hash_range

Test Plan: Unit test

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: shilin

FB internal diff: D1497250

folly/Hash.h

@@ -59,6 +59,19 @@ inline size_t hash_combine_generic() {
   return 0;
 }
 
+template <
+    class Iter,
+    class Hash = std::hash<typename std::iterator_traits<Iter>::value_type>>
+uint64_t hash_range(Iter begin,
+                    Iter end,
+                    uint64_t hash = 0,
+                    Hash hasher = Hash()) {
+  for (; begin != end; ++begin) {
+    hash = hash_128_to_64(hash, hasher(*begin));
+  }
+  return hash;
+}
+
 template <class Hasher, typename T, typename... Ts>
 size_t hash_combine_generic(const T& t, const Ts&... ts) {
   size_t seed = Hasher::hash(t);

folly/test/HashTest.cpp

@@ -238,6 +238,19 @@ TEST(Hash, std_tuple) {
   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());
+}
+}
+
+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);