text: ﻿// Copyright (c) 2011 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.
  
  #include "db/memtable.h"
  #include "db/dbformat.h"
  #include "leveldb/comparator.h"
  #include "leveldb/env.h"
  #include "leveldb/iterator.h"
  #include "util/coding.h"
  
  namespace leveldb {
  
  static Slice GetLengthPrefixedSlice(const char* data) {
    uint32_t len;
    const char* p = data;
    p = GetVarint32Ptr(p, p + 5, &len);  // +5: we assume "p" is not corrupted
    return Slice(p, len);
  }
  
  MemTable::MemTable(const InternalKeyComparator& cmp)
      : comparator_(cmp),
        refs_(0),
        table_(comparator_, &arena_) {
  }
  
  MemTable::~MemTable() {
    assert(refs_ == 0);
  }
  
  size_t MemTable::ApproximateMemoryUsage() { return arena_.MemoryUsage(); }
  
  int MemTable::KeyComparator::operator()(const char* aptr, const char* bptr)
      const {
    // Internal keys are encoded as length-prefixed strings.
    Slice a = GetLengthPrefixedSlice(aptr);
    Slice b = GetLengthPrefixedSlice(bptr);
    return comparator.Compare(a, b);
  }
  
  // Encode a suitable internal key target for "target" and return it.
  // Uses *scratch as scratch space, and the returned pointer will point
  // into this scratch space.
  static const char* EncodeKey(std::string* scratch, const Slice& target) {
    scratch->clear();
    PutVarint32(scratch, target.size());
    scratch->append(target.data(), target.size());
    return scratch->data();
  }
  
  class MemTableIterator: public Iterator {
   public:
    explicit MemTableIterator(MemTable::Table* table) : iter_(table) { }
  
    virtual bool Valid() const { return iter_.Valid(); }
    virtual void Seek(const Slice& k) { iter_.Seek(EncodeKey(&tmp_, k)); }
    virtual void SeekToFirst() { iter_.SeekToFirst(); }
    virtual void SeekToLast() { iter_.SeekToLast(); }
    virtual void Next() { iter_.Next(); }
    virtual void Prev() { iter_.Prev(); }
    virtual Slice key() const { return GetLengthPrefixedSlice(iter_.key()); }
    virtual Slice value() const {
      Slice key_slice = GetLengthPrefixedSlice(iter_.key());
      return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());
    }
  
    virtual Status status() const { return Status::OK(); }
  
   private:
    MemTable::Table::Iterator iter_;
    std::string tmp_;       // For passing to EncodeKey
  
    // No copying allowed
    MemTableIterator(const MemTableIterator&);
    void operator=(const MemTableIterator&);
  };
  
  Iterator* MemTable::NewIterator() {
    return new MemTableIterator(&table_);
  }
  
  void MemTable::Add(SequenceNumber s, ValueType type,
                     const Slice& key,
                     const Slice& value) {
    // Format of an entry is concatenation of:
    //  key_size     : varint32 of internal_key.size()
    //  key bytes    : char[internal_key.size()]
    //  value_size   : varint32 of value.size()
    //  value bytes  : char[value.size()]
    size_t key_size = key.size();
    size_t val_size = value.size();
    size_t internal_key_size = key_size + 8;
    const size_t encoded_len =
        VarintLength(internal_key_size) + internal_key_size +
        VarintLength(val_size) + val_size;
    char* buf = arena_.Allocate(encoded_len);
    char* p = EncodeVarint32(buf, internal_key_size);
    memcpy(p, key.data(), key_size);
    p += key_size;
    EncodeFixed64(p, (s << 8) | type);
    p += 8;
    p = EncodeVarint32(p, val_size);
    memcpy(p, value.data(), val_size);
    assert((p + val_size) - buf == encoded_len);
    table_.Insert(buf);
  }
  
  bool MemTable::Get(const LookupKey& key, std::string* value, Status* s) {
    Slice memkey = key.memtable_key();
    Table::Iterator iter(&table_);
    iter.Seek(memkey.data());
    if (iter.Valid()) {
      // entry format is:
      //    klength  varint32
      //    userkey  char[klength]
      //    tag      uint64
      //    vlength  varint32
      //    value    char[vlength]
      // Check that it belongs to same user key.  We do not check the
      // sequence number since the Seek() call above should have skipped
      // all entries with overly large sequence numbers.
      const char* entry = iter.key();
      uint32_t key_length;
      const char* key_ptr = GetVarint32Ptr(entry, entry+5, &key_length);
      if (comparator_.comparator.user_comparator()->Compare(
              Slice(key_ptr, key_length - 8),
              key.user_key()) == 0) {
        // Correct user key
        const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
        switch (static_cast<ValueType>(tag & 0xff)) {
          case kTypeValue: {
            Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
            value->assign(v.data(), v.size());
            return true;
          }
          case kTypeDeletion:
            *s = Status::NotFound(Slice());
            return true;
        }
      }
    }
    return false;
  }
  
  }  // namespace leveldb
  