rocksdb/utilities/merge_operators/string_append/stringappend2.cc

/**
 * @author Deon Nicholas (dnicholas@fb.com)
 * Copyright 2013 Facebook
 */

#include "stringappend2.h"

#include <memory>
#include <assert.h>

#include "leveldb/slice.h"
#include "leveldb/merge_operator.h"
#include "utilities/merge_operators.h"

namespace leveldb {

// Constructor: also specify the delimiter character.
StringAppendTESTOperator::StringAppendTESTOperator(char delim_char)
    : delim_(delim_char) {
}

// Implementation for the merge operation (concatenates two strings)
bool StringAppendTESTOperator::Merge(const Slice& key,
                                     const Slice* existing_value,
                                     const std::deque<std::string>& operands,
                                     std::string* new_value,
                                     Logger* logger) const {

  // Clear the *new_value for writing.
  assert(new_value);
  new_value->clear();

  // Compute the space needed for the final result.
  int numBytes = 0;
  for(auto it = operands.begin(); it != operands.end(); ++it) {
    numBytes += it->size() + 1;   // Plus 1 for the delimiter
  }

  // Only print the delimiter after the first entry has been printed
  bool printDelim = false;

  // Prepend the *existing_value if one exists.
  if (existing_value) {
    new_value->reserve(numBytes + existing_value->size());
    new_value->append(existing_value->data(), existing_value->size());
    printDelim = true;
  } else if (numBytes) {
    new_value->reserve(numBytes-1); // Minus 1 since we have one less delimiter
  }

  // Concatenate the sequence of strings (and add a delimiter between each)
  for(auto it = operands.begin(); it != operands.end(); ++it) {
    if (printDelim) {
      new_value->append(1,delim_);
    }
    new_value->append(*it);
    printDelim = true;
  }

  return true;
}

bool StringAppendTESTOperator::PartialMerge(const Slice& key,
                                            const Slice& left_operand,
                                            const Slice& right_operand,
                                            std::string* new_value,
                                            Logger* logger) const {
  return false;
}

// A version of PartialMerge that actually performs "partial merging".
// Use this to simulate the exact behaviour of the StringAppendOperator.
bool StringAppendTESTOperator::_AssocPartialMerge(const Slice& key,
                                            const Slice& left_operand,
                                            const Slice& right_operand,
                                            std::string* new_value,
                                            Logger* logger) const {
  // Clear the *new_value for writing.
  assert(new_value);
  new_value->clear();

  // Generic append
  // Reserve correct size for *new_value, and apply concatenation.
  new_value->reserve(left_operand.size() + 1 + right_operand.size());
  new_value->assign(left_operand.data(), left_operand.size());
  new_value->append(1,delim_);
  new_value->append(right_operand.data(), right_operand.size());

  return true;
}

const char* StringAppendTESTOperator::Name() const  {
  return "StringAppendTESTOperator";
}


std::shared_ptr<MergeOperator>
MergeOperators::CreateStringAppendTESTOperator() {
  return std::make_shared<StringAppendTESTOperator>(',');
}

} // namespace leveldb