LZWEncoder

import java.io.OutputStream;
import java.io.IOException;

//==============================================================================
//  Adapted from Jef Poskanzer's Java port by way of J. M. G. Elliott.
// source : http://www.docjar.com/docs/api/com/fmsware/index.html
//  K Weiner 12/00


final class LZWEncoder {

  private static final int EOF = -1;

  private int     imgW, imgH;
  private byte[]  pixAry;
  private int     initCodeSize;
  private int     remaining;
  private int     curPixel;


  // GIFCOMPR.C       - GIF Image compression routines
  //
  // Lempel-Ziv compression based on 'compress'.  GIF modifications by
  // David Rowley (mgardi@watdcsu.waterloo.edu)

  // General DEFINEs

  static final int BITS = 12;

  static final int HSIZE = 5003;       // 80% occupancy

  // GIF Image compression - modified 'compress'
  //
  // Based on: compress.c - File compression ala IEEE Computer, June 1984.
  //
  // By Authors:  Spencer W. Thomas      (decvax!harpo!utah-cs!utah-gr!thomas)
  //              Jim McKie              (decvax!mcvax!jim)
  //              Steve Davies           (decvax!vax135!petsd!peora!srd)
  //              Ken Turkowski          (decvax!decwrl!turtlevax!ken)
  //              James A. Woods         (decvax!ihnp4!ames!jaw)
  //              Joe Orost              (decvax!vax135!petsd!joe)

  int n_bits;                         // number of bits/code
  int maxbits = BITS;                 // user settable max # bits/code
  int maxcode;                        // maximum code, given n_bits
  int maxmaxcode = 1 << BITS; // should NEVER generate this code

  int[] htab = new int[HSIZE];
  int[] codetab = new int[HSIZE];

  int hsize = HSIZE;                  // for dynamic table sizing

  int free_ent = 0;                   // first unused entry

  // block compression parameters -- after all codes are used up,
  // and compression rate changes, start over.
  boolean clear_flg = false;

  // Algorithm:  use open addressing double hashing (no chaining) on the
  // prefix code / next character combination.  We do a variant of Knuth's
  // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
  // secondary probe.  Here, the modular division first probe is gives way
  // to a faster exclusive-or manipulation.  Also do block compression with
  // an adaptive reset, whereby the code table is cleared when the compression
  // ratio decreases, but after the table fills.  The variable-length output
  // codes are re-sized at this point, and a special CLEAR code is generated
  // for the decompressor.  Late addition:  construct the table according to
  // file size for noticeable speed improvement on small files.  Please direct
  // questions about this implementation to ames!jaw.

  int g_init_bits;

  int ClearCode;
  int EOFCode;

  // output
  //
  // Output the given code.
  // Inputs:
  //      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
  //              that n_bits =< wordsize - 1.
  // Outputs:
  //      Outputs code to the file.
  // Assumptions:
  //      Chars are 8 bits long.
  // Algorithm:
  //      Maintain a BITS character long buffer (so that 8 codes will
  // fit in it exactly).  Use the VAX insv instruction to insert each
  // code in turn.  When the buffer fills up empty it and start over.

  int cur_accum = 0;
  int cur_bits = 0;

  int masks[] = { 0x0000, 0x0001, 0x0003, 0x0007, 0x000F,
              0x001F, 0x003F, 0x007F, 0x00FF,
              0x01FF, 0x03FF, 0x07FF, 0x0FFF,
              0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF };

  // Number of characters so far in this 'packet'
  int a_count;

  // Define the storage for the packet accumulator
  byte[] accum = new byte[256];


  //----------------------------------------------------------------------------
  LZWEncoder(int width, int height, byte[] pixels, int color_depth)
  {
   imgW = width;
   imgH = height;
   pixAry = pixels;
   initCodeSize = Math.max(2, color_depth);
  }


  // Add a character to the end of the current packet, and if it is 254
  // characters, flush the packet to disk.
  void char_out( byte c, OutputStream outs ) throws IOException
     {
     accum[a_count++] = c;
     if ( a_count >= 254 )
        flush_char( outs );
     }


