org.apache.fop.layout.hyphenation
Class TernaryTree
- Direct Known Subclasses:
- HyphenationTree
- public class TernaryTree
- implements Cloneable, 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 requirenments 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.
- Author:
- cav@uniscope.co.jp
Method Summary |
void | balance() Balance the tree for best search performance |
Object | clone() |
int | find(String key) |
int | find(char[] key, int start) |
void | insert(String key, char val) Branches are initially compressed, needing
one node per key plus the size of the string
key. |
void | insert(char[] key, int start, char val) |
Enumeration | keys() |
boolean | knows(String key) |
static void | main(String[] args) |
void | printStats() |
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, int start) |
static int | strlen(char[] a) |
void | trimToSize() Each node stores a character (splitchar) which is part of
some key(s). |
TernaryTree
TernaryTree()
balance
public void balance()
- Balance the tree for best search performance
clone
public Object clone()
find
public int find(String key)
find
public int find(char[] key, int start)
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)
keys
public Enumeration keys()
knows
public boolean knows(String key)
main
public static void main(String[] args)
throws java.lang.Exception
printStats
public void printStats()
size
public int size()
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)
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!).
to Class org.apache.fop.layout.hyphenation.CharVector- This vector holds the trailing of the keys when the branch is compressed.