1 files changed, 155 insertions, 0 deletions
diff --git a/xpcom/ds/nsCheapSets.h b/xpcom/ds/nsCheapSets.h
new file mode 100644
index 0000000000..3d09ccfdb7
--- /dev/null
+++ b/xpcom/ds/nsCheapSets.h
@@ -0,0 +1,155 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __nsCheapSets_h__
+#define __nsCheapSets_h__
+
+#include "nsTHashtable.h"
+#include <stdint.h>
+
+enum nsCheapSetOperator {
+  OpNext = 0,    // enumerator says continue
+  OpRemove = 1,  // enumerator says remove and continue
+};
+
+/**
+ * A set that takes up minimal size when there are 0 or 1 entries in the set.
+ * Use for cases where sizes of 0 and 1 are even slightly common.
+ */
+template <typename EntryType>
+class nsCheapSet {
+ public:
+  typedef typename EntryType::KeyType KeyType;
+  typedef nsCheapSetOperator (*Enumerator)(EntryType* aEntry, void* userArg);
+
+  nsCheapSet() : mState(ZERO) { mUnion.table = nullptr; }
+  ~nsCheapSet() { Clear(); }
+
+  /**
+   * Remove all entries.
+   */
+  void Clear() {
+    switch (mState) {
+      case ZERO:
+        break;
+      case ONE:
+        GetSingleEntry()->~EntryType();
+        break;
+      case MANY:
+        delete mUnion.table;
+        break;
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        break;
+    }
+    mState = ZERO;
+  }
+
+  void Put(const KeyType aVal);
+
+  void Remove(const KeyType aVal);
+
+  bool Contains(const KeyType aVal) {
+    switch (mState) {
+      case ZERO:
+        return false;
+      case ONE:
+        return GetSingleEntry()->KeyEquals(EntryType::KeyToPointer(aVal));
+      case MANY:
+        return !!mUnion.table->GetEntry(aVal);
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        return false;
+    }
+  }
+
+  uint32_t EnumerateEntries(Enumerator aEnumFunc, void* aUserArg) {
+    switch (mState) {
+      case ZERO:
+        return 0;
+      case ONE:
+        if (aEnumFunc(GetSingleEntry(), aUserArg) == OpRemove) {
+          GetSingleEntry()->~EntryType();
+          mState = ZERO;
+        }
+        return 1;
+      case MANY: {
+        uint32_t n = mUnion.table->Count();
+        for (auto iter = mUnion.table->Iter(); !iter.Done(); iter.Next()) {
+          auto entry = static_cast<EntryType*>(iter.Get());
+          if (aEnumFunc(entry, aUserArg) == OpRemove) {
+            iter.Remove();
+          }
+        }
+        return n;
+      }
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        return 0;
+    }
+  }
+
+ private:
+  EntryType* GetSingleEntry() {
+    return reinterpret_cast<EntryType*>(&mUnion.singleEntry[0]);
+  }
+
+  enum SetState { ZERO, ONE, MANY };
+
+  union {
+    nsTHashtable<EntryType>* table;
+    char singleEntry[sizeof(EntryType)];
+  } mUnion;
+  enum SetState mState;
+};
+
+template <typename EntryType>
+void nsCheapSet<EntryType>::Put(const KeyType aVal) {
+  switch (mState) {
+    case ZERO:
+      new (GetSingleEntry()) EntryType(EntryType::KeyToPointer(aVal));
+      mState = ONE;
+      return;
+    case ONE: {
+      nsTHashtable<EntryType>* table = new nsTHashtable<EntryType>();
+      EntryType* entry = GetSingleEntry();
+      table->PutEntry(entry->GetKey());
+      entry->~EntryType();
+      mUnion.table = table;
+      mState = MANY;
+    }
+      [[fallthrough]];
+
+    case MANY:
+      mUnion.table->PutEntry(aVal);
+      return;
+    default:
+      MOZ_ASSERT_UNREACHABLE("bogus state");
+      return;
+  }
+}
+
+template <typename EntryType>
+void nsCheapSet<EntryType>::Remove(const KeyType aVal) {
+  switch (mState) {
+    case ZERO:
+      break;
+    case ONE:
+      if (Contains(aVal)) {
+        GetSingleEntry()->~EntryType();
+        mState = ZERO;
+      }
+      break;
+    case MANY:
+      mUnion.table->RemoveEntry(aVal);
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("bogus state");
+      break;
+  }
+}
+
+#endif