block.cc raw
1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file. See the AUTHORS file for names of contributors.
4 //
5 // Decodes the blocks generated by block_builder.cc.
6
7 #include "table/block.h"
8
9 #include <algorithm>
10 #include <cstdint>
11 #include <vector>
12
13 #include "leveldb/comparator.h"
14 #include "table/format.h"
15 #include "util/coding.h"
16 #include "util/logging.h"
17
18 namespace leveldb {
19
20 inline uint32_t Block::NumRestarts() const {
21 assert(size_ >= sizeof(uint32_t));
22 return DecodeFixed32(data_ + size_ - sizeof(uint32_t));
23 }
24
25 Block::Block(const BlockContents& contents)
26 : data_(contents.data.data()),
27 size_(contents.data.size()),
28 owned_(contents.heap_allocated) {
29 if (size_ < sizeof(uint32_t)) {
30 size_ = 0; // Error marker
31 } else {
32 size_t max_restarts_allowed = (size_ - sizeof(uint32_t)) / sizeof(uint32_t);
33 if (NumRestarts() > max_restarts_allowed) {
34 // The size is too small for NumRestarts()
35 size_ = 0;
36 } else {
37 restart_offset_ = size_ - (1 + NumRestarts()) * sizeof(uint32_t);
38 }
39 }
40 }
41
42 Block::~Block() {
43 if (owned_) {
44 delete[] data_;
45 }
46 }
47
48 // Helper routine: decode the next block entry starting at "p",
49 // storing the number of shared key bytes, non_shared key bytes,
50 // and the length of the value in "*shared", "*non_shared", and
51 // "*value_length", respectively. Will not dereference past "limit".
52 //
53 // If any errors are detected, returns nullptr. Otherwise, returns a
54 // pointer to the key delta (just past the three decoded values).
55 static inline const char* DecodeEntry(const char* p, const char* limit,
56 uint32_t* shared, uint32_t* non_shared,
57 uint32_t* value_length) {
58 if (limit - p < 3) return nullptr;
59 *shared = reinterpret_cast<const uint8_t*>(p)[0];
60 *non_shared = reinterpret_cast<const uint8_t*>(p)[1];
61 *value_length = reinterpret_cast<const uint8_t*>(p)[2];
62 if ((*shared | *non_shared | *value_length) < 128) {
63 // Fast path: all three values are encoded in one byte each
64 p += 3;
65 } else {
66 if ((p = GetVarint32Ptr(p, limit, shared)) == nullptr) return nullptr;
67 if ((p = GetVarint32Ptr(p, limit, non_shared)) == nullptr) return nullptr;
68 if ((p = GetVarint32Ptr(p, limit, value_length)) == nullptr) return nullptr;
69 }
70
71 if (static_cast<uint32_t>(limit - p) < (*non_shared + *value_length)) {
72 return nullptr;
73 }
74 return p;
75 }
76
77 class Block::Iter : public Iterator {
78 private:
79 const Comparator* const comparator_;
80 const char* const data_; // underlying block contents
81 uint32_t const restarts_; // Offset of restart array (list of fixed32)
82 uint32_t const num_restarts_; // Number of uint32_t entries in restart array
83
84 // current_ is offset in data_ of current entry. >= restarts_ if !Valid
85 uint32_t current_;
86 uint32_t restart_index_; // Index of restart block in which current_ falls
87 std::string key_;
88 Slice value_;
89 Status status_;
90
91 inline int Compare(const Slice& a, const Slice& b) const {
92 return comparator_->Compare(a, b);
93 }
94
95 // Return the offset in data_ just past the end of the current entry.
