text: /*
   * Copyright 2017-present Facebook, Inc.
   *
   * Licensed 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.
   */
  
  #include <folly/hash/Hash.h>
  
  #include <stdint.h>
  
  #include <unordered_map>
  #include <unordered_set>
  #include <utility>
  
  #include <folly/MapUtil.h>
  #include <folly/Range.h>
  #include <folly/portability/GTest.h>
  
  using namespace folly::hash;
  
  TEST(Hash, Fnv32) {
    const char* s1 = "hello, world!";
    const uint32_t s1_res = 3605494790UL;
    EXPECT_EQ(fnv32(s1), s1_res);
    EXPECT_EQ(fnv32(s1), fnv32_buf(s1, strlen(s1)));
  
    const char* s2 = "monkeys! m0nk3yz! ev3ry \\/\\/here~~~~";
    const uint32_t s2_res = 1270448334UL;
    EXPECT_EQ(fnv32(s2), s2_res);
    EXPECT_EQ(fnv32(s2), fnv32_buf(s2, strlen(s2)));
  
    const char* s3 = "";
    const uint32_t s3_res = 2166136261UL;
    EXPECT_EQ(fnv32(s3), s3_res);
    EXPECT_EQ(fnv32(s3), fnv32_buf(s3, strlen(s3)));
  
    const uint8_t s4_data[] = {0xFF, 0xFF, 0xFF, 0x00};
    const char* s4 = reinterpret_cast<const char*>(s4_data);
    const uint32_t s4_res = 2420936562UL;
    EXPECT_EQ(fnv32(s4), s4_res);
    EXPECT_EQ(fnv32(s4), fnv32_buf(s4, strlen(s4)));
  }
  
  TEST(Hash, Fnv64) {
    const char* s1 = "hello, world!";
    const uint64_t s1_res = 13991426986746681734ULL;
    EXPECT_EQ(fnv64(s1), s1_res);
    EXPECT_EQ(fnv64(s1), fnv64_buf(s1, strlen(s1)));
  
    const char* s2 = "monkeys! m0nk3yz! ev3ry \\/\\/here~~~~";
    const uint64_t s2_res = 6091394665637302478ULL;
    EXPECT_EQ(fnv64(s2), s2_res);
    EXPECT_EQ(fnv64(s2), fnv64_buf(s2, strlen(s2)));
  
    const char* s3 = "";
    const uint64_t s3_res = 14695981039346656037ULL;
    EXPECT_EQ(fnv64(s3), s3_res);
    EXPECT_EQ(fnv64(s3), fnv64_buf(s3, strlen(s3)));
  
    const uint8_t s4_data[] = {0xFF, 0xFF, 0xFF, 0x00};
    const char* s4 = reinterpret_cast<const char*>(s4_data);
    const uint64_t s4_res = 2787597222566293202ULL;
    EXPECT_EQ(fnv64(s4), s4_res);
    EXPECT_EQ(fnv64(s4), fnv64_buf(s4, strlen(s4)));
  
    // note: Use fnv64_buf to make a single hash value from multiple
    // fields/datatypes.
    const char* t4_a = "E Pluribus";
    int64_t t4_b = 0xF1E2D3C4B5A69788;
    int32_t t4_c = 0xAB12CD34;
    const char* t4_d = "Unum";
    uint64_t t4_res = 15571330457339273965ULL;
    uint64_t t4_hash1 = fnv64_buf(t4_a, strlen(t4_a));
    uint64_t t4_hash2 =
        fnv64_buf(reinterpret_cast<void*>(&t4_b), sizeof(int64_t), t4_hash1);
    uint64_t t4_hash3 =
        fnv64_buf(reinterpret_cast<void*>(&t4_c), sizeof(int32_t), t4_hash2);
    uint64_t t4_hash4 = fnv64_buf(t4_d, strlen(t4_d), t4_hash3);
    EXPECT_EQ(t4_hash4, t4_res);
    // note: These are probabalistic, not determinate, but c'mon.
    // These hash values should be different, or something's not
    // working.
    EXPECT_NE(t4_hash1, t4_hash4);
    EXPECT_NE(t4_hash2, t4_hash4);
    EXPECT_NE(t4_hash3, t4_hash4);
  }
  
