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 and Description
    --- | ---
    `class` | `TernaryTree.Iterator`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected static int` | `BLOCK_SIZE`
    `protected char[]` | `eq` Pointer to equal branch and to data when this node is a string terminator.
    `protected char` | `freenode`
    `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`
    `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`
    `protected char[]` | `sc` The character stored in this node: splitchar.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `balance()` Balance the tree for best search performance
    `TernaryTree` | `clone()`
    `int` | `find(char[] key, int start)`
    `int` | `find(String key)`
    `protected void` | `init()`
    `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.
    `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<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, 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 guaranteed 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

    - #### freenode

          protected char freenode

    - #### length

          protected int length

    - #### BLOCK_SIZE

          protected static final int BLOCK_SIZE

      See Also:
        Constant Field Values

  - ### Method Detail

    - #### init

          protected void init()

    - #### 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()

      Overrides:
        `clone` in class `Object`

    - #### 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.

    - #### 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)

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