/*
- * Copyright 2012 Facebook, Inc.
+ * Copyright 2014 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
#include <stdint.h>
#include <string>
#include <utility>
+#include <tuple>
-#include "folly/SpookyHashV1.h"
-#include "folly/SpookyHashV2.h"
+#include <folly/SpookyHashV1.h>
+#include <folly/SpookyHashV2.h>
/*
* Various hashing functions.
namespace folly { namespace hash {
// This is a general-purpose way to create a single hash from multiple
-// hashable objects. It relies on std::hash<T> being available for all
-// relevant types and combines those hashes in an order-dependent way
-// to yield a new hash.
+// hashable objects. hash_combine_generic takes a class Hasher implementing
+// hash<T>; hash_combine uses a default hasher StdHasher that uses std::hash.
+// hash_combine_generic hashes each argument and combines those hashes in
+// an order-dependent way to yield a new hash.
-// Never used, but gcc demands it.
-inline size_t hash_combine() {
- return 0;
-}
// This is the Hash128to64 function from Google's cityhash (available
// under the MIT License). We use it to reduce multiple 64 bit hashes
return b;
}
-template <typename T, typename... Ts>
-size_t hash_combine(const T& t, const Ts&... ts) {
- size_t seed = std::hash<T>()(t);
+// Never used, but gcc demands it.
+template <class Hasher>
+inline size_t hash_combine_generic() {
+ return 0;
+}
+
+template <class Hasher, typename T, typename... Ts>
+size_t hash_combine_generic(const T& t, const Ts&... ts) {
+ size_t seed = Hasher::hash(t);
if (sizeof...(ts) == 0) {
return seed;
}
- size_t remainder = hash_combine(ts...);
+ size_t remainder = hash_combine_generic<Hasher>(ts...);
return hash_128_to_64(seed, remainder);
}
+// Simply uses std::hash to hash. Note that std::hash is not guaranteed
+// to be a very good hash function; provided std::hash doesn't collide on
+// the individual inputs, you are fine, but that won't be true for, say,
+// strings or pairs
+class StdHasher {
+ public:
+ template <typename T>
+ static size_t hash(const T& t) {
+ return std::hash<T>()(t);
+ }
+};
+
+template <typename T, typename... Ts>
+size_t hash_combine(const T& t, const Ts&... ts) {
+ return hash_combine_generic<StdHasher>(t, ts...);
+}
+
//////////////////////////////////////////////////////////////////////
/*
* http://www.isthe.com/chongo/tech/comp/fnv/
*/
-const uint32_t FNV_32_HASH_START = 216613626UL;
+const uint32_t FNV_32_HASH_START = 2166136261UL;
const uint64_t FNV_64_HASH_START = 14695981039346656037ULL;
inline uint32_t fnv32(const char* s,
}
inline uint32_t fnv32(const std::string& str,
- uint64_t hash = FNV_32_HASH_START) {
+ uint32_t hash = FNV_32_HASH_START) {
return fnv32_buf(str.data(), str.size(), hash);
}
}
};
+// recursion
+template <size_t index, typename... Ts>
+struct TupleHasher {
+ size_t operator()(std::tuple<Ts...> const& key) const {
+ return hash::hash_combine(
+ TupleHasher<index - 1, Ts...>()(key),
+ std::get<index>(key));
+ }
+};
+
+// base
+template <typename... Ts>
+struct TupleHasher<0, Ts...> {
+ size_t operator()(std::tuple<Ts...> const& key) const {
+ // we could do std::hash here directly, but hash_combine hides all the
+ // ugly templating implicitly
+ return hash::hash_combine(std::get<0>(key));
+ }
+};
+
} // namespace folly
// Custom hash functions.
return folly::hash::hash_combine(x.first, x.second);
}
};
+
+ // Hash function for tuples. Requires default hash functions for all types.
+ template <typename... Ts>
+ struct hash<std::tuple<Ts...>> {
+ size_t operator()(std::tuple<Ts...> const& key) const {
+ folly::TupleHasher<
+ std::tuple_size<std::tuple<Ts...>>::value - 1, // start index
+ Ts...> hasher;
+
+ return hasher(key);
+ }
+ };
} // namespace std
#endif