1// Copyright 2018 The Abseil Authors.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// https://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15#include "absl/container/internal/raw_hash_set.h"
16
17#include <atomic>
18#include <cstddef>
19
20#include "absl/base/config.h"
21
22namespace absl {
23namespace container_internal {
24
25constexpr size_t Group::kWidth;
26
27// Returns "random" seed.
28inline size_t RandomSeed() {
29#if ABSL_HAVE_THREAD_LOCAL
30 static thread_local size_t counter = 0;
31 size_t value = ++counter;
32#else // ABSL_HAVE_THREAD_LOCAL
33 static std::atomic<size_t> counter(0);
34 size_t value = counter.fetch_add(1, std::memory_order_relaxed);
35#endif // ABSL_HAVE_THREAD_LOCAL
36 return value ^ static_cast<size_t>(reinterpret_cast<uintptr_t>(&counter));
37}
38
39bool ShouldInsertBackwards(size_t hash, ctrl_t* ctrl) {
40 // To avoid problems with weak hashes and single bit tests, we use % 13.
41 // TODO(kfm,sbenza): revisit after we do unconditional mixing
42 return (H1(hash, ctrl) ^ RandomSeed()) % 13 > 6;
43}
44
45} // namespace container_internal
46} // namespace absl
47