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// Copyright (C) 2019 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef ICING_INDEX_HIT_HIT_H_
#define ICING_INDEX_HIT_HIT_H_
#include <array>
#include <cstdint>
#include <limits>
#include "icing/legacy/core/icing-packed-pod.h"
#include "icing/schema/section.h"
#include "icing/store/document-id.h"
namespace icing {
namespace lib {
// BasicHit is a specific encoding that refers to content within a document. A
// basic hit consists of:
// - a DocumentId
// - a SectionId
// referring to the document and section that the hit corresponds to.
//
// The hit is the most basic unit of the index and, when grouped together by
// term, can be used to encode what terms appear in what documents.
//
// BasicHit is for indices (e.g. numeric index) that don't require term
// frequency.
class BasicHit {
public:
// The datatype used to encode BasicHit information: the document_id and
// section_id.
using Value = uint32_t;
// WARNING: Changing this value will invalidate any pre-existing posting lists
// on user devices.
//
// kInvalidValue contains:
// - 0 for unused bits. Note that unused bits are always 0 for both valid and
// invalid BasicHit values.
// - Inverted kInvalidDocumentId
// - SectionId 0 (valid), which is ok because inverted kInvalidDocumentId has
// already invalidated the value. In fact, we currently use all 2^6 section
// ids and there is no "invalid section id", so it doesn't matter what
// SectionId we set for kInvalidValue.
static constexpr Value kInvalidValue = 0;
explicit BasicHit(SectionId section_id, DocumentId document_id);
explicit BasicHit() : value_(kInvalidValue) {}
bool is_valid() const { return value_ != kInvalidValue; }
Value value() const { return value_; }
DocumentId document_id() const;
SectionId section_id() const;
bool operator<(const BasicHit& h2) const { return value_ < h2.value_; }
bool operator==(const BasicHit& h2) const { return value_ == h2.value_; }
private:
// Value bits layout: 4 unused + 22 document_id + 6 section id.
Value value_;
} __attribute__((packed));
static_assert(sizeof(BasicHit) == 4, "");
// Hit is a specific encoding that refers to content within a document. A hit
// consists of:
// - a DocumentId
// - a SectionId
// referring to the document and section that the hit corresponds to, as well as
// metadata about the hit:
// - whether the Hit has a TermFrequency other than the default value
// - whether the Hit does not appear exactly in the document, but instead
// represents a term that is a prefix of a term in the document
// - whether the Hit came from a section that has prefix expansion enabled
// and a term frequency for the hit.
//
// The hit is the most basic unit of the index and, when grouped together by
// term, can be used to encode what terms appear in what documents.
class Hit {
public:
// The datatype used to encode Hit information: the document_id, section_id
// and the has_term_frequency, prefix hit and in prefix section flags.
using Value = uint32_t;
// WARNING: Changing this value will invalidate any pre-existing posting lists
// on user devices.
//
// WARNING:
// - Hit::kInvalidValue should contain inverted kInvalidDocumentId, which is
// b'00...0. However, currently we set it as UINT32_MAX and actually it
// contains b'11...1, which is the inverted document_id 0.
// - It means Hit::kInvalidValue contains valid (document_id, section_id,
// flags), so we potentially cannot distinguish if a Hit is invalid or not.
// The invalidity is an essential feature for posting list since we use it
// to determine the state of the posting list.
// - The reason why it won't break the current posting list is because the
// unused bit(s) are set as 1 for Hit::kInvalidValue and 0 for all valid
// Hits. In other words, the unused bit(s) are actually serving as "invalid
// flag".
// - If we want to exhaust all unused bits in the future, then we have to
// change Hit::kInvalidValue to set the inverted document_id section
// correctly (b'00...0, refer to BasicHit::kInvalidValue as an example).
// - Also this problem is guarded by static_assert in hit.cc. If exhausting
// all unused bits, then the static_assert will detect and fail. We can
// safely remove the static_assert check after following the above process
// to resolve the incorrect Hit::kInvalidValue issue.
static constexpr Value kInvalidValue = std::numeric_limits<Value>::max();
// Docs are sorted in reverse, and 0 is never used as the inverted
// DocumentId (because it is the inverse of kInvalidValue), so it is always
// the max in a descending sort.
static constexpr Value kMaxDocumentIdSortValue = 0;
// The Term Frequency of a Hit.
using TermFrequency = uint8_t;
using TermFrequencyArray = std::array<Hit::TermFrequency, kTotalNumSections>;
// Max TermFrequency is 255.
static constexpr TermFrequency kMaxTermFrequency =
std::numeric_limits<TermFrequency>::max();
static constexpr TermFrequency kDefaultTermFrequency = 1;
static constexpr TermFrequency kNoTermFrequency = 0;
explicit Hit(Value value = kInvalidValue,
TermFrequency term_frequency = kDefaultTermFrequency)
: value_(value), term_frequency_(term_frequency) {}
Hit(SectionId section_id, DocumentId document_id,
TermFrequency term_frequency, bool is_in_prefix_section = false,
bool is_prefix_hit = false);
bool is_valid() const { return value() != kInvalidValue; }
Value value() const { return value_; }
DocumentId document_id() const;
SectionId section_id() const;
// Whether or not the hit contains a valid term frequency.
bool has_term_frequency() const;
TermFrequency term_frequency() const { return term_frequency_; }
bool is_prefix_hit() const;
bool is_in_prefix_section() const;
// Creates a new hit based on old_hit but with new_document_id set.
static Hit TranslateHit(Hit old_hit, DocumentId new_document_id);
bool operator<(const Hit& h2) const { return value() < h2.value(); }
bool operator==(const Hit& h2) const { return value() == h2.value(); }
struct EqualsDocumentIdAndSectionId {
bool operator()(const Hit& hit1, const Hit& hit2) const;
};
private:
// Value and TermFrequency must be in this order.
// Value bits layout: 1 unused + 22 document_id + 6 section id + 3 flags.
Value value_;
TermFrequency term_frequency_;
} __attribute__((packed));
static_assert(sizeof(Hit) == 5, "");
// TODO(b/138991332) decide how to remove/replace all is_packed_pod assertions.
static_assert(icing_is_packed_pod<Hit>::value, "go/icing-ubsan");
} // namespace lib
} // namespace icing
#endif // ICING_INDEX_HIT_HIT_H_
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