[base] Template hashmap on key and value

src/base/hashmap-entry.h

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_BASE_HASHMAP_ENTRY_H_
+#define V8_BASE_HASHMAP_ENTRY_H_
+
+#include <cstdint>
+
+namespace v8 {
+namespace base {
+
+// HashMap entries are (key, value, hash) triplets, with a boolean indicating if
+// they are an empty entry. Some clients may not need to use the value slot
+// (e.g. implementers of sets, where the key is the value).
+template <typename Key, typename Value>
+struct TemplateHashMapEntry {
+  Key key;
+  Value value;
+  uint32_t hash;  // The full hash value for key
+
+  TemplateHashMapEntry(Key key, Value value, uint32_t hash)
+      : key(key), value(value), hash(hash), exists_(true) {}
+
+  bool exists() const { return exists_; }
+
+  void clear() { exists_ = false; }
+
+ private:
+  bool exists_;
+};
+
+// Specialization for pointer-valued keys
+template <typename Key, typename Value>
+struct TemplateHashMapEntry<Key*, Value> {
+  Key* key;
+  Value value;
+  uint32_t hash;  // The full hash value for key
+
+  TemplateHashMapEntry(Key* key, Value value, uint32_t hash)
+      : key(key), value(value), hash(hash) {}
+
+  bool exists() const { return key != nullptr; }
+
+  void clear() { key = nullptr; }
+};
+
+// TODO(leszeks): There could be a specialisation for void values (e.g. for
+// sets), which omits the value field
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_HASHMAP_ENTRY_H_