  TEST(Hash, Hsieh32) {
    const char* s1 = "hello, world!";
    const uint32_t s1_res = 2918802987ul;
    EXPECT_EQ(hsieh_hash32(s1), s1_res);
    EXPECT_EQ(hsieh_hash32(s1), hsieh_hash32_buf(s1, strlen(s1)));
  
    const char* s2 = "monkeys! m0nk3yz! ev3ry \\/\\/here~~~~";
    const uint32_t s2_res = 47373213ul;
    EXPECT_EQ(hsieh_hash32(s2), s2_res);
    EXPECT_EQ(hsieh_hash32(s2), hsieh_hash32_buf(s2, strlen(s2)));
  
    const char* s3 = "";
    const uint32_t s3_res = 0;
    EXPECT_EQ(hsieh_hash32(s3), s3_res);
    EXPECT_EQ(hsieh_hash32(s3), hsieh_hash32_buf(s3, strlen(s3)));
  }
  
  TEST(Hash, TWang_Mix64) {
    uint64_t i1 = 0x78a87873e2d31dafULL;
    uint64_t i1_res = 3389151152926383528ULL;
    EXPECT_EQ(i1_res, twang_mix64(i1));
    EXPECT_EQ(i1, twang_unmix64(i1_res));
  
    uint64_t i2 = 0x0123456789abcdefULL;
    uint64_t i2_res = 3061460455458984563ull;
    EXPECT_EQ(i2_res, twang_mix64(i2));
    EXPECT_EQ(i2, twang_unmix64(i2_res));
  }
  
  namespace {
  void checkTWang(uint64_t r) {
    uint64_t result = twang_mix64(r);
    EXPECT_EQ(r, twang_unmix64(result));
  }
  } // namespace
  
  TEST(Hash, TWang_Unmix64) {
    // We'll try (1 << i), (1 << i) + 1, (1 << i) - 1
    for (int i = 1; i < 64; i++) {
      checkTWang((uint64_t(1) << i) - 1);
      checkTWang(uint64_t(1) << i);
      checkTWang((uint64_t(1) << i) + 1);
    }
  }
  
  TEST(Hash, TWang_32From64) {
    uint64_t i1 = 0x78a87873e2d31dafULL;
    uint32_t i1_res = 1525586863ul;
    EXPECT_EQ(twang_32from64(i1), i1_res);
  
    uint64_t i2 = 0x0123456789abcdefULL;
    uint32_t i2_res = 2918899159ul;
    EXPECT_EQ(twang_32from64(i2), i2_res);
  }
  
  TEST(Hash, Jenkins_Rev_Mix32) {
    uint32_t i1 = 3805486511ul;
    uint32_t i1_res = 381808021ul;
    EXPECT_EQ(i1_res, jenkins_rev_mix32(i1));
    EXPECT_EQ(i1, jenkins_rev_unmix32(i1_res));
  
    uint32_t i2 = 2309737967ul;
    uint32_t i2_res = 1834777923ul;
    EXPECT_EQ(i2_res, jenkins_rev_mix32(i2));
    EXPECT_EQ(i2, jenkins_rev_unmix32(i2_res));
  }
  
  namespace {
  void checkJenkins(uint32_t r) {
    uint32_t result = jenkins_rev_mix32(r);
    EXPECT_EQ(r, jenkins_rev_unmix32(result));
  }
  } // namespace
  
  TEST(Hash, Jenkins_Rev_Unmix32) {
    // We'll try (1 << i), (1 << i) + 1, (1 << i) - 1
    for (int i = 1; i < 32; i++) {
      checkJenkins((1U << i) - 1);
      checkJenkins(1U << i);
      checkJenkins((1U << i) + 1);
    }
  }
  
