Allow std::unordered_*.

base/hash.h

Index: base/hash.h
diff --git a/base/hash.h b/base/hash.h
index ed8d9fd4cc9b987272f9d54ffa854c2d86f7129e..97e251cdf1ed8031e1b1adc79d999fd10e303d39 100644
--- a/base/hash.h
+++ b/base/hash.h
@@ -10,6 +10,7 @@
 
 #include <limits>
 #include <string>
+#include <utility>
 
 #include "base/base_export.h"
 #include "base/logging.h"
@@ -35,6 +36,87 @@ inline uint32_t Hash(const std::string& str) {
   return Hash(str.data(), str.size());
 }
 
+// Implement hashing for pairs of at-most 32 bit integer values.
+// When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using
+// multiply-add hashing. This algorithm, as described in
+// Theorem 4.3.3 of the thesis "Über die Komplexität der Multiplikation in
+// eingeschränkten Branchingprogrammmodellen" by Woelfel, is:
+//
+//   h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32
+//
+// Contact danakj@chromium.org for any questions.
+inline size_t HashInts32(uint32_t value1, uint32_t value2) {
+  uint64_t value1_64 = value1;
+  uint64_t hash64 = (value1_64 << 32) | value2;
+
+  if (sizeof(size_t) >= sizeof(uint64_t))
+    return static_cast<size_t>(hash64);
+
+  uint64_t odd_random = 481046412LL << 32 | 1025306955LL;
+  uint32_t shift_random = 10121U << 16;
+
+  hash64 = hash64 * odd_random + shift_random;
+  size_t high_bits =
+      static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
+  return high_bits;
+}
+
+// Implement hashing for pairs of up-to 64-bit integer values.
+// We use the compound integer hash method to produce a 64-bit hash code, by
+// breaking the two 64-bit inputs into 4 32-bit values:
+// http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000
+// Then we reduce our result to 32 bits if required, similar to above.
+inline size_t HashInts64(uint64_t value1, uint64_t value2) {
+  uint32_t short_random1 = 842304669U;
+  uint32_t short_random2 = 619063811U;
+  uint32_t short_random3 = 937041849U;
+  uint32_t short_random4 = 3309708029U;
+
+  uint32_t value1a = static_cast<uint32_t>(value1 & 0xffffffff);
+  uint32_t value1b = static_cast<uint32_t>((value1 >> 32) & 0xffffffff);
+  uint32_t value2a = static_cast<uint32_t>(value2 & 0xffffffff);
+  uint32_t value2b = static_cast<uint32_t>((value2 >> 32) & 0xffffffff);
+
+  uint64_t product1 = static_cast<uint64_t>(value1a) * short_random1;
+  uint64_t product2 = static_cast<uint64_t>(value1b) * short_random2;
+  uint64_t product3 = static_cast<uint64_t>(value2a) * short_random3;
+  uint64_t product4 = static_cast<uint64_t>(value2b) * short_random4;
+
+  uint64_t hash64 = product1 + product2 + product3 + product4;
+
+  if (sizeof(size_t) >= sizeof(uint64_t))
+    return static_cast<size_t>(hash64);
+
+  uint64_t odd_random = 1578233944LL << 32 | 194370989LL;
+  uint32_t shift_random = 20591U << 16;
+
+  hash64 = hash64 * odd_random + shift_random;
+  size_t high_bits =
+      static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
+  return high_bits;
+}
+
+template <typename T1, typename T2>
+inline size_t HashInts(T1 value1, T2 value2) {
+  // This condition is expected to be compile-time evaluated and optimised away
+  // in release builds.
+  if (sizeof(T1) > sizeof(uint32_t) || (sizeof(T2) > sizeof(uint32_t)))
+    return HashInts64(value1, value2);
+
+  return HashInts32(value1, value2);
+}
+
+// A templated hasher for pairs of integer types.
+template <typename T>
+struct IntPairHash;
+
+template <typename Type1, typename Type2>
+struct IntPairHash<std::pair<Type1, Type2>> {
+  size_t operator()(std::pair<Type1, Type2> value) const {
+    return HashInts(value.first, value.second);
+  }
+};
+
 }  // namespace base
 
 #endif  // BASE_HASH_H_