Package com.itextpdf.layout.hyphenation

Class TernaryTree

java.lang.Object
com.itextpdf.layout.hyphenation.TernaryTree
Direct Known Subclasses:
HyphenationTree
public class TernaryTree extends Object

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 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 (cav@uniscope.co.jp).

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    protected static final 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

    Modifier and Type
    Method
    Description
    void
    balance()
    Balance the tree for best search performance
    int
    find(char[] key, int start)
    Find key.
    int
    find(String key)
    Find key.
    protected void
    init()
    initialize
    void
    insert(char[] key, int start, char val)
    Insert key.
    void
    insert(String key, char val)
    Branches are initially compressed, needing one node per key plus the size of the string key.
    protected void
    insertBalanced(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.
    Enumeration
    keys()
     
    boolean
    knows(String key)
     
    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

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

  • Field Details

    • 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:

  • Method Details

    • init

      protected void init()
      initialize

    • 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.
      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 (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 (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 (String key)
      Parameters:
      key - a key
      Returns:
      trye if key present

    • size

      public int size()
      Returns:
      length

    • insertBalanced

      protected void insertBalanced (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 Enumeration keys()
      Returns:
      the keys