Change rocksDB merging code

tools/merge/merge.cc

+/*
+ *  Author : Ashwin <ashwin2007ray@gmail.com>
+ *
+ *  compile : g++ rocks_merge.cc <path_to_rocksDB_installation>/librocksdb.so.4.1 --std=c++11 -lstdc++fs
+ *  usage : ./<executable> <folder_having_rocksDB_directories_to_be_merged> <destination_folder>
+ *
+ */
+
+#include <fstream>
+#include <cstdio>
+#include <iostream>
+#include <string>
+
+#include "rocksdb/db.h"
+#include "rocksdb/options.h"
+
+#include <experimental/filesystem>
+
+using namespace rocksdb;
+namespace fs = std::experimental::filesystem;
+
+int main(int argc, char* argv[]) {
+  if(argc != 3) {
+    std::cerr << "Wrong number of arguments\nusage : ./<executable> <folder_having_rocksDB_directories_to_be_merged> <destination_folder>\n";
+    return 1;
+  }
+  std::string destinationDB = argv[2];
+  DB* db;
+  Options options;
+  // Optimize RocksDB. This is the easiest way to get RocksDB to perform well
+  options.IncreaseParallelism();
+  options.OptimizeLevelStyleCompaction();
+  // create the DB if it's not already present
+  options.create_if_missing = true;
+             
+  // open DB
+  Status s = DB::Open(options, destinationDB, &db);
+  assert(s.ok());
+
+  for (auto& dirEntry : fs::directory_iterator(argv[1])) {
+    std::cout << dirEntry << "\n" ;
+    DB* cur_db;
+    Options options;
+    options.IncreaseParallelism();
+    options.OptimizeLevelStyleCompaction();
+    // Don't create the DB if it's not already present
+    options.create_if_missing = false;
+    
+    // open DB
+    Status s1 = DB::Open(options, dirEntry.path().c_str(), &cur_db);
+    assert(s1.ok());
+
+    rocksdb::Iterator* it = cur_db->NewIterator(rocksdb::ReadOptions());
+    for (it->SeekToFirst(); it->Valid(); it->Next()) {
+      std::cout << it->key().ToString() << ": " << it->value().ToString() << std::endl;
+      s = db->Put(WriteOptions(), it->key().ToString(), it->value().ToString());
+      assert(s.ok());
+    }
+    assert(it->status().ok()); // Check for any errors found during the scan
+    delete it;
+    delete cur_db;
+  }
+
+  delete db;  
+  return 0;
+}