com.itextpdf.io.codec.LZWStringTable Maven / Gradle / Ivy
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Authors: Bruno Lowagie, Paulo Soares, et al.
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package com.itextpdf.io.codec;
import java.io.PrintStream;
/**
* General purpose LZW String Table.
* Extracted from GIFEncoder by Adam Doppelt
* Comments added by Robin Luiten
* expandCode added by Robin Luiten
* The strLen_ table to give quick access to the lenght of an expanded
* code for use by the expandCode method added by Robin.
*/
public class LZWStringTable {
/**
* codesize + Reserved Codes
*/
private static final int RES_CODES = 2;
//0xFFFF
private static final short HASH_FREE = -1;
//0xFFFF
private static final short NEXT_FIRST = -1;
private static final int MAXBITS = 12;
private static final int MAXSTR = (1 << MAXBITS);
private static final short HASHSIZE = 9973;
private static final short HASHSTEP = 2039;
// after predecessor character
byte[] strChr_;
// predecessor string
short[] strNxt_;
// hash table to find predecessor + char pairs
short[] strHsh_;
// next code if adding new prestring + char
short numStrings_;
/**
* each entry corresponds to a code and contains the length of data
* that the code expands to when decoded.
**/
int[] strLen_;
/**
* Constructor allocate memory for string store data
**/
public LZWStringTable() {
strChr_ = new byte[MAXSTR];
strNxt_ = new short[MAXSTR];
strLen_ = new int[MAXSTR];
strHsh_ = new short[HASHSIZE];
}
/**
* @param index value of -1 indicates no predecessor [used in initialization]
* @param b the byte [character] to add to the string store which follows
* the predecessor string specified the index.
* @return 0xFFFF if no space in table left for addition of predecessor
* index and byte b. Else return the code allocated for combination index + b.
**/
public int AddCharString(short index, byte b) {
int hshidx;
// if used up all codes
if (numStrings_ >= MAXSTR)
{
return 0xFFFF;
}
hshidx = Hash(index, b);
while (strHsh_[hshidx] != HASH_FREE)
hshidx = (hshidx + HASHSTEP) % HASHSIZE;
strHsh_[hshidx] = numStrings_;
strChr_[numStrings_] = b;
if (index == HASH_FREE) {
strNxt_[numStrings_] = NEXT_FIRST;
strLen_[numStrings_] = 1;
} else {
strNxt_[numStrings_] = index;
strLen_[numStrings_] = strLen_[index] + 1;
}
// return the code and inc for next code
return numStrings_++;
}
/**
* @param index index to prefix string
* @param b the character that follws the index prefix
* @return b if param index is HASH_FREE. Else return the code
* for this prefix and byte successor
**/
public short FindCharString(short index, byte b) {
int hshidx, nxtidx;
if (index == HASH_FREE)
// Rob fixed used to sign extend
return (short) (b & 0xFF);
hshidx = Hash(index, b);
// search
while ((nxtidx = strHsh_[hshidx]) != HASH_FREE)
{
if (strNxt_[nxtidx] == index && strChr_[nxtidx] == b)
return (short) nxtidx;
hshidx = (hshidx + HASHSTEP) % HASHSIZE;
}
//return (short) 0xFFFF;
return -1;
}
/**
* @param codesize the size of code to be preallocated for the
* string store.
**/
public void ClearTable(int codesize) {
numStrings_ = 0;
for (int q = 0; q < HASHSIZE; q++)
strHsh_[q] = HASH_FREE;
int w = (1 << codesize) + RES_CODES;
for (int q = 0; q < w; q++) {
// init with no prefix
AddCharString((short)-1, (byte) q);
}
}
public static int Hash(short index, byte lastbyte) {
return (((short) (lastbyte << 8) ^ index) & 0xFFFF) % HASHSIZE;
}
/**
* If expanded data doesn't fit into array only what will fit is written
* to buf and the return value indicates how much of the expanded code has
* been written to the buf. The next call to expandCode() should be with
* the same code and have the skip parameter set the negated value of the
* previous return. Successive negative return values should be negated and
* added together for next skip parameter value with same code.
*
* @param buf buffer to place expanded data into
* @param offset offset to place expanded data
* @param code the code to expand to the byte array it represents.
* PRECONDITION This code must already be in the LZSS
* @param skipHead is the number of bytes at the start of the expanded code to
* be skipped before data is written to buf. It is possible that skipHead is
* equal to codeLen.
* @return the length of data expanded into buf. If the expanded code is longer
* than space left in buf then the value returned is a negative number which when
* negated is equal to the number of bytes that were used of the code being expanded.
* This negative value also indicates the buffer is full.
**/
public int expandCode(byte[] buf, int offset, short code, int skipHead) {
if (offset == -2) {
if (skipHead == 1) skipHead = 0;
}
// code == -1 is checked just in case.
//-1 ~ 0xFFFF
if (code == -1 ||
// DONE no more unpacked
skipHead == strLen_[code])
return 0;
// how much data we are actually expanding
int expandLen;
// length of expanded code left
int codeLen = strLen_[code] - skipHead;
// how much space left
int bufSpace = buf.length - offset;
if (bufSpace > codeLen) {
// only got this many to unpack
expandLen = codeLen;
} else {
expandLen = bufSpace;
}
// only > 0 if codeLen > bufSpace [left overs]
int skipTail = codeLen - expandLen;
// initialise to exclusive end address of buffer area
int idx = offset + expandLen;
// NOTE: data unpacks in reverse direction and we are placing the
// unpacked data directly into the array in the correct location.
while ((idx > offset) && (code != -1)) {
// skip required of expanded data
if (--skipTail < 0)
{
buf[--idx] = strChr_[code];
}
// to predecessor code
code = strNxt_[code];
}
if (codeLen > expandLen) {
// indicate what part of codeLen used
return -expandLen;
} else {
// indicate length of dat unpacked
return expandLen;
}
}
public void dump(PrintStream output) {
int i;
for (i = 258; i < numStrings_; ++i)
output.println(" strNxt_[" + i + "] = " + strNxt_[i]
+ " strChr_ " + Integer.toHexString(strChr_[i] & 0xFF)
+ " strLen_ " + Integer.toHexString(strLen_[i]));
}
}