1 files changed, 64 insertions, 0 deletions

diff --git a/crates/ra_analysis/src/loc2id.rs b/crates/ra_analysis/src/loc2id.rs
new file mode 100644
index 000000000..a53ce8348
--- /dev/null
+++ b/crates/ra_analysis/src/loc2id.rs
@@ -0,0 +1,64 @@
+use std::hash::Hash;
+
+use rustc_hash::FxHashMap;
+
+/// There are two principle ways to refer to things:
+///   - by their locatinon (module in foo/bar/baz.rs at line 42)
+///   - by their numeric id (module `ModuleId(42)`)
+///
+/// The first one is more powerful (you can actually find the thing in question
+/// by id), but the second one is so much more compact.
+///
+/// `Loc2IdMap` allows us to have a cake an eat it as well: by maintaining a
+/// bidirectional mapping between positional and numeric ids, we can use compact
+/// representation wich still allows us to get the actual item
+#[derive(Debug)]
+pub(crate) struct Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    loc2id: FxHashMap<L, ID>,
+    id2loc: FxHashMap<ID, L>,
+}
+
+impl<L, ID> Default for Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    fn default() -> Self {
+        Loc2IdMap {
+            loc2id: FxHashMap::default(),
+            id2loc: FxHashMap::default(),
+        }
+    }
+}
+
+impl<L, ID> Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    pub fn loc2id(&mut self, loc: &L) -> ID {
+        match self.loc2id.get(loc) {
+            Some(id) => return id.clone(),
+            None => (),
+        }
+        let id = self.loc2id.len();
+        assert!(id < u32::max_value() as usize);
+        let id = ID::from_u32(id as u32);
+        self.loc2id.insert(loc.clone(), id.clone());
+        self.id2loc.insert(id.clone(), loc.clone());
+        id
+    }
+
+    pub fn id2loc(&self, id: &ID) -> L {
+        self.id2loc[id].clone()
+    }
+}
+
+pub(crate) trait NumericId: Clone + Eq + Hash {
+    fn from_u32(id: u32) -> Self;
+    fn to_u32(self) -> u32;
+}