ReverseKeyComparator.cpp

#include <iostream>
#include <assert.h>
#include <string>
#include "rocksdb/db.h"
#include "rocksdb/comparator.h"
 
using namespace std;
using namespace rocksdb;
 
class ReverseKeyComparator : public rocksdb::Comparator {
   public:
    int Compare(const rocksdb::Slice& a, const rocksdb::Slice& b) const {
	return -a.compare(b);
    }
 
    // Ignore the following methods for now:
    const char* Name() const { return "ReverseKeyComparator"; }
    void FindShortestSeparator(std::string*, const rocksdb::Slice&) const { }
    void FindShortSuccessor(std::string*) const { }
  };
 
int main() {
	DB* db;
	Options options;
	options.create_if_missing = true;
	ReverseKeyComparator cmp;
	options.comparator = &cmp;
 
	// open a database with custom comparator
	Status status = rocksdb::DB::Open(options, "/tmp/testdb", &db);
	assert(status.ok());
 
	// populate the database
	Slice key1 = "key1_2143214";
	Slice key2 = "key1_123214";
	Slice key3 = "key2_341434324";
	string val1 = "one";
	string val2 = "two";
	string val3 = "three";
	db->Put(WriteOptions(), key1, val1);
	db->Put(WriteOptions(), key2, val2);
	db->Put(WriteOptions(), key3, val3);

	//iterate the database
	auto it = db->NewIterator(rocksdb::ReadOptions()); 

        // Prefix initialization for seek operation
        Slice prefix = "key1_33333";

        // Seek for first key1 ordered by numeric value in descending order
        it->Seek(prefix);
        if (it->Valid()){
	  cout << it->key().ToString() << ": " << it->value().ToString() << endl;      
        }
	cout << endl << endl;
	
	// Iterate from first to last key/value pair in index
	for (it->SeekToFirst(); it->Valid(); it->Next()) {
	  cout << it->key().ToString() << ": " << it->value().ToString() << endl;
	}
        // delete iterator
        delete it;
	// close the database
	delete db;
	return 0;
}