Allow Mojo structs as map keys

mojo/public/cpp/bindings/lib/hash_util.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MOJO_PUBLIC_CPP_BINDINGS_LIB_HASH_UTIL_H_
+#define MOJO_PUBLIC_CPP_BINDINGS_LIB_HASH_UTIL_H_
+
+#include <cstring>
+#include <functional>
+#include <type_traits>
+#include <vector>
+
+#include "mojo/public/cpp/bindings/lib/template_util.h"
+
+namespace mojo {
+namespace internal {
+
+template <typename T>
+size_t HashCombine(size_t seed, const T& value) {
+  // Based on proposal in:
+  // http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1756.pdf
+  return seed ^ (std::hash<T>()(value) + (seed << 6) + (seed >> 2));
+}
+
+template <typename T>
+struct HasHashMethod {
+  template <typename U>
+  static char Test(decltype(&U::Hash));
+  template <typename U>
+  static int Test(...);
+  static const bool value = sizeof(Test<T>(0)) == sizeof(char);
+
+ private:
+  EnsureTypeIsComplete<T> check_t_;
+};
+
+template <typename T, bool has_hash_method = HasHashMethod<T>::value>
+struct HashTraits;
+
+template <typename T>
+size_t Hash(size_t seed, const T& value);
+
+template <typename T>
+struct HashTraits<T, true> {
+  static size_t Hash(size_t seed, const T& value) { return value.Hash(seed); }
+};
+
+template <typename T>
+struct HashTraits<T, false> {
+  static size_t Hash(size_t seed, const T& value) {
+    return HashCombine(seed, value);
+  }
+};
+
+template <typename T>
+struct HashTraits<std::vector<T>, false> {
+  static size_t Hash(size_t seed, const std::vector<T>& value) {
+    for (const auto& element : value) {
+      seed = HashCombine(seed, element);
+    }
+    return seed;
+  }
+};
+
+template <typename T>
+size_t Hash(size_t seed, const T& value) {
+  return HashTraits<T>::Hash(seed, value);
+}
+
+}  // namespace internal
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_HASH_UTIL_H_