Bcursor Class Reference

A cursor pointing to a position in a Btree table, for reading several entries in order, or finding approximate matches. More...

#include <bcursor.h>

Inheritance diagram for Bcursor:

[legend]Collaboration diagram for Bcursor:

[legend]List of all members.


Public Member Functions
	Bcursor (Btree *B)
	Create a cursor attached to a Btree.
	~Bcursor ()
	Destroy the Bcursor.
void	read_tag ()
	Read the tag from the table and store it in current_tag.
bool	next ()
	Advance to the next key.
bool	prev ()
	Move to the previous key.
bool	find_entry (const string &key)
	Find an entry, or a near match, in the table.
bool	after_end ()
	Determine whether cursor is off the end of table.
void	del ()
	Delete the current key/tag pair, leaving the cursor on the next entry.
Public Attributes
string	current_key
	Current key pointed to by cursor.
string	current_tag
	Current tag pointed to by cursor.
Private Member Functions
	Bcursor (const Bcursor &)
	Copying not allowed.
Bcursor &	operator= (const Bcursor &)
	Assignment not allowed.
bool	get_key (string *key) const
	Get the key.
Private Attributes
bool	is_positioned
	Whether the cursor is positioned at a valid entry.
bool	is_after_end
	Whether the cursor is off the end of the table.
bool	have_read_tag
	Have we read the tag for the current key?
Btree *	B
	The Btree table.
Cursor *	C
	Pointer to an array of Cursors.
int	level
	The value of level in the Btree structure.

Detailed Description

A cursor pointing to a position in a Btree table, for reading several entries in order, or finding approximate matches.

Definition at line 67 of file bcursor.h.

Constructor & Destructor Documentation

Bcursor::Bcursor ( const Bcursor & ) [private]

Copying not allowed.

Bcursor::Bcursor ( Btree * B )

Create a cursor attached to a Btree.

Creates a cursor, which can be used to remember a position inside the B-tree. The position is simply the item (key and tag) to which the cursor points. A cursor is either positioned or unpositioned, and is initially unpositioned.

NB: You must not try to use a Bcursor after the Btree it is attached to is destroyed. It's safe to destroy the Bcursor after the Btree though, you just may not use the Bcursor.

Definition at line 49 of file bcursor.cc.

References B, BLK_UNUSED, Btree::block_size, Btree::C, C, level, Cursor::n, and Cursor::p.

Bcursor::~Bcursor ( )

Destroy the Bcursor.

Definition at line 66 of file bcursor.cc.

References C, and level.

Member Function Documentation

Bcursor& Bcursor::operator= ( const Bcursor & ) [private]

Assignment not allowed.

bool Bcursor::get_key ( string * key ) const [private]

Get the key.

If the cursor is unpositioned, the result is false.

If the cursor is positioned, the key of the item at the cursor is copied into key and the result is then true.

e.g.

Bcursor BC(&btree); string key;

// Now do something to each key in the Btree BC.find_entry(""); // must give result true

while (BC.next()) { BC.get_key(&key)) { do_something_to(key); }

Definition at line 199 of file bcursor.cc.

References Assert, B, C, is_positioned, level, and Btree::level.

Referenced by find_entry(), next(), and prev().

void Bcursor::read_tag ( )

Read the tag from the table and store it in current_tag.

Some cursor users don't need the tag, so for efficiency we only read it when asked to.

Definition at line 210 of file bcursor.cc.

References Assert, B, C, current_tag, DEBUGCALL, DEBUGLINE, have_read_tag, hex_encode(), is_positioned, level, Btree::level, Btree::next(), and Btree::read_tag().

Referenced by check_table_values_empty(), check_table_values_hello(), PositionCursor::next(), DocIDKeyedCursor::next(), PostlistCursor::next(), test_cursor1(), test_cursor2(), test_cursor3(), and test_table3().

bool Bcursor::next ( )

Advance to the next key.

If cursor is unpositioned, the result is simply false.

If cursor is positioned, and points to the very last item in the Btree the cursor is made unpositioned, and the result is false. Otherwise the cursor is moved to the next item in the B-tree, and the result is true.

Effectively, Bcursor::next() loses the position of BC when it drops off the end of the list of items. If this is awkward, one can always arrange for a key to be present which has a rightmost position in a set of keys,

Reimplemented in PostlistCursor, DocIDKeyedCursor, PositionCursor, and PostlistCursor.

Definition at line 121 of file bcursor.cc.

References Assert, B, C, current_key, DEBUGCALL, DEBUGLINE, get_key(), have_read_tag, hex_encode(), is_after_end, is_positioned, level, Btree::level, and Btree::next().

Referenced by del(), PositionCursor::next(), DocIDKeyedCursor::next(), PostlistCursor::next(), test_cursor1(), test_cursor3(), and test_table3().

bool Bcursor::prev ( )

Move to the previous key.

This is like Bcursor::next, but BC is taken to the previous rather than next item.

Definition at line 77 of file bcursor.cc.

References Assert, B, C, current_key, DEBUGCALL, DEBUGLINE, find_entry(), get_key(), have_read_tag, hex_encode(), is_after_end, is_positioned, level, Btree::level, and Btree::prev().

bool Bcursor::find_entry ( const string & key )

Find an entry, or a near match, in the table.

If the exact key is found in the table, the cursor will be set to point to it, and the method will return true.

If the key is not found, the cursor will be set to point to the key preceding that asked for, and the method will return false.

If there is no key preceding that asked for, the cursor will point to a null key.

Note: Since the B-tree always contains a null key, which precedes everything, a call to Bcursor::find_entry always results in a valid key being pointed to by the cursor.

Note: Calling this method with a null key, then calling next() will leave the cursor pointing to the first key.

Parameters:

key

The key to look for in the table.