Package org.apache.lucene.analysis.compound.hyphenation

Class TernaryTree

java.lang.Object

org.apache.lucene.analysis.compound.hyphenation.TernaryTree

All Implemented Interfaces:

Cloneable

Direct Known Subclasses:

HyphenationTree

public class TernaryTree
extends Object
implements Cloneable

Ternary Search Tree.

A ternary search tree is a hybrid between a binary tree and a digital search tree (trie). Keys are limited to strings. A data value of type char is stored in each leaf node. It can be used as an index (or pointer) to the data. Branches that only contain one key are compressed to one node by storing a pointer to the trailer substring of the key. This class is intended to serve as base class or helper class to implement Dictionary collections or the like. Ternary trees have some nice properties as the following: the tree can be traversed in sorted order, partial matches (wildcard) can be implemented, retrieval of all keys within a given distance from the target, etc. The storage requirements are higher than a binary tree but a lot less than a trie. Performance is comparable with a hash table, sometimes it outperforms a hash function (most of the time can determine a miss faster than a hash).

The main purpose of this java port is to serve as a base for implementing TeX's hyphenation algorithm (see The TeXBook, appendix H). Each language requires from 5000 to 15000 hyphenation patterns which will be keys in this tree. The strings patterns are usually small (from 2 to 5 characters), but each char in the tree is stored in a node. Thus memory usage is the main concern. We will sacrifice 'elegance' to keep memory requirements to the minimum. Using java's char type as pointer (yes, I know pointer it is a forbidden word in java) we can keep the size of the node to be just 8 bytes (3 pointers and the data char). This gives room for about 65000 nodes. In my tests the english patterns took 7694 nodes and the german patterns 10055 nodes, so I think we are safe.

All said, this is a map with strings as keys and char as value. Pretty limited!. It can be extended to a general map by using the string representation of an object and using the char value as an index to an array that contains the object values.

This class has been taken from the Apache FOP project (http://xmlgraphics.apache.org/fop/). They have been slightly modified.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
class	TernaryTree.Iterator

Method Summary

Modifier and Type	Method	Description
void	balance()	Balance the tree for best search performance
TernaryTree	clone()
int	find(char[] key, int start)
int	find(String key)
void	insert(char[] key, int start, char val)
void	insert(String key, char val)	Branches are initially compressed, needing one node per key plus the size of the string key.
Enumeration<String>	keys()
boolean	knows(String key)
void	printStats(PrintStream out)
int	size()
static int	strcmp(char[] a, int startA, char[] b, int startB)	Compares 2 null terminated char arrays
static int	strcmp(String str, char[] a, int start)	Compares a string with null terminated char array
static void	strcpy(char[] dst, int di, char[] src, int si)
static int	strlen(char[] a)
static int	strlen(char[] a, int start)
void	trimToSize()	Each node stores a character (splitchar) which is part of some key(s).

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

insert

public void insert(String key,
                   char val)

Branches are initially compressed, needing one node per key plus the size of the string key. They are decompressed as needed when another key with same prefix is inserted. This saves a lot of space, specially for long keys.

insert

public void insert(char[] key,
                   int start,
                   char val)

strcmp

public static int strcmp(char[] a,
                         int startA,
                         char[] b,
                         int startB)

Compares 2 null terminated char arrays

strcmp

public static int strcmp(String str,
                         char[] a,
                         int start)

Compares a string with null terminated char array

strcpy

public static void strcpy(char[] dst,
                          int di,
                          char[] src,
                          int si)

strlen

public static int strlen(char[] a,
                         int start)

strlen

public static int strlen(char[] a)

find

public int find(String key)

find

public int find(char[] key,
                int start)

knows

public boolean knows(String key)

size

public int size()

clone

public TernaryTree clone()

balance

public void balance()

Balance the tree for best search performance

trimToSize

public void trimToSize()

Each node stores a character (splitchar) which is part of some key(s). In a compressed branch (one that only contain a single string key) the trailer of the key which is not already in nodes is stored externally in the kv array. As items are inserted, key substrings decrease. Some substrings may completely disappear when the whole branch is totally decompressed. The tree is traversed to find the key substrings actually used. In addition, duplicate substrings are removed using a map (implemented with a TernaryTree!).

keys

public Enumeration<String> keys()

printStats

public void printStats(PrintStream out)