  TEST(Hash, hasher) {
    // Basically just confirms that things compile ok.
    std::unordered_map<int32_t, int32_t, folly::hasher<int32_t>> m;
    m.insert(std::make_pair(4, 5));
    EXPECT_EQ(get_default(m, 4), 5);
  }
  
  TEST(Hash, integral_types) {
    // Basically just confirms that things compile ok.
    std::unordered_set<size_t> hashes;
    folly::Hash hasher;
    hashes.insert(hasher((char)1));
    hashes.insert(hasher((signed char)2));
    hashes.insert(hasher((unsigned char)3));
    hashes.insert(hasher((short)4));
    hashes.insert(hasher((signed short)5));
    hashes.insert(hasher((unsigned short)6));
    hashes.insert(hasher((int)7));
    hashes.insert(hasher((signed int)8));
    hashes.insert(hasher((unsigned int)9));
    hashes.insert(hasher((long)10));
    hashes.insert(hasher((signed long)11));
    hashes.insert(hasher((unsigned long)12));
    hashes.insert(hasher((long long)13));
    hashes.insert(hasher((signed long long)14));
    hashes.insert(hasher((unsigned long long)15));
    hashes.insert(hasher((int8_t)16));
    hashes.insert(hasher((uint8_t)17));
    hashes.insert(hasher((int16_t)18));
    hashes.insert(hasher((uint16_t)19));
    hashes.insert(hasher((int32_t)20));
    hashes.insert(hasher((uint32_t)21));
    hashes.insert(hasher((int64_t)22));
    hashes.insert(hasher((uint64_t)23));
    hashes.insert(hasher((size_t)24));
  
    size_t setSize = 24;
  #if FOLLY_HAVE_INT128_T
    hashes.insert(hasher((__int128_t)25));
    hashes.insert(hasher((__uint128_t)26));
    setSize += 2;
  #endif
    EXPECT_EQ(setSize, hashes.size());
  }
  
  namespace {
  enum class TestEnum {
    MIN = 0,
    ITEM = 1,
    MAX = 2,
  };
  
  enum class TestBigEnum : uint64_t {
    ITEM = 1,
  };
  
  struct TestStruct {};
  } // namespace
  
  namespace std {
  template <>
  struct hash<TestEnum> {
    std::size_t operator()(TestEnum const& e) const noexcept {
      return hash<int>()(static_cast<int>(e));
    }
  };
  
  template <>
  struct hash<TestStruct> {
    std::size_t operator()(TestStruct const&) const noexcept {
      return 0;
    }
  };
  } // namespace std
  
  namespace {
  thread_local size_t allocatedMemorySize{0};
  
  template <class T>
  class TestAlloc {
   public:
    using Alloc = std::allocator<T>;
    using value_type = typename Alloc::value_type;
  
    using pointer = typename Alloc::pointer;
    using const_pointer = typename Alloc::const_pointer;
    using reference = typename Alloc::reference;
    using const_reference = typename Alloc::const_reference;
    using size_type = typename Alloc::size_type;
  
    using propagate_on_container_swap = std::true_type;
    using propagate_on_container_copy_assignment = std::true_type;
    using propagate_on_container_move_assignment = std::true_type;
  
    TestAlloc() {}
  
    template <class T2>
    TestAlloc(TestAlloc<T2> const& other) noexcept : a_(other.a_) {}
  
    template <class T2>
    TestAlloc& operator=(TestAlloc<T2> const& other) noexcept {
      a_ = other.a_;
      return *this;
    }
  
    template <class T2>
    TestAlloc(TestAlloc<T2>&& other) noexcept : a_(std::move(other.a_)) {}
  
    template <class T2>
    TestAlloc& operator=(TestAlloc<T2>&& other) noexcept {
      a_ = std::move(other.a_);
      return *this;
    }
  
    static size_t getAllocatedMemorySize() {
      return allocatedMemorySize;
    }
  
    static void resetTracking() {
      allocatedMemorySize = 0;
    }
  
    T* allocate(size_t n) {
      allocatedMemorySize += n * sizeof(T);
      return a_.allocate(n);
    }
    void deallocate(T* p, size_t n) {
      allocatedMemorySize -= n * sizeof(T);
      a_.deallocate(p, n);
    }
  
