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  #include "db/memtable.h"
  6  #include "db/dbformat.h"
  7  #include "leveldb/comparator.h"
  8  #include "leveldb/env.h"
  9  #include "leveldb/iterator.h"
 10  #include "util/coding.h"
 11  
 12  namespace leveldb {
 13  
 14  static Slice GetLengthPrefixedSlice(const char* data) {
 15    uint32_t len;
 16    const char* p = data;
 17    p = GetVarint32Ptr(p, p + 5, &len);  // +5: we assume "p" is not corrupted
 18    return Slice(p, len);
 19  }
 20  
 21  MemTable::MemTable(const InternalKeyComparator& comparator)
 22      : comparator_(comparator), refs_(0), table_(comparator_, &arena_) {}
 23  
 24  MemTable::~MemTable() { assert(refs_ == 0); }
 25  
 26  size_t MemTable::ApproximateMemoryUsage() { return arena_.MemoryUsage(); }
 27  
 28  int MemTable::KeyComparator::operator()(const char* aptr,
 29                                          const char* bptr) const {
 30    // Internal keys are encoded as length-prefixed strings.
 31    Slice a = GetLengthPrefixedSlice(aptr);
 32    Slice b = GetLengthPrefixedSlice(bptr);
 33    return comparator.Compare(a, b);
 34  }
 35  
 36  // Encode a suitable internal key target for "target" and return it.
 37  // Uses *scratch as scratch space, and the returned pointer will point
 38  // into this scratch space.
 39  static const char* EncodeKey(std::string* scratch, const Slice& target) {
 40    scratch->clear();
 41    PutVarint32(scratch, target.size());
 42    scratch->append(target.data(), target.size());
 43    return scratch->data();
 44  }
 45  
 46  class MemTableIterator : public Iterator {
 47   public:
 48    explicit MemTableIterator(MemTable::Table* table) : iter_(table) {}
 49  
 50    MemTableIterator(const MemTableIterator&) = delete;
 51    MemTableIterator& operator=(const MemTableIterator&) = delete;
 52  
 53    ~MemTableIterator() override = default;
 54  
 55    bool Valid() const override { return iter_.Valid(); }
 56    void Seek(const Slice& k) override { iter_.Seek(EncodeKey(&tmp_, k)); }
 57    void SeekToFirst() override { iter_.SeekToFirst(); }
 58    void SeekToLast() override { iter_.SeekToLast(); }
 59    void Next() override { iter_.Next(); }
 60    void Prev() override { iter_.Prev(); }
 61    Slice key() const override { return GetLengthPrefixedSlice(iter_.key()); }
 62    Slice value() const override {
 63      Slice key_slice = GetLengthPrefixedSlice(iter_.key());
 64      return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());
 65    }
 66  
 67    Status status() const override { return Status::OK(); }
 68  
 69   private:
 70    MemTable::Table::Iterator iter_;
 71    std::string tmp_;  // For passing to EncodeKey
 72  };
 73  
 74  Iterator* MemTable::NewIterator() { return new MemTableIterator(&table_); }
 75  
 76  void MemTable::Add(SequenceNumber s, ValueType type, const Slice& key,
 77                     const Slice& value) {
 78    // Format of an entry is concatenation of:
 79    //  key_size     : varint32 of internal_key.size()
 80    //  key bytes    : char[internal_key.size()]
 81    //  value_size   : varint32 of value.size()
 82    //  value bytes  : char[value.size()]
 83    size_t key_size = key.size();
 84    size_t val_size = value.size();
 85    size_t internal_key_size = key_size + 8;
 86    const size_t encoded_len = VarintLength(internal_key_size) +
 87                               internal_key_size + VarintLength(val_size) +
 88                               val_size;
 89    char* buf = arena_.Allocate(encoded_len);
 90    char* p = EncodeVarint32(buf, internal_key_size);
 91    memcpy(p, key.data(), key_size);
 92    p += key_size;
 93    EncodeFixed64(p, (s << 8) | type);
 94    p += 8;
 95    p = EncodeVarint32(p, val_size);
 96    memcpy(p, value.data(), val_size);
 97    assert(p + val_size == buf + encoded_len);
 98    table_.Insert(buf);
 99  }
100  
101  bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
102    Slice memkey = key.memtable_key();
103    Table::Iterator iter(&table_);
104    iter.Seek(memkey.data());
105    if (iter.Valid()) {
106      // entry format is:
107      //    klength  varint32
108      //    userkey  char[klength]
109      //    tag      uint64
110      //    vlength  varint32
111      //    value    char[vlength]
112      // Check that it belongs to same user key.  We do not check the
113      // sequence number since the Seek() call above should have skipped
114      // all entries with overly large sequence numbers.
115      const char* entry = iter.key();
116      uint32_t key_length;
117      const char* key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
118      if (comparator_.comparator.user_comparator()->Compare(
119              Slice(key_ptr, key_length - 8), key.user_key()) == 0) {
120        // Correct user key
121        const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
122        switch (static_cast<ValueType>(tag & 0xff)) {
123          case kTypeValue: {
124            Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
125            value->assign(v.data(), v.size());
126            return true;
127          }
128          case kTypeDeletion:
129            *s = Status::NotFound(Slice());
130            return true;
131        }
132      }
133    }
134    return false;
135  }
136  
137  }  // namespace leveldb