expose commutative accumulator as commutative_hash_128_to_64

Summary:
So it can be used in cases where data is already hashed, and
order-independent way to reduce the set of hashes is required.

Reviewed By: ot

Differential Revision: D28382187

fbshipit-source-id: 4327ddd67b49a61d0172c4676090b2f96e84c065

folly/hash/Hash.h

@@ -71,6 +71,15 @@ constexpr uint64_t hash_128_to_64(
   return b;
 }
 
+// Order-independent way to reduce multiple 64 bit hashes into a single hash.
+FOLLY_DISABLE_UNDEFINED_BEHAVIOR_SANITIZER("unsigned-integer-overflow")
+constexpr uint64_t commutative_hash_128_to_64(
+    const uint64_t upper, const uint64_t lower) noexcept {
+  // Commutative accumulator taken from this paper:
+  // https://www.preprints.org/manuscript/201710.0192/v1/download
+  return 3860031 + (upper + lower) * 2779 + (upper * lower * 2);
+}
+
 //  twang_mix64
 //
 //  Thomas Wang 64 bit mix hash function.
@@ -643,9 +652,7 @@ uint64_t commutative_hash_combine_value_generic(
     uint64_t seed, Hash const& hasher, Value const& value) {
   auto const x = hasher(value);
   auto const y = IsAvalanchingHasher<Hash, Value>::value ? x : twang_mix64(x);
-  // Commutative accumulator taken from this paper:
-  // https://www.preprints.org/manuscript/201710.0192/v1/download
-  return 3860031 + (seed + y) * 2779 + (seed * y * 2);
+  return commutative_hash_128_to_64(seed, y);
 }
 
 // hash_range combines hashes of items in the range [first, last) in an

folly/hash/test/HashTest.cpp

@@ -31,6 +31,15 @@
 
 using namespace folly::hash;
 
+TEST(Hash, Test128To64) {
+  constexpr uint64_t upper = 12345678910111213UL;
+  constexpr uint64_t lower = 141516171819202122UL;
+  EXPECT_NE(hash_128_to_64(upper, lower), hash_128_to_64(lower, upper));
+  EXPECT_EQ(
+      commutative_hash_128_to_64(upper, lower),
+      commutative_hash_128_to_64(lower, upper));
+}
+
 TEST(Hash, Fnv32) {
   const char* s1 = "hello, world!";
   const uint32_t s1_res = 3605494790UL;