Index: base/hash.h |
diff --git a/base/hash.h b/base/hash.h |
index ed8d9fd4cc9b987272f9d54ffa854c2d86f7129e..134b10e7eafbaef432deb123436dcb471ab2f253 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 +36,84 @@ 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 Type1, typename Type2> |
+struct IntPairHash { |
+ size_t operator()(std::pair<Type1, Type2> value) const { |
+ return HashInts(value.first, value.second); |
+ } |
+}; |
+ |
} // namespace base |
#endif // BASE_HASH_H_ |