   private:
    std::allocator<T> a_;
  
    template <class U>
    friend class TestAlloc;
  };
  
  template <class T1, class T2>
  bool operator==(TestAlloc<T1> const&, TestAlloc<T2> const&) {
    return true;
  }
  
  template <class T1, class T2>
  bool operator!=(TestAlloc<T1> const&, TestAlloc<T2> const&) {
    return false;
  }
  
  template <class M, class A>
  std::vector<size_t> getStats(size_t iter) {
    std::vector<size_t> ret;
    ret.reserve(iter);
    A::resetTracking();
    M m;
    ret.push_back(A::getAllocatedMemorySize());
    for (size_t i = 1; i < iter; ++i) {
      m.insert(std::make_pair(
          folly::to<typename M::key_type>(i), typename M::mapped_type{}));
      ret.push_back(A::getAllocatedMemorySize());
    }
    return ret;
  }
  
  template <typename K, typename V, typename H>
  void testNoCachedHashCode() {
    using A = TestAlloc<std::pair<const K, V>>;
    using M = std::unordered_map<K, V, std::hash<K>, std::equal_to<K>, A>;
    using MActual = std::unordered_map<K, V, H, std::equal_to<K>, A>;
    constexpr int kIter = 10;
    auto expected = getStats<M, A>(kIter);
    auto actual = getStats<MActual, A>(kIter);
    ASSERT_EQ(expected.size(), actual.size());
    for (size_t i = 0; i < expected.size(); ++i) {
      ASSERT_EQ(expected[i], actual[i]);
    }
  }
  } // namespace
  
  TEST(Hash, noCachedHashCode) {
    testNoCachedHashCode<bool, char, folly::hasher<bool>>();
    testNoCachedHashCode<int, char, folly::hasher<int>>();
    testNoCachedHashCode<double, char, folly::hasher<double>>();
    testNoCachedHashCode<TestEnum, char, folly::hasher<TestEnum>>();
  
    testNoCachedHashCode<bool, std::string, folly::Hash>();
    testNoCachedHashCode<int, std::string, folly::Hash>();
    testNoCachedHashCode<double, std::string, folly::Hash>();
    testNoCachedHashCode<TestEnum, std::string, folly::Hash>();
  }
  
  TEST(Hash, integer_conversion) {
    folly::hasher<uint64_t> h;
    uint64_t k = 10;
    EXPECT_EQ(h(k), h(10));
  }
  
  #if FOLLY_HAVE_INT128_T
  TEST(Hash, int128_std_hash) {
    std::unordered_set<__int128> hs;
    hs.insert(__int128_t{1});
    hs.insert(__int128_t{2});
    EXPECT_EQ(2, hs.size());
  
    std::set<unsigned __int128> s;
    s.insert(static_cast<unsigned __int128>(1));
    s.insert(static_cast<unsigned __int128>(2));
    EXPECT_EQ(2, s.size());
  }
  #endif
  
  TEST(Hash, float_types) {
    folly::Hash hasher;
  
    EXPECT_EQ(hasher(0.0f), hasher(-0.0f));
    EXPECT_EQ(hasher(0.0), hasher(-0.0));
  
    // Basically just confirms that things compile ok.
    std::unordered_set<size_t> hashes;
    hashes.insert(hasher(0.0f));
    hashes.insert(hasher(0.1f));
    hashes.insert(hasher(0.2));
    hashes.insert(hasher(0.2f));
    hashes.insert(hasher(-0.3));
    hashes.insert(hasher(-0.3f));
  
    EXPECT_EQ(6, hashes.size());
  }
  
  // Not a full hasher since only handles one type
  class TestHasher {
   public:
    static size_t hash(const std::pair<int, int>& p) {
      return p.first + p.second;
    }
  };
  
  template <typename T, typename... Ts>
  size_t hash_combine_test(const T& t, const Ts&... ts) {
    return hash_combine_generic<TestHasher>(t, ts...);
  }
  
