, T> {
+    fn default() -> Self {
+        ArenaMap {
+            v: Vec::new(),
+            _ty: PhantomData,
+        }
+    }
+}