96 inline uint32_t NextEntryOffset() const {
97 return (value_.data() + value_.size()) - data_;
98 }
99
100 uint32_t GetRestartPoint(uint32_t index) {
101 assert(index < num_restarts_);
102 return DecodeFixed32(data_ + restarts_ + index * sizeof(uint32_t));
103 }
104
105 void SeekToRestartPoint(uint32_t index) {
106 key_.clear();
107 restart_index_ = index;
108 // current_ will be fixed by ParseNextKey();
109
110 // ParseNextKey() starts at the end of value_, so set value_ accordingly
111 uint32_t offset = GetRestartPoint(index);
112 value_ = Slice(data_ + offset, 0);
113 }
114
115 public:
116 Iter(const Comparator* comparator, const char* data, uint32_t restarts,
117 uint32_t num_restarts)
118 : comparator_(comparator),
119 data_(data),
120 restarts_(restarts),
121 num_restarts_(num_restarts),
122 current_(restarts_),
123 restart_index_(num_restarts_) {
124 assert(num_restarts_ > 0);
125 }
126
127 bool Valid() const override { return current_ < restarts_; }
128 Status status() const override { return status_; }
129 Slice key() const override {
130 assert(Valid());
131 return key_;
132 }
133 Slice value() const override {
134 assert(Valid());
135 return value_;
136 }
137
138 void Next() override {
139 assert(Valid());
140 ParseNextKey();
141 }
142
143 void Prev() override {
144 assert(Valid());
145
146 // Scan backwards to a restart point before current_
147 const uint32_t original = current_;
148 while (GetRestartPoint(restart_index_) >= original) {
149 if (restart_index_ == 0) {
150 // No more entries
151 current_ = restarts_;
152 restart_index_ = num_restarts_;
153 return;
154 }
155 restart_index_--;
156 }
157
158 SeekToRestartPoint(restart_index_);
159 do {
160 // Loop until end of current entry hits the start of original entry
161 } while (ParseNextKey() && NextEntryOffset() < original);
162 }
163
164 void Seek(const Slice& target) override {
165 // Binary search in restart array to find the last restart point
166 // with a key < target
167 uint32_t left = 0;
168 uint32_t right = num_restarts_ - 1;
169 while (left < right) {
170 uint32_t mid = (left + right + 1) / 2;
171 uint32_t region_offset = GetRestartPoint(mid);
172 uint32_t shared, non_shared, value_length;
173 const char* key_ptr =
174 DecodeEntry(data_ + region_offset, data_ + restarts_, &shared,
175 &non_shared, &value_length);
176 if (key_ptr == nullptr || (shared != 0)) {
177 CorruptionError();
178 return;
179 }
180 Slice mid_key(key_ptr, non_shared);
181 if (Compare(mid_key, target) < 0) {
182 // Key at "mid" is smaller than "target". Therefore all
183 // blocks before "mid" are uninteresting.
184 left = mid;
185 } else {
186 // Key at "mid" is >= "target". Therefore all blocks at or
187 // after "mid" are uninteresting.
188 right = mid - 1;
189 }
190 }
191
192 // Linear search (within restart block) for first key >= target
193 SeekToRestartPoint(left);
194 while (true) {
195 if (!ParseNextKey()) {
196 return;
197 }
198 if (Compare(key_, target) >= 0) {
199 return;
200 }
201 }
202 }
203
204 void SeekToFirst() override {
205 SeekToRestartPoint(0);
206 ParseNextKey();
207 }
208
209 void SeekToLast() override {
210 SeekToRestartPoint(num_restarts_ - 1);
211 while (ParseNextKey() && NextEntryOffset() < restarts_) {
212 // Keep skipping
213 }
214 }
215
216 private:
217 void CorruptionError() {
218 current_ = restarts_;
219 restart_index_ = num_restarts_;
220 status_ = Status::Corruption("bad entry in block");
221 key_.clear();
222 value_.clear();
223 }
224
225 bool ParseNextKey() {
226 current_ = NextEntryOffset();
227 const char* p = data_ + current_;
228 const char* limit = data_ + restarts_; // Restarts come right after data
229 if (p >= limit) {
230 // No more entries to return. Mark as invalid.
231 current_ = restarts_;
232 restart_index_ = num_restarts_;
233 return false;
234 }
235
236 // Decode next entry
237 uint32_t shared, non_shared, value_length;
238 p = DecodeEntry(p, limit, &shared, &non_shared, &value_length);
239 if (p == nullptr || key_.size() < shared) {
240 CorruptionError();
241 return false;
242 } else {
243 key_.resize(shared);
244 key_.append(p, non_shared);
245 value_ = Slice(p + non_shared, value_length);
246 while (restart_index_ + 1 < num_restarts_ &&
247 GetRestartPoint(restart_index_ + 1) < current_) {
248 ++restart_index_;
249 }
250 return true;
251 }
252 }
253 };
254
255 Iterator* Block::NewIterator(const Comparator* comparator) {
256 if (size_ < sizeof(uint32_t)) {
257 return NewErrorIterator(Status::Corruption("bad block contents"));
258 }
259 const uint32_t num_restarts = NumRestarts();
260 if (num_restarts == 0) {
261 return NewEmptyIterator();
262 } else {
263 return new Iter(comparator, data_, restart_offset_, num_restarts);
264 }
265 }
266
267 } // namespace leveldb
268