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+// Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+// Copyright (c) 2012 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#pragma once
+
+#include <atomic>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "rocksdb/filter_policy.h"
+#include "rocksdb/table.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+class Slice;
+
+// Exposes any extra information needed for testing built-in
+// FilterBitsBuilders
+class BuiltinFilterBitsBuilder : public FilterBitsBuilder {
+ public:
+  // Calculate number of bytes needed for a new filter, including
+  // metadata. Passing the result to CalculateNumEntry should
+  // return >= the num_entry passed in.
+  virtual uint32_t CalculateSpace(const int num_entry) = 0;
+
+  // Returns an estimate of the FP rate of the returned filter if
+  // `keys` keys are added and the filter returned by Finish is `bytes`
+  // bytes.
+  virtual double EstimatedFpRate(size_t keys, size_t bytes) = 0;
+};
+
+// RocksDB built-in filter policy for Bloom or Bloom-like filters.
+// This class is considered internal API and subject to change.
+// See NewBloomFilterPolicy.
+class BloomFilterPolicy : public FilterPolicy {
+ public:
+  // An internal marker for operating modes of BloomFilterPolicy, in terms
+  // of selecting an implementation. This makes it easier for tests to track
+  // or to walk over the built-in set of Bloom filter implementations. The
+  // only variance in BloomFilterPolicy by mode/implementation is in
+  // GetFilterBitsBuilder(), so an enum is practical here vs. subclasses.
+  //
+  // This enum is essentially the union of all the different kinds of return
+  // value from GetFilterBitsBuilder, or "underlying implementation", and
+  // higher-level modes that choose an underlying implementation based on
+  // context information.
+  enum Mode {
+    // Legacy implementation of Bloom filter for full and partitioned filters.
+    // Set to 0 in case of value confusion with bool use_block_based_builder
+    // NOTE: TESTING ONLY as this mode does not use best compatible
+    // implementation
+    kLegacyBloom = 0,
+    // Deprecated block-based Bloom filter implementation.
+    // Set to 1 in case of value confusion with bool use_block_based_builder
+    // NOTE: DEPRECATED but user exposed
+    kDeprecatedBlock = 1,
+    // A fast, cache-local Bloom filter implementation. See description in
+    // FastLocalBloomImpl.
+    // NOTE: TESTING ONLY as this mode does not check format_version
+    kFastLocalBloom = 2,
+    // Automatically choose from the above (except kDeprecatedBlock) based on
+    // context at build time, including compatibility with format_version.
+    // NOTE: This is currently the only recommended mode that is user exposed.
+    kAuto = 100,
+  };
+  // All the different underlying implementations that a BloomFilterPolicy
+  // might use, as a mode that says "always use this implementation."
+  // Only appropriate for unit tests.
+  static const std::vector<Mode> kAllFixedImpls;
+
+  // All the different modes of BloomFilterPolicy that are exposed from
+  // user APIs. Only appropriate for higher-level unit tests. Integration
+  // tests should prefer using NewBloomFilterPolicy (user-exposed).
+  static const std::vector<Mode> kAllUserModes;
+
+  explicit BloomFilterPolicy(double bits_per_key, Mode mode);
+
+  ~BloomFilterPolicy() override;
+
+  const char* Name() const override;
+
+  // Deprecated block-based filter only
+  void CreateFilter(const Slice* keys, int n, std::string* dst) const override;
+
+  // Deprecated block-based filter only
+  bool KeyMayMatch(const Slice& key, const Slice& bloom_filter) const override;
+
+  FilterBitsBuilder* GetFilterBitsBuilder() const override;
+
+  // To use this function, call GetBuilderFromContext().
+  //
+  // Neither the context nor any objects therein should be saved beyond
+  // the call to this function, unless it's shared_ptr.
+  FilterBitsBuilder* GetBuilderWithContext(
+      const FilterBuildingContext&) const override;
+
+  // Returns a new FilterBitsBuilder from the filter_policy in
+  // table_options of a context, or nullptr if not applicable.
+  // (An internal convenience function to save boilerplate.)
+  static FilterBitsBuilder* GetBuilderFromContext(const FilterBuildingContext&);
+
+  // Read metadata to determine what kind of FilterBitsReader is needed
+  // and return a new one. This must successfully process any filter data
+  // generated by a built-in FilterBitsBuilder, regardless of the impl
+  // chosen for this BloomFilterPolicy. Not compatible with CreateFilter.
+  FilterBitsReader* GetFilterBitsReader(const Slice& contents) const override;
+
+  // Essentially for testing only: configured millibits/key
+  int GetMillibitsPerKey() const { return millibits_per_key_; }
+  // Essentially for testing only: legacy whole bits/key
+  int GetWholeBitsPerKey() const { return whole_bits_per_key_; }
+
+ private:
+  // Newer filters support fractional bits per key. For predictable behavior
+  // of 0.001-precision values across floating point implementations, we
+  // round to thousandths of a bit (on average) per key.
+  int millibits_per_key_;
+
+  // Older filters round to whole number bits per key. (There *should* be no
+  // compatibility issue with fractional bits per key, but preserving old
+  // behavior with format_version < 5 just in case.)
+  int whole_bits_per_key_;
+
+  // Selected mode (a specific implementation or way of selecting an
+  // implementation) for building new SST filters.
+  Mode mode_;
+
+  // Whether relevant warnings have been logged already. (Remember so we
+  // only report once per BloomFilterPolicy instance, to keep the noise down.)
+  mutable std::atomic<bool> warned_;
+
+  // For newer Bloom filter implementation(s)
+  FilterBitsReader* GetBloomBitsReader(const Slice& contents) const;
+};
+
+}  // namespace ROCKSDB_NAMESPACE