introduce OpenTHashSet.getOrAdd, which may be used to avoid hashing twice when interning

GitOrigin-RevId: 21f3bc68974f4fd3213f72139c80e017b2295aaf

platform/util/src/com/intellij/util/containers/HashSetInterner.java

@@ -33,15 +33,7 @@ public class HashSetInterner<T> extends Interner<T> {
   @Override
   @NotNull
   public T intern(@NotNull T name) {
-    T interned = mySet.get(name);
-    if (interned != null) {
-      return interned;
-    }
-
-    boolean added = mySet.add(name);
-    assert added;
-
-    return name;
+    return mySet.getOrAdd(name);
   }
 
   @Override

platform/util/src/com/intellij/util/containers/OpenTHashSet.java

@@ -17,6 +17,7 @@ package com.intellij.util.containers;
 
 import gnu.trove.THashSet;
 import gnu.trove.TObjectHashingStrategy;
+import org.jetbrains.annotations.Contract;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
@@ -83,4 +84,22 @@ public class OpenTHashSet<T> extends THashSet<T> {
     final int index = index(obj);
     return index < 0 ? null : get(index);
   }
+
+  /**
+   * Returns an element of this set equal to the given one, or adds the given one to the set and returns it.
+   * Can be used for interning objects to save memory.
+   */
+  @Contract("!null -> !null")
+  public T getOrAdd(final T obj) {
+    final int index = insertionIndex(obj);
+
+    boolean alreadyStored = index < 0;
+    if (alreadyStored) return get(-index - 1);
+
+    Object old = _set[index];
+    _set[index] = obj;
+
+    postInsertHook(old == null);
+    return obj;
+  }
 }