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diff --git a/contrib/frozen/include/frozen/unordered_set.h b/contrib/frozen/include/frozen/unordered_set.h
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+++ b/contrib/frozen/include/frozen/unordered_set.h
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+/*
+ * Frozen
+ * Copyright 2016 QuarksLab
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+#ifndef FROZEN_LETITGO_UNORDERED_SET_H
+#define FROZEN_LETITGO_UNORDERED_SET_H
+
+#include "frozen/bits/basic_types.h"
+#include "frozen/bits/constexpr_assert.h"
+#include "frozen/bits/elsa.h"
+#include "frozen/bits/pmh.h"
+#include "frozen/bits/version.h"
+#include "frozen/random.h"
+
+#include <utility>
+
+namespace frozen {
+
+namespace bits {
+
+struct Get {
+  template <class T> constexpr T const &operator()(T const &key) const {
+    return key;
+  }
+};
+
+} // namespace bits
+
+template <class Key, std::size_t N, typename Hash = elsa<Key>,
+          class KeyEqual = std::equal_to<Key>>
+class unordered_set {
+  static constexpr std::size_t storage_size =
+      bits::next_highest_power_of_two(N) * (N < 32 ? 2 : 1); // size adjustment to prevent high collision rate for small sets
+  using container_type = bits::carray<Key, N>;
+  using tables_type = bits::pmh_tables<storage_size, Hash>;
+
+  KeyEqual const equal_;
+  container_type keys_;
+  tables_type tables_;
+
+public:
+  /* typedefs */
+  using key_type = Key;
+  using value_type = Key;
+  using size_type = typename container_type::size_type;
+  using difference_type = typename container_type::difference_type;
+  using hasher = Hash;
+  using key_equal = KeyEqual;
+  using const_reference = typename container_type::const_reference;
+  using reference = const_reference;
+  using const_pointer = typename container_type::const_pointer;
+  using pointer = const_pointer;
+  using const_iterator = const_pointer;
+  using iterator = const_iterator;
+
+public:
+  /* constructors */
+  unordered_set(unordered_set const &) = default;
+  constexpr unordered_set(container_type keys, Hash const &hash,
+                          KeyEqual const &equal)
+      : equal_{equal}
+      , keys_{keys}
+      , tables_{bits::make_pmh_tables<storage_size>(
+            keys_, hash, bits::Get{}, default_prg_t{})} {}
+  explicit constexpr unordered_set(container_type keys)
+      : unordered_set{keys, Hash{}, KeyEqual{}} {}
+
+  constexpr unordered_set(std::initializer_list<Key> keys)
+      : unordered_set{keys, Hash{}, KeyEqual{}} {}
+
+  constexpr unordered_set(std::initializer_list<Key> keys, Hash const & hash, KeyEqual const & equal)
+      : unordered_set{container_type{keys}, hash, equal} {
+        constexpr_assert(keys.size() == N, "Inconsistent initializer_list size and type size argument");
+      }
+
+  /* iterators */
+  constexpr const_iterator begin() const { return keys_.begin(); }
+  constexpr const_iterator end() const { return keys_.end(); }
+  constexpr const_iterator cbegin() const { return keys_.cbegin(); }
+  constexpr const_iterator cend() const { return keys_.cend(); }
+
+  /* capacity */
+  constexpr bool empty() const { return !N; }
+  constexpr size_type size() const { return N; }
+  constexpr size_type max_size() const { return N; }
+
+  /* lookup */
+  constexpr std::size_t count(Key const &key) const {
+    auto const k = lookup(key);
+    return equal_(k, key);
+  }
+  constexpr const_iterator find(Key const &key) const {
+    auto const &k = lookup(key);
+    if (equal_(k, key))
+      return &k;
+    else
+      return keys_.end();
+  }
+
+  constexpr std::pair<const_iterator, const_iterator> equal_range(Key const &key) const {
+    auto const &k = lookup(key);
+    if (equal_(k, key))
+      return {&k, &k + 1};
+    else
+      return {keys_.end(), keys_.end()};
+  }
+
+  /* bucket interface */
+  constexpr std::size_t bucket_count() const { return storage_size; }
+  constexpr std::size_t max_bucket_count() const { return storage_size; }
+
+  /* observers*/
+  constexpr hasher hash_function() const { return tables_.hash_; }
+  constexpr key_equal key_eq() const { return equal_; }
+
+private:
+  constexpr auto const &lookup(Key const &key) const {
+    return keys_[tables_.lookup(key)];
+  }
+};
+
+template <typename T, std::size_t N>
+constexpr auto make_unordered_set(T const (&keys)[N]) {
+  return unordered_set<T, N>{keys};
+}
+
+} // namespace frozen
+
+#endif