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LZWStringTable.javaAPI DocJava SE 6 API7168Tue Jun 10 00:21:52 BST 2008com.sun.imageio.plugins.common

LZWStringTable

public class LZWStringTable extends Object
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.

Fields Summary
private static final int
RES_CODES
codesize + Reserved Codes
private static final short
HASH_FREE
private static final short
NEXT_FIRST
private static final int
MAXBITS
private static final int
MAXSTR
private static final short
HASHSIZE
private static final short
HASHSTEP
byte[]
strChr
short[]
strNxt
short[]
strHsh
short
numStrings
int[]
strLen
Constructors Summary
public LZWStringTable()


    /*
     * Constructor allocate memory for string store data
     */
      
        strChr = new byte[MAXSTR];
        strNxt = new short[MAXSTR];
        strLen = new int[MAXSTR];
        strHsh = new short[HASHSIZE];
Methods Summary
public intaddCharString(short index, byte b)

        int hshidx;
        
        if (numStrings >= MAXSTR) { // if used up all codes
            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 numStrings++; // return the code and inc for next code
public voidclearTable(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++) {
            addCharString((short)0xFFFF, (byte)q); // init with no prefix
        }
public voiddump(java.io.PrintStream out)

        int i;
        for (i = 258; i < numStrings; ++i) {
            out.println(" strNxt[" + i + "] = " + strNxt[i]
                        + " strChr " + Integer.toHexString(strChr[i] & 0xFF)
                        + " strLen " + Integer.toHexString(strLen[i]));
        }
public intexpandCode(byte[] buf, int offset, short code, int skipHead)

        if (offset == -2) {
            if (skipHead == 1) {
                skipHead = 0;
            }
        }
        if (code == (short)0xFFFF ||    // just in case
            skipHead == strLen[code])  // DONE no more unpacked
        {
            return 0;
        }

        int expandLen;  // how much data we are actually expanding
        int codeLen = strLen[code] - skipHead; // length of expanded code left
        int bufSpace = buf.length - offset;  // how much space left
        if (bufSpace > codeLen) {
            expandLen = codeLen; // only got this many to unpack
        } else {
            expandLen = bufSpace;
        }

        int skipTail = codeLen - expandLen;  // only > 0 if codeLen > bufSpace [left overs]

        int idx = offset + expandLen;   // initialise to exclusive end address of buffer area

        // 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 != (short)0xFFFF)) {
            if (--skipTail < 0) { // skip required of expanded data
                buf[--idx] = strChr[code];
            }
            code = strNxt[code];    // to predecessor code
        }

        if (codeLen > expandLen) {
            return -expandLen; // indicate what part of codeLen used
        } else {
            return expandLen;     // indicate length of dat unpacked
        }
public shortfindCharString(short index, byte b)

        int hshidx, nxtidx;
        
        if (index == HASH_FREE) {
            return (short)(b & 0xFF);    // Rob fixed used to sign extend
        }

        hshidx = hash(index, b);
        while ((nxtidx = strHsh[hshidx]) != HASH_FREE) { // search
            if (strNxt[nxtidx] == index && strChr[nxtidx] == b) {
                return (short)nxtidx;
            }
            hshidx = (hshidx + HASHSTEP) % HASHSIZE;
        }
        
        return (short)0xFFFF;
public static inthash(short index, byte lastbyte)

        return ((int)((short)(lastbyte << 8) ^ index) & 0xFFFF) % HASHSIZE;