stdlib: Modify union-find to guarantee that root nodes are always less than or equal to leaf nodes; add a "prune" method

Author: pcwalton

src/lib/ufind.rs

@@ -2,6 +2,8 @@ import option::none;
 import option::some;
 
 // A very naive implementation of union-find with unsigned integer nodes.
+// Maintains the invariant that the root of a node is always equal to or less
+// than the node itself.
 
 type node = option::t[uint];
 type ufind = rec(mutable vec[mutable node] nodes);
@@ -20,18 +22,27 @@ fn make_set(&ufind ufnd) -> uint {
 
 fn find(&ufind ufnd, uint n) -> uint {
     alt (ufnd.nodes.(n)) {
-    case (none[uint]) { ret n; }
-    case (some[uint](?m)) {
-        // TODO: "be"
-        ret find(ufnd, m);
-    }
+        case (none[uint]) { ret n; }
+        case (some[uint](?m)) { be find(ufnd, m); }
     }
 }
 
 fn union(&ufind ufnd, uint m, uint n) {
     auto m_root = find(ufnd, m);
     auto n_root = find(ufnd, n);
-    auto ptr = some[uint](n_root);
-    ufnd.nodes.(m_root) = ptr;
+    if (m_root < n_root) {
+        ufnd.nodes.(n_root) = some[uint](m_root);
+    } else {
+        ufnd.nodes.(m_root) = some[uint](n_root);
+    }
+}
+
+// Removes all sets with IDs greater than or equal to the given value.
+fn prune(&ufind ufnd, uint n) {
+    // TODO: Use "slice" once we get rid of "mutable?"
+    while (n != 0u) {
+        vec::pop[node](ufnd.nodes);
+        n -= 1u;
+    }
 }