  TEST(Hash, pair) {
    auto a = std::make_pair(1, 2);
    auto b = std::make_pair(3, 4);
    auto c = std::make_pair(1, 2);
    auto d = std::make_pair(2, 1);
    EXPECT_EQ(hash_combine(a), hash_combine(c));
    EXPECT_NE(hash_combine(b), hash_combine(c));
    EXPECT_NE(hash_combine(d), hash_combine(c));
  
    // With composition
    EXPECT_EQ(hash_combine(a, b), hash_combine(c, b));
    // Test order dependence
    EXPECT_NE(hash_combine(a, b), hash_combine(b, a));
  
    // Test with custom hasher
    EXPECT_EQ(hash_combine_test(a), hash_combine_test(c));
    // 3 + 4 != 1 + 2
    EXPECT_NE(hash_combine_test(b), hash_combine_test(c));
    // This time, thanks to a terrible hash function, these are equal
    EXPECT_EQ(hash_combine_test(d), hash_combine_test(c));
    // With composition
    EXPECT_EQ(hash_combine_test(a, b), hash_combine_test(c, b));
    // Test order dependence
    EXPECT_NE(hash_combine_test(a, b), hash_combine_test(b, a));
    // Again, 1 + 2 == 2 + 1
    EXPECT_EQ(hash_combine_test(a, b), hash_combine_test(d, b));
  }
  
  TEST(Hash, hash_combine) {
    EXPECT_NE(hash_combine(1, 2), hash_combine(2, 1));
  }
  
  TEST(Hash, hash_bool) {
    const auto hash = folly::Hash();
    EXPECT_NE(hash(true), hash(false));
  }
  
  TEST(Hash, hash_bool10) {
    const auto hash = folly::Hash();
    std::set<size_t> values;
    for (bool b1 : {false, true}) {
      for (bool b2 : {false, true}) {
        for (bool b3 : {false, true}) {
          for (bool b4 : {false, true}) {
            for (bool b5 : {false, true}) {
              for (bool b6 : {false, true}) {
                for (bool b7 : {false, true}) {
                  for (bool b8 : {false, true}) {
                    for (bool b9 : {false, true}) {
                      for (bool b10 : {false, true}) {
                        values.insert(
                            hash(b1, b2, b3, b4, b5, b6, b7, b8, b9, b10));
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
    EXPECT_EQ(values.size(), 1 << 10);
  }
  
  TEST(Hash, std_tuple) {
    typedef std::tuple<int64_t, std::string, int32_t> tuple3;
    tuple3 t(42, "foo", 1);
  
    std::unordered_map<tuple3, std::string> m;
    m[t] = "bar";
    EXPECT_EQ("bar", m[t]);
  }
  
  TEST(Hash, enum_type) {
    const auto hash = folly::Hash();
  
    enum class Enum32 : int32_t { Foo, Bar };
    EXPECT_EQ(hash(static_cast<int32_t>(Enum32::Foo)), hash(Enum32::Foo));
    EXPECT_EQ(hash(static_cast<int32_t>(Enum32::Bar)), hash(Enum32::Bar));
    EXPECT_NE(hash(Enum32::Foo), hash(Enum32::Bar));
  
    std::unordered_map<Enum32, std::string, folly::Hash> m32;
    m32[Enum32::Foo] = "foo";
    EXPECT_EQ("foo", m32[Enum32::Foo]);
  
    enum class Enum64 : int64_t { Foo, Bar };
    EXPECT_EQ(hash(static_cast<int64_t>(Enum64::Foo)), hash(Enum64::Foo));
    EXPECT_EQ(hash(static_cast<int64_t>(Enum64::Bar)), hash(Enum64::Bar));
    EXPECT_NE(hash(Enum64::Foo), hash(Enum64::Bar));
  
    std::unordered_map<Enum64, std::string, folly::Hash> m64;
    m64[Enum64::Foo] = "foo";
    EXPECT_EQ("foo", m64[Enum64::Foo]);
  }
  
