8b8a2e9f05
Summary: * Fully optimized StandardHasher, in terms of efficiently generating Start, CoeffRow, and ResultRow from a stock hash value, with sufficient independence between them to have no measurably degraded behavior. (Degraded behavior would be an FP rate higher than explainable by 2^-b and, if using a 32-bit stock hash function, expected stock hash collisions.) Details in code comments. * Our standard 64-bit and 32-bit hash functions do not exhibit sufficient independence on sequential seeds (for one Ribbon construction attempt to have independent probability from the next). I have worked around this in the Ribbon code by "pre-mixing" "ordinal seeds," sequentially tried and appropriate for storage in persisted metadata, into "raw seeds," ready for application and appropriate for in-memory storage. This way the pre-mixing step (though fast) is only applied on loading or configuring the structure, not on each query or banding add. * Fix a subtle flaw in which backtracking not clearing ResultRow data could lead to elevated FP rate on keys that were backtracked on and should (for generality) exhibit the same FP rate as novel keys. * Added a basic test for PhsfQuery and construction algorithms (map or "retrieval structure" rather than set or filter), and made a few trivial related fixes. * Better random configuration generation in unit tests * Some other minor cleanup / clarification / etc. Pull Request resolved: https://github.com/facebook/rocksdb/pull/7635 Test Plan: unit tests included Reviewed By: jay-zhuang Differential Revision: D24738978 Pulled By: pdillinger fbshipit-source-id: f9d03599d9e2ca3e30e9d3e7d81cd936b56f76f0
97 lines
3.4 KiB
C++
97 lines
3.4 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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//
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// Common hash functions with convenient interfaces. If hashing a
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// statically-sized input in a performance-critical context, consider
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// calling a specific hash implementation directly, such as
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// XXH3p_64bits from xxhash.h.
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//
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// Since this is a very common header, implementation details are kept
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// out-of-line. Out-of-lining also aids in tracking the time spent in
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// hashing functions. Inlining is of limited benefit for runtime-sized
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// hash inputs.
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#pragma once
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#include <cstddef>
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#include <cstdint>
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#include "rocksdb/slice.h"
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#include "util/fastrange.h"
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namespace ROCKSDB_NAMESPACE {
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// Stable/persistent 64-bit hash. Higher quality and generally faster than
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// Hash(), especially for inputs > 24 bytes.
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// KNOWN FLAW: incrementing seed by 1 might not give sufficiently independent
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// results from previous seed. Recommend incrementing by a large odd number.
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extern uint64_t Hash64(const char* data, size_t n, uint64_t seed);
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// Specific optimization without seed (same as seed = 0)
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extern uint64_t Hash64(const char* data, size_t n);
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// Non-persistent hash. Must only used for in-memory data structure.
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// The hash results are thus applicable to change. (Thus, it rarely makes
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// sense to specify a seed for this function.)
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// KNOWN FLAW: incrementing seed by 1 might not give sufficiently independent
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// results from previous seed. Recommend incrementing by a large odd number.
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inline uint64_t NPHash64(const char* data, size_t n, uint32_t seed) {
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// Currently same as Hash64
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return Hash64(data, n, seed);
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}
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// Specific optimization without seed (same as seed = 0)
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inline uint64_t NPHash64(const char* data, size_t n) {
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// Currently same as Hash64
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return Hash64(data, n);
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}
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// Stable/persistent 32-bit hash. Moderate quality and high speed on
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// small inputs.
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// TODO: consider rename to Hash32
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// KNOWN FLAW: incrementing seed by 1 might not give sufficiently independent
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// results from previous seed. Recommend pseudorandom or hashed seeds.
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extern uint32_t Hash(const char* data, size_t n, uint32_t seed);
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// TODO: consider rename to LegacyBloomHash32
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inline uint32_t BloomHash(const Slice& key) {
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return Hash(key.data(), key.size(), 0xbc9f1d34);
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}
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inline uint64_t GetSliceHash64(const Slice& key) {
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return Hash64(key.data(), key.size());
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}
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inline uint64_t GetSliceNPHash64(const Slice& s) {
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return NPHash64(s.data(), s.size());
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}
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inline size_t GetSliceRangedNPHash(const Slice& s, size_t range) {
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return FastRange64(NPHash64(s.data(), s.size()), range);
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}
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// TODO: consider rename to GetSliceHash32
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inline uint32_t GetSliceHash(const Slice& s) {
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return Hash(s.data(), s.size(), 397);
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}
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// Useful for splitting up a 64-bit hash
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inline uint32_t Upper32of64(uint64_t v) {
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return static_cast<uint32_t>(v >> 32);
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}
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inline uint32_t Lower32of64(uint64_t v) { return static_cast<uint32_t>(v); }
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// std::hash compatible interface.
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// TODO: consider rename to SliceHasher32
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struct SliceHasher {
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uint32_t operator()(const Slice& s) const { return GetSliceHash(s); }
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};
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} // namespace ROCKSDB_NAMESPACE
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