// 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.

//

// BlockBuilder generates blocks where keys are prefix-compressed:

//

// When we store a key, we drop the prefix shared with the previous

// string.  This helps reduce the space requirement significantly.

// Furthermore, once every K keys, we do not apply the prefix

// compression and store the entire key.  We call this a "restart

// point".  The tail end of the block stores the offsets of all of the

// restart points, and can be used to do a binary search when looking

// for a particular key.  Values are stored as-is (without compression)

// immediately following the corresponding key.

//

// An entry for a particular key-value pair has the form:

//     shared_bytes: varint32

//     unshared_bytes: varint32

//     value_length: varint32

//     key_delta: char[unshared_bytes]

//     value: char[value_length]

// shared_bytes == 0 for restart points.

//

// The trailer of the block has the form:

//     restarts: uint32[num_restarts]

//     num_restarts: uint32

// restarts[i] contains the offset within the block of the ith restart point.

#include "table/block_builder.h"

#include <algorithm>

#include <cassert>

#include "leveldb/comparator.h"

#include "leveldb/options.h"

#include "util/coding.h"

namespace leveldb {

BlockBuilder::BlockBuilder(const Options* options)

    : options_(options), restarts_(), counter_(0), finished_(false) {

  assert(options->block_restart_interval >= 1);

  restarts_.push_back(0);  // First restart point is at offset 0

}

void BlockBuilder::Reset() {

  buffer_.clear();

  restarts_.clear();

  restarts_.push_back(0);  // First restart point is at offset 0

  counter_ = 0;

  finished_ = false;

  last_key_.clear();

}

size_t BlockBuilder::CurrentSizeEstimate() const {

  return (buffer_.size() +                       // Raw data buffer

          restarts_.size() * sizeof(uint32_t) +  // Restart array

          sizeof(uint32_t));                     // Restart array length

}

Slice BlockBuilder::Finish() {

  // Append restart array

  for (size_t i = 0; i < restarts_.size(); i++) {

    PutFixed32(&buffer_, restarts_[i]);

  }

  PutFixed32(&buffer_, restarts_.size());

  finished_ = true;

  return Slice(buffer_);

}

void BlockBuilder::Add(const Slice& key, const Slice& value) {

  Slice last_key_piece(last_key_);

  assert(!finished_);

  assert(counter_ <= options_->block_restart_interval);

  assert(buffer_.empty()  // No values yet?

         || options_->comparator->Compare(key, last_key_piece) > 0);

  size_t shared = 0;

  if (counter_ < options_->block_restart_interval) {

    // See how much sharing to do with previous string

    const size_t min_length = std::min(last_key_piece.size(), key.size());

    while ((shared < min_length) && (last_key_piece[shared] == key[shared])) {

      shared++;

    }

  } else {

    // Restart compression

    restarts_.push_back(buffer_.size());

    counter_ = 0;

  }

  const size_t non_shared = key.size() - shared;

  // Add "<shared><non_shared><value_size>" to buffer_

  PutVarint32(&buffer_, shared);

  PutVarint32(&buffer_, non_shared);

  PutVarint32(&buffer_, value.size());

  // Add string delta to buffer_ followed by value

  buffer_.append(key.data() + shared, non_shared);

  buffer_.append(value.data(), value.size());

  // Update state

  last_key_.resize(shared);

  last_key_.append(key.data() + shared, non_shared);

  assert(Slice(last_key_) == key);

  counter_++;

}

}  // namespace leveldb