// 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& comparator)

    : comparator_(comparator), 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) {}

  MemTableIterator(const MemTableIterator&) = delete;

  MemTableIterator& operator=(const MemTableIterator&) = delete;

  ~MemTableIterator() override = default;

  bool Valid() const override { return iter_.Valid(); }

  void Seek(const Slice& k) override { iter_.Seek(EncodeKey(&tmp_, k)); }

  void SeekToFirst() override { iter_.SeekToFirst(); }

  void SeekToLast() override { iter_.SeekToLast(); }

  void Next() override { iter_.Next(); }

  void Prev() override { iter_.Prev(); }

  Slice key() const override { return GetLengthPrefixedSlice(iter_.key()); }

  Slice value() const override {

    Slice key_slice = GetLengthPrefixedSlice(iter_.key());

    return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());

  }

  Status status() const override { return Status::OK(); }

 private:

  MemTable::Table::Iterator iter_;

  std::string tmp_;  // For passing to EncodeKey

};

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()]

  //  tag          : uint64((sequence << 8) | type)

  //  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);

  std::memcpy(p, key.data(), key_size);

  p += key_size;

  EncodeFixed64(p, (s << 8) | type);

  p += 8;

  p = EncodeVarint32(p, val_size);

  std::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