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author | davidben <davidben@chromium.org> | 2016-01-20 17:08:38 -0800 |
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committer | Commit bot <commit-bot@chromium.org> | 2016-01-21 01:10:07 +0000 |
commit | 3f37f7f1459e7b5a452c0e433493e0a6e9649ca7 (patch) | |
tree | d37f361db93e0f91fe3dedee38851941f5586020 /base/hash.h | |
parent | 26a2b4883c04424a86abcda3d7333505eb1d032d (diff) | |
download | chromium_src-3f37f7f1459e7b5a452c0e433493e0a6e9649ca7.zip chromium_src-3f37f7f1459e7b5a452c0e433493e0a6e9649ca7.tar.gz chromium_src-3f37f7f1459e7b5a452c0e433493e0a6e9649ca7.tar.bz2 |
Allow std::unordered_*.
base::hash_* is, as a transition step, implemented in terms of
std::unordered_*. Later commits will convert existing uses.
Also fix a host of IWYU problems that arose from this CL.
(NOPRESUBMIT because the wstring presubmit check is overzealous
and complains about the reference to wstring in the comment.)
NOPRESUBMIT=true
BUG=576864
TBR=derat@chromium.org,blundell@chromium.org,jbauman@chromium.org,dalecurtis@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review URL: https://codereview.chromium.org/1502373009
Cr-Commit-Position: refs/heads/master@{#370553}
Diffstat (limited to 'base/hash.h')
-rw-r--r-- | base/hash.h | 81 |
1 files changed, 81 insertions, 0 deletions
diff --git a/base/hash.h b/base/hash.h index ed8d9fd..d5dc549 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 +36,86 @@ 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_ |