com.itextpdf.layout.hyphenation.TernaryTree Maven / Gradle / Ivy
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
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* See the License for the specific language governing permissions and
* limitations under the License.
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package com.itextpdf.layout.hyphenation;
import java.io.Serializable;
import java.util.Enumeration;
/**
* 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]).
*/
public class TernaryTree implements Serializable {
/**
* We use 4 arrays to represent a node. I guess I should have created
* a proper node class, but somehow Knuth's pascal code made me forget
* we now have a portable language with virtual memory management and
* automatic garbage collection! And now is kind of late, furthermore,
* if it ain't broken, don't fix it.
*/
private static final long serialVersionUID = 3175412271203716160L;
/**
* 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[] lo;
/**
* Pointer to high branch.
*/
protected char[] hi;
/**
* Pointer to equal branch and to data when this node is a string terminator.
*/
protected char[] eq;
/**
* 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.
*/
protected char[] sc;
/**
* This vector holds the trailing of the keys when the branch is compressed.
*/
protected CharVector kv;
/** root */
protected char root;
/** free node */
protected char freenode;
/** number of items in tree */
protected int length;
/** allocation size for arrays */
protected static final int BLOCK_SIZE = 2048;
/** default constructor */
TernaryTree() {
init();
}
TernaryTree(TernaryTree tt) {
this.root = tt.root;
this.freenode = tt.freenode;
this.length = tt.length;
this.lo = (char[]) tt.lo.clone();
this.hi = (char[]) tt.hi.clone();
this.eq = (char[]) tt.eq.clone();
this.sc = (char[]) tt.sc.clone();
this.kv = new CharVector(tt.kv);
}
/** initialize */
protected void init() {
root = 0;
freenode = 1;
length = 0;
lo = new char[BLOCK_SIZE];
hi = new char[BLOCK_SIZE];
eq = new char[BLOCK_SIZE];
sc = new char[BLOCK_SIZE];
kv = new CharVector();
}
/**
* 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.
* @param key the key
* @param val a value
*/
public void insert(String key, char val) {
// make sure we have enough room in the arrays
int len = key.length()
+ 1; // maximum number of nodes that may be generated
if (freenode + len > eq.length) {
redimNodeArrays(eq.length + BLOCK_SIZE);
}
char[] strkey = new char[len--];
key.getChars(0, len, strkey, 0);
strkey[len] = 0;
root = insert(root, strkey, 0, val);
}
/**
* Insert key.
* @param key the key
* @param start offset into key array
* @param val a value
*/
public void insert(char[] key, int start, char val) {
int len = strlen(key) + 1;
if (freenode + len > eq.length) {
redimNodeArrays(eq.length + BLOCK_SIZE);
}
root = insert(root, key, start, val);
}
/**
* The actual insertion function, recursive version.
*/
private char insert(char p, char[] key, int start, char val) {
int len = strlen(key, start);
if (p == 0) {
// this means there is no branch, this node will start a new branch.
// Instead of doing that, we store the key somewhere else and create
// only one node with a pointer to the key
p = freenode++;
eq[p] = val; // holds data
length++;
hi[p] = 0;
if (len > 0) {
sc[p] = 0xFFFF; // indicates branch is compressed
lo[p] = (char)kv.alloc(len
+ 1); // use 'lo' to hold pointer to key
strcpy(kv.getArray(), lo[p], key, start);
} else {
sc[p] = 0;
lo[p] = 0;
}
return p;
}
if (sc[p] == 0xFFFF) {
// branch is compressed: need to decompress
// this will generate garbage in the external key array
// but we can do some garbage collection later
char pp = freenode++;
lo[pp] = lo[p]; // previous pointer to key
eq[pp] = eq[p]; // previous pointer to data
lo[p] = 0;
if (len > 0) {
sc[p] = kv.get(lo[pp]);
eq[p] = pp;
lo[pp]++;
if (kv.get(lo[pp]) == 0) {
// key completly decompressed leaving garbage in key array
lo[pp] = 0;
sc[pp] = 0;
hi[pp] = 0;
} else {
// we only got first char of key, rest is still there
sc[pp] = 0xFFFF;
}
} else {
// In this case we can save a node by swapping the new node
// with the compressed node
sc[pp] = 0xFFFF;
hi[p] = pp;
sc[p] = 0;
eq[p] = val;
length++;
return p;
}
}
char s = key[start];
if (s < sc[p]) {
lo[p] = insert(lo[p], key, start, val);
} else if (s == sc[p]) {
if (s != 0) {
eq[p] = insert(eq[p], key, start + 1, val);
} else {
// key already in tree, overwrite data
eq[p] = val;
}
} else {
hi[p] = insert(hi[p], key, start, val);
}
return p;
}
/**
* Compares 2 null terminated char arrays
* @param a a character array
* @param startA an index into character array
* @param b a character array
* @param startB an index into character array
* @return an integer
*/
public static int strcmp(char[] a, int startA, char[] b, int startB) {
for (; a[startA] == b[startB]; startA++, startB++) {
if (a[startA] == 0) {
return 0;
}
}
return a[startA] - b[startB];
}
/**
* Compares a string with null terminated char array
* @param str a string
* @param a a character array
* @param start an index into character array
* @return an integer
*/
public static int strcmp(String str, char[] a, int start) {
int i;
int d;
int len = str.length();
for (i = 0; i < len; i++) {
d = (int)str.charAt(i) - a[start + i];
if (d != 0) {
return d;
}
if (a[start + i] == 0) {
return d;
}
}
if (a[start + i] != 0) {
return -a[start + i];
}
return 0;
}
/**
* @param dst a character array
* @param di an index into character array
* @param src a character array
* @param si an index into character array
*/
public static void strcpy(char[] dst, int di, char[] src, int si) {
while (src[si] != 0) {
dst[di++] = src[si++];
}
dst[di] = 0;
}
/**
* @param a a character array
* @param start an index into character array
* @return an integer
*/
public static int strlen(char[] a, int start) {
int len = 0;
for (int i = start; i < a.length && a[i] != 0; i++) {
len++;
}
return len;
}
/**
* @param a a character array
* @return an integer
*/
public static int strlen(char[] a) {
return strlen(a, 0);
}
/**
* Find key.
