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      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& cmp)
     22     : comparator_(cmp),
     23       refs_(0),
     24       table_(comparator_, &arena_) {
     25 }
     26 
     27 MemTable::~MemTable() {
     28   assert(refs_ == 0);
     29 }
     30 
     31 size_t MemTable::ApproximateMemoryUsage() { return arena_.MemoryUsage(); }
     32 
     33 int MemTable::KeyComparator::operator()(const char* aptr, const char* bptr)
     34     const {
     35   // Internal keys are encoded as length-prefixed strings.
     36   Slice a = GetLengthPrefixedSlice(aptr);
     37   Slice b = GetLengthPrefixedSlice(bptr);
     38   return comparator.Compare(a, b);
     39 }
     40 
     41 // Encode a suitable internal key target for "target" and return it.
     42 // Uses *scratch as scratch space, and the returned pointer will point
     43 // into this scratch space.
     44 static const char* EncodeKey(std::string* scratch, const Slice& target) {
     45   scratch->clear();
     46   PutVarint32(scratch, target.size());
     47   scratch->append(target.data(), target.size());
     48   return scratch->data();
     49 }
     50 
     51 class MemTableIterator: public Iterator {
     52  public:
     53   explicit MemTableIterator(MemTable::Table* table) : iter_(table) { }
     54 
     55   virtual bool Valid() const { return iter_.Valid(); }
     56   virtual void Seek(const Slice& k) { iter_.Seek(EncodeKey(&tmp_, k)); }
     57   virtual void SeekToFirst() { iter_.SeekToFirst(); }
     58   virtual void SeekToLast() { iter_.SeekToLast(); }
     59   virtual void Next() { iter_.Next(); }
     60   virtual void Prev() { iter_.Prev(); }
     61   virtual Slice key() const { return GetLengthPrefixedSlice(iter_.key()); }
     62   virtual Slice value() const {
     63     Slice key_slice = GetLengthPrefixedSlice(iter_.key());
     64     return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());
     65   }
     66 
     67   virtual Status status() const { return Status::OK(); }
     68 
     69  private:
     70   MemTable::Table::Iterator iter_;
     71   std::string tmp_;       // For passing to EncodeKey
     72 
     73   // No copying allowed
     74   MemTableIterator(const MemTableIterator&);
     75   void operator=(const MemTableIterator&);
     76 };
     77 
     78 Iterator* MemTable::NewIterator() {
     79   return new MemTableIterator(&table_);
     80 }
     81 
     82 void MemTable::Add(SequenceNumber s, ValueType type,
     83                    const Slice& key,
     84                    const Slice& value) {
     85   // Format of an entry is concatenation of:
     86   //  key_size     : varint32 of internal_key.size()
     87   //  key bytes    : char[internal_key.size()]
     88   //  value_size   : varint32 of value.size()
     89   //  value bytes  : char[value.size()]
     90   size_t key_size = key.size();
     91   size_t val_size = value.size();
     92   size_t internal_key_size = key_size + 8;
     93   const size_t encoded_len =
     94       VarintLength(internal_key_size) + internal_key_size +
     95       VarintLength(val_size) + val_size;
     96   char* buf = arena_.Allocate(encoded_len);
     97   char* p = EncodeVarint32(buf, internal_key_size);
     98   memcpy(p, key.data(), key_size);
     99   p += key_size;
    100   EncodeFixed64(p, (s << 8) | type);
    101   p += 8;
    102   p = EncodeVarint32(p, val_size);
    103   memcpy(p, value.data(), val_size);
    104   assert((p + val_size) - buf == encoded_len);
    105   table_.Insert(buf);
    106 }
    107 
    108 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
    109   Slice memkey = key.memtable_key();
    110   Table::Iterator iter(&table_);
    111   iter.Seek(memkey.data());
    112   if (iter.Valid()) {
    113     // entry format is:
    114     //    klength  varint32
    115     //    userkey  char[klength]
    116     //    tag      uint64
    117     //    vlength  varint32
    118     //    value    char[vlength]
    119     // Check that it belongs to same user key.  We do not check the
    120     // sequence number since the Seek() call above should have skipped
    121     // all entries with overly large sequence numbers.
    122     const char* entry = iter.key();
    123     uint32_t key_length;
    124     const char* key_ptr = GetVarint32Ptr(entry, entry+5, &key_length);
    125     if (comparator_.comparator.user_comparator()->Compare(
    126             Slice(key_ptr, key_length - 8),
    127             key.user_key()) == 0) {
    128       // Correct user key
    129       const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
    130       switch (static_cast<ValueType>(tag & 0xff)) {
    131         case kTypeValue: {
    132           Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
    133           value->assign(v.data(), v.size());
    134           return true;
    135         }
    136         case kTypeDeletion:
    137           *s = Status::NotFound(Slice());
    138           return true;
    139       }
    140     }
    141   }
    142   return false;
    143 }
    144 
    145 }  // namespace leveldb
    146