org.apache.fop.hyphenation
Class TernaryTree

java.lang.Object
  org.apache.fop.hyphenation.TernaryTree

All Implemented Interfaces:

java.io.Serializable, java.lang.Cloneable

Direct Known Subclasses:

HyphenationTree

public class TernaryTree
extends java.lang.Object
implements java.lang.Cloneable, java.io.Serializable

Ternary Search Tree.

A ternary search tree is a hibrid 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 sacrify '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 work was authored by Carlos Villegas ([email protected]).

See Also:

Serialized Form

Nested Class Summary

class

TernaryTree.Iterator an iterator

Field Summary

protected static int	BLOCK_SIZE allocation size for arrays
protected char[]	eq Pointer to equal branch and to data when this node is a string terminator.
protected char	freenode free node
protected char[]	hi Pointer to high branch.
protected CharVector	kv This vector holds the trailing of the keys when the branch is compressed.
protected int	length number of items in tree
protected char[]	lo Pointer to low branch and to rest of the key when it is stored directly in this node, we don't have unions in java!
protected char	root root
protected char[]	sc The character stored in this node: splitchar.

Method Summary

void	balance() Balance the tree for best search performance
java.lang.Object	clone()
int	find(char[] key, int start) Find key.
int	find(java.lang.String key) Find key.
protected void	init() initialize
void	insert(char[] key, int start, char val) Insert key.
void	insert(java.lang.String key, char val) Branches are initially compressed, needing one node per key plus the size of the string key.
protected void	insertBalanced(java.lang.String[] k, char[] v, int offset, int n) Recursively insert the median first and then the median of the lower and upper halves, and so on in order to get a balanced tree.
java.util.Enumeration	keys()
boolean	knows(java.lang.String key)
static void	main(java.lang.String[] args) Main entry point for testing.
void	printStats() Print stats (for testing).
int	size()
static int	strcmp(char[] a, int startA, char[] b, int startB) Compares 2 null terminated char arrays
static int	strcmp(java.lang.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, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

lo

protected char[] lo

Pointer to low branch and to rest of the key when it is stored directly in this node, we don't have unions in java!

hi

protected char[] hi

Pointer to high branch.

eq

protected char[] eq

Pointer to equal branch and to data when this node is a string terminator.

sc

protected char[] sc

The character stored in this node: splitchar. Two special values are reserved:

• 0x0000 as string terminator
• 0xFFFF to indicate that the branch starting at this node is compressed

This shouldn't be a problem if we give the usual semantics to strings since 0xFFFF is garanteed not to be an Unicode character.

kv

protected CharVector kv

This vector holds the trailing of the keys when the branch is compressed.

root

protected char root

root

freenode

protected char freenode

free node

length

protected int length

number of items in tree

BLOCK_SIZE

protected static final int BLOCK_SIZE

allocation size for arrays

See Also:

Constant Field Values

Method Detail

init

protected void init()

initialize

insert

public void insert(java.lang.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.

Parameters:

key - the key

val - a value

insert

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

Insert key.

Parameters:

key - the key

start - offset into key array

val - a value

strcmp

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

Compares 2 null terminated char arrays

Parameters:

a - a character array

startA - an index into character array

b - a character array

startB - an index into character array

Returns:

an integer

strcmp

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

Compares a string with null terminated char array

Parameters:

str - a string

a - a character array

start - an index into character array

Returns:

an integer

strcpy

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

Parameters:

dst - a character array

di - an index into character array

src - a character array

si - an index into character array

strlen

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

Parameters:

a - a character array

start - an index into character array

Returns:

an integer

strlen

public static int strlen(char[] a)

Parameters:

a - a character array

Returns:

an integer

find

public int find(java.lang.String key)

Find key.

Parameters:

key - the key

Returns:

result

find

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

Find key.

Parameters:

key - the key

start - offset into key array

Returns:

result

knows

public boolean knows(java.lang.String key)

Parameters:

key - a key

Returns:

trye if key present

size

public int size()

Returns:

length

clone

public java.lang.Object clone()
                       throws java.lang.CloneNotSupportedException

Overrides:

clone in class java.lang.Object

Throws:

java.lang.CloneNotSupportedException

insertBalanced

protected void insertBalanced(java.lang.String[] k,
                              char[] v,
                              int offset,
                              int n)

Recursively insert the median first and then the median of the lower and upper halves, and so on in order to get a balanced tree. The array of keys is assumed to be sorted in ascending order.

Parameters:

k - array of keys

v - array of values

offset - where to insert

n - count to insert

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 java.util.Enumeration keys()

Returns:

the keys

printStats

public void printStats()

Print stats (for testing).

main

public static void main(java.lang.String[] args)
                 throws java.lang.Exception

Main entry point for testing.

Parameters:

args - not used

Throws:

java.lang.Exception - if not caught