  TEST(Hash, pair_folly_hash) {
    typedef std::pair<int64_t, int32_t> pair2;
    pair2 p(42, 1);
  
    std::unordered_map<pair2, std::string, folly::Hash> m;
    m[p] = "bar";
    EXPECT_EQ("bar", m[p]);
  }
  
  TEST(Hash, tuple_folly_hash) {
    typedef std::tuple<int64_t, int32_t, int32_t> tuple3;
    tuple3 t(42, 1, 1);
  
    std::unordered_map<tuple3, std::string, folly::Hash> m;
    m[t] = "bar";
    EXPECT_EQ("bar", m[t]);
  }
  
  namespace {
  template <class T>
  size_t hash_vector(const std::vector<T>& v) {
    return hash_range(v.begin(), v.end());
  }
  } // namespace
  
  TEST(Hash, hash_range) {
    EXPECT_EQ(hash_vector<int32_t>({1, 2}), hash_vector<int16_t>({1, 2}));
    EXPECT_NE(hash_vector<int>({2, 1}), hash_vector<int>({1, 2}));
    EXPECT_EQ(hash_vector<int>({}), hash_vector<float>({}));
  }
  
  TEST(Hash, std_tuple_different_hash) {
    typedef std::tuple<int64_t, std::string, int32_t> tuple3;
    tuple3 t1(42, "foo", 1);
    tuple3 t2(9, "bar", 3);
    tuple3 t3(42, "foo", 3);
  
    EXPECT_NE(std::hash<tuple3>()(t1), std::hash<tuple3>()(t2));
    EXPECT_NE(std::hash<tuple3>()(t1), std::hash<tuple3>()(t3));
  }
  
  TEST(Hash, Strings) {
    using namespace folly;
  
    StringPiece a1 = "10050517", b1 = "51107032", a2 = "10050518",
                b2 = "51107033", a3 = "10050519", b3 = "51107034",
                a4 = "10050525", b4 = "51107040";
    Range<const wchar_t*> w1 = range(L"10050517"), w2 = range(L"51107032"),
                          w3 = range(L"10050518"), w4 = range(L"51107033");
    Hash h2;
    EXPECT_NE(h2(a1), h2(b1));
    EXPECT_NE(h2(a1), h2(b1));
    EXPECT_NE(h2(a2), h2(b2));
    EXPECT_NE(h2(a3), h2(b3));
    EXPECT_NE(h2(ByteRange(a1)), h2(ByteRange(b1)));
    EXPECT_NE(h2(ByteRange(a2)), h2(ByteRange(b2)));
    EXPECT_NE(h2(ByteRange(a3)), h2(ByteRange(b3)));
    EXPECT_NE(h2(ByteRange(a4)), h2(ByteRange(b4)));
    EXPECT_NE(h2(w1), h2(w2));
    EXPECT_NE(h2(w1), h2(w3));
    EXPECT_NE(h2(w2), h2(w4));
  
    // Check compatibility with std::string.
    EXPECT_EQ(h2(a1), h2(a1.str()));
    EXPECT_EQ(h2(a2), h2(a2.str()));
    EXPECT_EQ(h2(a3), h2(a3.str()));
    EXPECT_EQ(h2(a4), h2(a4.str()));
  }
  
  struct FNVTestParam {
    std::string in;
    uint64_t out;
  };
  
  class FNVTest : public ::testing::TestWithParam<FNVTestParam> {};
  
  TEST_P(FNVTest, Fnva64Buf) {
    EXPECT_EQ(
        GetParam().out, fnva64_buf(GetParam().in.data(), GetParam().in.size()));
  }
  
  TEST_P(FNVTest, Fnva64) {
    EXPECT_EQ(GetParam().out, fnva64(GetParam().in));
  }
  