  // Clear out the hash table

  // table clear for block compress
  void cl_block( OutputStream outs ) throws IOException
     {
     cl_hash( hsize );
     free_ent = ClearCode + 2;
     clear_flg = true;

     output( ClearCode, outs );
     }


  // reset code table
  void cl_hash( int hsize )
     {
     for ( int i = 0; i < hsize; ++i )
        htab[i] = -1;
     }


  void compress( int init_bits, OutputStream outs ) throws IOException
     {
     int fcode;
     int i /* = 0 */;
     int c;
     int ent;
     int disp;
     int hsize_reg;
     int hshift;

     // Set up the globals:  g_init_bits - initial number of bits
     g_init_bits = init_bits;

     // Set up the necessary values
     clear_flg = false;
     n_bits = g_init_bits;
     maxcode = MAXCODE( n_bits );

     ClearCode = 1 << ( init_bits - 1 );
     EOFCode = ClearCode + 1;
     free_ent = ClearCode + 2;

     a_count = 0;  // clear packet

     ent = nextPixel();

     hshift = 0;
     for ( fcode = hsize; fcode < 65536; fcode *= 2 )
        ++hshift;
     hshift = 8 - hshift;         // set hash code range bound

     hsize_reg = hsize;
     cl_hash( hsize_reg );        // clear hash table

     output( ClearCode, outs );

     outer_loop:
     while ( (c = nextPixel()) != EOF )
        {
        fcode = ( c << maxbits ) + ent;
        i = ( c << hshift ) ^ ent;   // xor hashing

        if ( htab[i] == fcode )
           {
           ent = codetab[i];
           continue;
           }
        else if ( htab[i] >= 0 )     // non-empty slot
           {
           disp = hsize_reg - i;  // secondary hash (after G. Knott)
           if ( i == 0 )
              disp = 1;
           do
              {
              if ( (i -= disp) < 0 )
                 i += hsize_reg;

              if ( htab[i] == fcode )
                 {
                 ent = codetab[i];
                 continue outer_loop;
                 }
              }
           while ( htab[i] >= 0 );
           }
        output( ent, outs );
        ent = c;
        if ( free_ent < maxmaxcode )
           {
           codetab[i] = free_ent++;  // code -> hashtable
           htab[i] = fcode;
           }
        else
           cl_block( outs );
        }
     // Put out the final code.
     output( ent, outs );
     output( EOFCode, outs );
     }


  //----------------------------------------------------------------------------
  void encode(OutputStream os) throws IOException
  {
   os.write(initCodeSize);         // write "initial code size" byte

   remaining = imgW * imgH;        // reset navigation variables
   curPixel = 0;

   compress(initCodeSize + 1, os); // compress and write the pixel data

   os.write(0);                    // write block terminator
  }


  // Flush the packet to disk, and reset the accumulator
  void flush_char( OutputStream outs ) throws IOException
     {
     if ( a_count > 0 )
        {
        outs.write( a_count );
        outs.write( accum, 0, a_count );
        a_count = 0;
        }
     }


  final int MAXCODE( int n_bits )
     {
     return ( 1 << n_bits ) - 1;
     }


  //----------------------------------------------------------------------------
  // Return the next pixel from the image
  //----------------------------------------------------------------------------
  private int nextPixel()
  {
   if (remaining == 0)
     return EOF;

   --remaining;

   byte pix = pixAry[curPixel++];

   return pix & 0xff;
  }


  void output( int code, OutputStream outs ) throws IOException
     {
     cur_accum &= masks[cur_bits];

     if ( cur_bits > 0 )
        cur_accum |= ( code << cur_bits );
     else
        cur_accum = code;

     cur_bits += n_bits;

     while ( cur_bits >= 8 )
        {
        char_out( (byte) ( cur_accum & 0xff ), outs );
        cur_accum >>= 8;
        cur_bits -= 8;
        }

     // If the next entry is going to be too big for the code size,
     // then increase it, if possible.
    if ( free_ent > maxcode || clear_flg )
        {
        if ( clear_flg )
           {
           maxcode = MAXCODE(n_bits = g_init_bits);
           clear_flg = false;
           }
        else
           {
           ++n_bits;
           if ( n_bits == maxbits )
              maxcode = maxmaxcode;
           else
              maxcode = MAXCODE(n_bits);
           }
        }

     if ( code == EOFCode )
        {
        // At EOF, write the rest of the buffer.
        while ( cur_bits > 0 )
           {
           char_out( (byte) ( cur_accum & 0xff ), outs );
           cur_accum >>= 8;
           cur_bits -= 8;
           }

        flush_char( outs );
        }
     }
}