Move HashCombine to HashUtils and make it work better on 64bits

HashCombine will be used in more than just BindGroupLayout caching so we
extract it to a separate header. Add a better mixing constant for 64bit
systems.

src/common/CMakeLists.txt

@@ -21,6 +21,7 @@ list(APPEND COMMON_SOURCES
     ${COMMON_DIR}/Compiler.h
     ${COMMON_DIR}/DynamicLib.cpp
     ${COMMON_DIR}/DynamicLib.h
+    ${COMMON_DIR}/HashUtils.h
     ${COMMON_DIR}/Math.cpp
     ${COMMON_DIR}/Math.h
     ${COMMON_DIR}/Platform.h

src/common/HashUtils.h

@@ -0,0 +1,55 @@
+// Copyright 2018 The NXT Authors
+//
+// 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.
+
+#ifndef COMMON_HASHUTILS_H_
+#define COMMON_HASHUTILS_H_
+
+#include "common/Platform.h"
+
+#include <functional>
+
+// Wrapper around std::hash to make it a templated function instead of a functor. It is marginally
+// nicer, and avoids adding to the std namespace to add hashing of other types.
+template <typename T>
+size_t Hash(const T& value) {
+    return std::hash<T>()(value);
+}
+
+// When hashing sparse structures we want to iteratively build a hash value with only parts of the
+// data. HashCombine "hashes" together an existing hash and hashable values.
+//
+// Example usage to compute the hash of a mask and values corresponding to the mask:
+//
+//    size_t hash = Hash(mask):
+//    for (uint32_t i : IterateBitSet(mask)) { HashCombine(&hash, hashables[i]); }
+//    return hash;
+template <typename T>
+void HashCombine(size_t* hash, const T& value) {
+#if defined(NXT_PLATFORM_64_BIT)
+    const size_t offset = 0x9e3779b97f4a7c16;
+#elif defined(NXT_PLATFORM_32_BIT)
+    const size_t offset = 0x9e3779b9;
+#else
+#    error "Unsupported platform"
+#endif
+    *hash ^= Hash(value) + offset + (*hash << 6) + (*hash >> 2);
+}
+
+template <typename T, typename... Args>
+void HashCombine(size_t* hash, const T& value, const Args&... args) {
+    HashCombine(hash, value);
+    HashCombine(hash, args...);
+}
+
+#endif  // COMMON_HASHUTILS_H_