  TEST_P(FNVTest, Fnva64Partial) {
    size_t partialLen = GetParam().in.size() / 2;
    auto data = GetParam().in.data();
    auto partial = fnva64_buf(data, partialLen);
    EXPECT_EQ(
        GetParam().out,
        fnva64_buf(
            data + partialLen, GetParam().in.size() - partialLen, partial));
  }
  
  // Taken from http://www.isthe.com/chongo/src/fnv/test_fnv.c
  INSTANTIATE_TEST_CASE_P(
      FNVTesting,
      FNVTest,
      ::testing::Values(
          FNVTestParam{"foobar", // 11
                       0x85944171f73967e8},
          FNVTestParam{"chongo was here!\n", // 39
                       0x46810940eff5f915},
          FNVTestParam{"127.0.0.3", // 106,
                       0xaabafc7104d91158},
          FNVTestParam{"http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash", // 126
                       0xd9b957fb7fe794c5},
          FNVTestParam{"http://norvig.com/21-days.html", // 136
                       0x07aaa640476e0b9a}));
  
  //////// static checks
  
  static constexpr bool k32Bit = sizeof(std::size_t) == 4;
  
  static_assert(!folly::IsAvalanchingHasher<std::hash<int>, int>::value, "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<char const*>, char const*>::value,
      "");
  static_assert(!folly::IsAvalanchingHasher<std::hash<float>, float>::value, "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<double>, double>::value,
      "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<long double>, long double>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<std::hash<std::string>, std::string>::value,
      "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<TestEnum>, TestEnum>::value,
      "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<TestStruct>, TestStruct>::value,
      "");
  
  static_assert(
      !folly::IsAvalanchingHasher<folly::transparent<std::hash<int>>, int>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::transparent<std::hash<std::string>>,
          std::string>::value,
      "");
  
  // these come from folly/hash/Hash.h
  static_assert(
      folly::IsAvalanchingHasher<
          std::hash<std::pair<int, int>>,
          std::pair<int, int>>::value,
      "");
  static_assert(
      !folly::IsAvalanchingHasher<std::hash<std::tuple<int>>, std::tuple<int>>::
          value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          std::hash<std::tuple<std::string>>,
          std::tuple<std::string>>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          std::hash<std::tuple<int, int>>,
          std::tuple<int, int>>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          std::hash<std::tuple<int, int, int>>,
          std::tuple<int, int, int>>::value,
      "");
  
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, uint8_t>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, char>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, uint16_t>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, int16_t>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, uint32_t>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, int32_t>::value,
      "");
  static_assert(folly::IsAvalanchingHasher<folly::Hash, uint64_t>::value, "");
  static_assert(folly::IsAvalanchingHasher<folly::Hash, int64_t>::value, "");
  static_assert(
      folly::IsAvalanchingHasher<folly::Hash, folly::StringPiece>::value,
      "");
  static_assert(folly::IsAvalanchingHasher<folly::Hash, std::string>::value, "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::Hash, TestEnum>::value,
      "");
  static_assert(folly::IsAvalanchingHasher<folly::Hash, TestBigEnum>::value, "");
  
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<uint8_t>, uint8_t>::value,
      "");
  static_assert(
      k32Bit == folly::IsAvalanchingHasher<folly::hasher<char>, char>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<uint16_t>, uint16_t>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<int16_t>, int16_t>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<uint32_t>, uint32_t>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<int32_t>, int32_t>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<uint64_t>, uint64_t>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<int64_t>, int64_t>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<float>, float>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<double>, double>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<std::string>, std::string>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<folly::StringPiece>, std::string>::
          value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::hasher<folly::StringPiece>,
          folly::StringPiece>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::transparent<folly::hasher<folly::StringPiece>>,
          folly::StringPiece>::value,
      "");
  
