lzwstringtable.java

java1.6众多例子参考

/*
 * @(#)LZWStringTable.java	1.2 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package com.sun.imageio.plugins.common;

import java.io.PrintStream;

/** 
 * General purpose LZW String Table.
 * Extracted from GIFEncoder by Adam Doppelt
 * Comments added by Robin Luiten
 * <code>expandCode</code> added by Robin Luiten
 * The strLen table to give quick access to the lenght of an expanded
 * code for use by the <code>expandCode</code> method added by Robin.
 **/
public class LZWStringTable {
    /** codesize + Reserved Codes */
    private final static int RES_CODES = 2;

    private final static short HASH_FREE = (short)0xFFFF;
    private final static short NEXT_FIRST = (short)0xFFFF;
    
    private final static int MAXBITS = 12;
    private final static int MAXSTR = (1 << MAXBITS);
    
    private final static short HASHSIZE = 9973;
    private final static short HASHSTEP = 2039;

    byte[]  strChr;  // after predecessor character
    short[] strNxt;  // predecessor string 
    short[] strHsh;  // hash table to find  predecessor + char pairs
    short numStrings;  // next code if adding new prestring + char
    
    /*
     * 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 initialisation]
     * @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 predecesor
     * index and byte b. Else return the code allocated for combination index + b.
     */
    public int addCharString(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
    }

    /*
     * @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) {
            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;
    }

    /*
     * @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++) {
            addCharString((short)0xFFFF, (byte)q); // init with no prefix
        }
    }

    static public int hash(short index, byte lastbyte) {
        return ((int)((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. Succesive 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;
            }
        }
        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 void dump(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]));
        }
    }
}