* @param key the key
* @return result
*/
public int find(String key) {
int len = key.length();
char[] strkey = new char[len + 1];
key.getChars(0, len, strkey, 0);
strkey[len] = 0;
return find(strkey, 0);
}
/**
* Find key.
* @param key the key
* @param start offset into key array
* @return result
*/
public int find(char[] key, int start) {
int d;
char p = root;
int i = start;
char c;
while (p != 0) {
if (sc[p] == 0xFFFF) {
if (strcmp(key, i, kv.getArray(), lo[p]) == 0) {
return eq[p];
} else {
return -1;
}
}
c = key[i];
d = c - sc[p];
if (d == 0) {
if (c == 0) {
return eq[p];
}
i++;
p = eq[p];
} else if (d < 0) {
p = lo[p];
} else {
p = hi[p];
}
}
return -1;
}
/**
* @param key a key
* @return trye if key present
*/
public boolean knows(String key) {
return (find(key) >= 0);
}
// redimension the arrays
private void redimNodeArrays(int newsize) {
int len = newsize < lo.length ? newsize : lo.length;
char[] na = new char[newsize];
System.arraycopy(lo, 0, na, 0, len);
lo = na;
na = new char[newsize];
System.arraycopy(hi, 0, na, 0, len);
hi = na;
na = new char[newsize];
System.arraycopy(eq, 0, na, 0, len);
eq = na;
na = new char[newsize];
System.arraycopy(sc, 0, na, 0, len);
sc = na;
}
/** @return length */
public int size() {
return length;
}
/**
* 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.
* @param k array of keys
* @param v array of values
* @param offset where to insert
* @param n count to insert
*/
protected void insertBalanced(String[] k, char[] v, int offset, int n) {
int m;
if (n < 1) {
return;
}
m = n >> 1;
insert(k[m + offset], v[m + offset]);
insertBalanced(k, v, offset, m);
insertBalanced(k, v, offset + m + 1, n - m - 1);
}
/**
* Balance the tree for best search performance
*/
public void balance() {
// System.out.print("Before root splitchar = "); System.out.println(sc[root]);
int i = 0;
int n = length;
String[] k = new String[n];
char[] v = new char[n];
TernaryTreeIterator iter = new TernaryTreeIterator(this);
while (iter.hasMoreElements()) {
v[i] = iter.getValue();
k[i++] = (String)iter.nextElement();
}
init();
insertBalanced(k, v, 0, n);
// With uniform letter distribution sc[root] should be around 'm'
// System.out.print("After root splitchar = "); System.out.println(sc[root]);
}
/**
* 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!).
*
*/
public void trimToSize() {
// first balance the tree for best performance
balance();
// redimension the node arrays
redimNodeArrays(freenode);
// ok, compact kv array
CharVector kx = new CharVector();
kx.alloc(1);
TernaryTree map = new TernaryTree();
compact(kx, map, root);
kv = kx;
kv.trimToSize();
}
private void compact(CharVector kx, TernaryTree map, char p) {
int k;
if (p == 0) {
return;
}
if (sc[p] == 0xFFFF) {
k = map.find(kv.getArray(), lo[p]);
if (k < 0) {
k = kx.alloc(strlen(kv.getArray(), lo[p]) + 1);
strcpy(kx.getArray(), k, kv.getArray(), lo[p]);
map.insert(kx.getArray(), k, (char)k);
}
lo[p] = (char)k;
} else {
compact(kx, map, lo[p]);
if (sc[p] != 0) {
compact(kx, map, eq[p]);
}
compact(kx, map, hi[p]);
}
}
/** @return the keys */
public Enumeration keys() {
return new TernaryTreeIterator(this);
}
}