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 Details

    • 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)