1 files changed, 40 insertions, 0 deletions
diff --git a/src/libsyntax/util/interner.rs b/src/libsyntax/util/interner.rs
new file mode 100644
index 00000000000..89078bfaa36
--- /dev/null
+++ b/src/libsyntax/util/interner.rs
@@ -0,0 +1,40 @@
+// An "interner" is a data structure that associates values with uint tags and
+// allows bidirectional lookup; i.e. given a value, one can easily find the
+// type, and vice versa.
+import std::map;
+import std::map::{hashmap, hashfn, eqfn};
+import dvec::{dvec, extensions};
+
+type interner<T> =
+    {map: hashmap<T, uint>,
+     vect: dvec<T>,
+     hasher: hashfn<T>,
+     eqer: eqfn<T>};
+
+fn mk<T: copy>(hasher: hashfn<T>, eqer: eqfn<T>) -> interner<T> {
+    let m = map::hashmap::<T, uint>(hasher, eqer);
+    ret {map: m, vect: dvec(), hasher: hasher, eqer: eqer};
+}
+
+fn intern<T: copy>(itr: interner<T>, val: T) -> uint {
+    alt itr.map.find(val) {
+      some(idx) { ret idx; }
+      none {
+        let new_idx = itr.vect.len();
+        itr.map.insert(val, new_idx);
+        itr.vect.push(val);
+        ret new_idx;
+      }
+    }
+}
+
+// |get| isn't "pure" in the traditional sense, because it can go from
+// failing to returning a value as items are interned. But for typestate,
+// where we first check a pred and then rely on it, ceasing to fail is ok.
+pure fn get<T: copy>(itr: interner<T>, idx: uint) -> T {
+    unchecked {
+        itr.vect.get_elt(idx)
+    }
+}
+
+fn len<T>(itr: interner<T>) -> uint { ret itr.vect.len(); }