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<std::string>, std::string>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::hasher<std::pair<int, int>>,
          std::pair<int, int>>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<
              folly::hasher<std::tuple<int>>,
              std::tuple<int>>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::hasher<std::tuple<std::string>>,
          std::tuple<std::string>>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::hasher<std::tuple<int, int>>,
          std::tuple<int, int>>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<
          folly::hasher<std::tuple<int, int, int>>,
          std::tuple<int, int, int>>::value,
      "");
  static_assert(
      k32Bit ==
          folly::IsAvalanchingHasher<folly::hasher<TestEnum>, TestEnum>::value,
      "");
  static_assert(
      folly::IsAvalanchingHasher<folly::hasher<TestBigEnum>, TestBigEnum>::value,
      "");
  
  //////// dynamic checks
  
  namespace {
  template <typename H, typename T, typename F>
  void verifyAvalanching(T initialValue, F const& advance) {
    // This doesn't check probabilities, but does verify that every bit
    // changed independently of every other bit, in both directions, when
    // traversing a sequence of dependent changes.  Note that it is NOT
    // sufficient to just use a random sequence here, because even the
    // identity function will pass.  As constructed this will require
    // 2^63 steps to complete for an identity hash, because none of the
    // transitions with on == 63 will occur until then.
    H const hasher;
    constexpr std::size_t N = sizeof(decltype(hasher(initialValue))) * 8;
  
    // seen[i][j] if we have seen i flip on at the same time as j went off
    bool seen[N][N] = {};
    std::size_t unseenCount = N * (N - 1);
    auto v = initialValue;
    auto h = hasher(v);
    std::size_t steps = 0;
    // wait for 95% coverage
    while (unseenCount > (N * (N - 1)) / 95) {
      ++steps;
      auto hPrev = h;
      advance(v);
      h = hasher(v);
  
      uint64_t delta = hPrev ^ h;
      for (std::size_t i = 0; i < N - 1; ++i) {
        if (((delta >> i) & 1) == 0) {
          continue;
        }
        // we know i flipped
        for (std::size_t j = i + 1; j < N; ++j) {
          if (((delta >> j) & 1) == 0) {
            continue;
          }
          // we know j flipped
          bool iOn = ((hPrev >> i) & 1) == 0;
          bool jOn = ((hPrev >> j) & 1) == 0;
          if (iOn != jOn) {
            auto on = iOn ? i : j;
            auto off = iOn ? j : i;
            if (!seen[on][off]) {
              seen[on][off] = true;
              --unseenCount;
            }
          }
        }
      }
  
      // we should actually only need a couple hundred
      ASSERT_LT(steps, 1000) << unseenCount << " of " << (N * (N - 1))
                             << " pair transitions unseen";
    }
  }
  } // namespace
  
  TEST(Traits, stdHashPairAvalances) {
    verifyAvalanching<std::hash<std::pair<int, int>>>(
        std::make_pair(0, 0), [](std::pair<int, int>& v) { v.first++; });
  }
  
  TEST(Traits, stdHashTuple2Avalances) {
    verifyAvalanching<std::hash<std::tuple<int, int>>>(
        std::make_tuple(0, 0),
        [](std::tuple<int, int>& v) { std::get<0>(v) += 1; });
  }
  
  TEST(Traits, stdHashStringAvalances) {
    verifyAvalanching<std::hash<std::string>, std::string>(
        "00000000000000000000000000000", [](std::string& str) {
          std::size_t i = 0;
          while (str[i] == '1') {
            str[i] = '0';
            ++i;
          }
          str[i] = '1';
        });
  }
  
  TEST(Traits, follyHashUint64Avalances) {
    verifyAvalanching<folly::Hash>(uint64_t{0}, [](uint64_t& v) { v++; });
  }
  
  TEST(Traits, follyHasherInt64Avalances) {
    verifyAvalanching<folly::hasher<int64_t>>(
        int64_t{0}, [](int64_t& v) { v++; });
  }
  
  TEST(Traits, follyHasherFloatAvalanches) {
    verifyAvalanching<folly::hasher<float>>(0.0f, [](float& v) { v += 1; });
  }
  
  TEST(Traits, follyHasherDoubleAvalanches) {
    verifyAvalanching<folly::hasher<double>>(0.0, [](double& v) { v += 1; });
  }
  