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/* $Id: esort.h 1649 2009-10-19 14:35:01Z terpstra $
*
* esort.h - Public interface to libesort
*
* Copyright (C) 2002 - Wesley W. Terpstra
*
* License: GPL
*
* Authors: 'Wesley W. Terpstra' <wesley@terpstra.ca>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; version 2.1.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
#ifndef ESORT_H
#define ESORT_H
#include <string>
#include <memory>
/* What is ESort? -- An Online External Sort
*
* ESort provides a very primitive database API: An insert-only set of keys.
*
* There are very different trade-offs when compared with a standard database.
*
* N = number of keys in the set, M = number of operations
* The seeks needed to for each repeated operation in one transaction:
*
* ESort BTree Hash
* Insertion O(1) O(M*log(N)) O(M)
* Read in sequence O(1) O(M) impossible
* Seek to key O(M*log^2(N)) O(M*log(N)) O(M)
* Delete impossible O(M*log(N)) O(M)
*
* From this table, one can conclude that ESort allows blindly fast insertion
* and sequential read, but pays in the cost to seek. This is what makes it
* highly appropriate for keyword indexes.
*
* An additional restriction is that ESort only supports a single-writer.
* An additional advantage is that ESort readers get snap-shots.
*/
namespace ESort
{
using std::string;
using std::auto_ptr;
/** These parameters specify minimum values required for a database.
* If an existing database has too small a value, an error is returned.
* The unique flag toggles whether the database is a set or multiset.
*/
class Parameters
{
protected:
unsigned int version_;
unsigned long blockSize_;
unsigned long keySize_;
bool unique_;
bool synced_;
unsigned int keyWidth_;
public:
// keySize & blockSize are the number of bytes maximum
Parameters(
bool synced = true,
bool unique = true,
unsigned int version = 1,
unsigned long blockSize = 8192,
unsigned long keySize = 255);
bool isWider(const Parameters& o);
unsigned int version () const { return version_; }
unsigned long blockSize() const { return blockSize_; }
unsigned long keySize () const { return keySize_; }
bool unique () const { return unique_; }
bool synced () const { return synced_; }
unsigned int keyWidth () const { return keyWidth_; }
static Parameters minimize(const Parameters& x)
{
return Parameters(x.synced(), x.unique(), 1, 8, 1);
}
};
/** The direction in which the Walker walks the database.
*
* A Forward Query will find the first key >= the request
* A Backward Query will find the last key < the request
*
* The Forward Query moves the iterator so the key increases
* The Backward Query moves the iterator so the key decreases
*
* In this manner Forward + Backward patition the database's keys
*/
enum Direction
{
Forward,
Backward
};
/** The Walker class walks the database in sorted order.
* As from the chart above, each invokation of advance() pays 0 seeks.
*/
class Walker
{
public:
/** The current key which the walker points to.
* This is not valid until advance is called once.
*/
string key;
/** Advance to the next key in the database.
* Returned is the number of bytes in common with the last key.
* -1 is returned on error, errno = 0 -> reached EOF.
*/
virtual int advance() = 0;
virtual ~Walker();
};
/** A Reader object allows you to obtain Walkers.
*
* A Reader is atomic. When you obtain a Reader, the contents of the set
* remain fixed for as long as you keep a hold of the handle. Furthermore,
* a Reader will take snapshots of the database only between Writer commits.
*/
class Reader
{
public:
virtual ~Reader();
/** Obtain a Walker as described in Direction.
* This costs O(log^2(N)) seeks, so try not to call it too often.
*
* This operation never fails; you must call advance() before use.
*/
virtual auto_ptr<Walker> seek(const string& k, Direction dir) = 0;
/** Obtain a record Walker over only those records which are
* prefixed by 'prefix'. When the last such is passed, eof is
* returned.
*
* This operation never failes; you must call advance() before use.
*/
virtual auto_ptr<Walker> seek(const string& prefix, const string& k, Direction dir) = 0;
/** Open an existing database for reading.
* 0 is returned and errno set on error.
*/
static auto_ptr<Reader> opendb(
const string& db,
const Parameters& p = Parameters::minimize(Parameters()));
};
/** The Writer object allows you to insert keys.
*
* The Writer is atomic. When a key is inserted, it immediately becomes
* available in all Walkers & Readers obtained from the Writer. However,
* all open Readers never see the inserted keys, and new Readers do not
* see them until the Writer calls commit.
*/
class Writer : public Reader
{
public:
/** Insert a key into the database.
* If an error occures, -1 is returned and errno set, else 0.
* Be sure the key is smaller than keySize on pain of EINVAL.
*/
virtual int insert(const string& k) = 0;
/** Commit the changes to the database.
* If an error occures, -1 is returned and errno set, else 0.
*/
virtual int commit() = 0;
/** Open a database for writing.
* It is created if it did not exist.
*
* The mode is identical to open(2).
*
* 0 is returned and errno set on error.
*/
static auto_ptr<Writer> opendb(
const string& db, const Parameters& p = Parameters(),
int mode = 0666);
};
}
#endif
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