328 lines
8.5 KiB
JavaScript
328 lines
8.5 KiB
JavaScript
/**
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* This class handles LZW encoding
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* Adapted from Jef Poskanzer's Java port by way of J. M. G. Elliott.
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* @author Kevin Weiner (original Java version - kweiner@fmsware.com)
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* @author Thibault Imbert (AS3 version - bytearray.org)
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* @version 0.1 AS3 implementation
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*/
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//import flash.utils.ByteArray;
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LZWEncoder = function()
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{
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var exports = {};
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/*private_static*/ var EOF/*int*/ = -1;
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/*private*/ var imgW/*int*/;
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/*private*/ var imgH/*int*/
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/*private*/ var pixAry/*ByteArray*/;
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/*private*/ var initCodeSize/*int*/;
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/*private*/ var remaining/*int*/;
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/*private*/ var curPixel/*int*/;
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// GIFCOMPR.C - GIF Image compression routines
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// Lempel-Ziv compression based on 'compress'. GIF modifications by
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// David Rowley (mgardi@watdcsu.waterloo.edu)
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// General DEFINEs
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/*private_static*/ var BITS/*int*/ = 12;
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/*private_static*/ var HSIZE/*int*/ = 5003; // 80% occupancy
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// GIF Image compression - modified 'compress'
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// Based on: compress.c - File compression ala IEEE Computer, June 1984.
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// By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas)
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// Jim McKie (decvax!mcvax!jim)
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// Steve Davies (decvax!vax135!petsd!peora!srd)
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// Ken Turkowski (decvax!decwrl!turtlevax!ken)
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// James A. Woods (decvax!ihnp4!ames!jaw)
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// Joe Orost (decvax!vax135!petsd!joe)
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/*private*/ var n_bits/*int*/ // number of bits/code
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/*private*/ var maxbits/*int*/ = BITS; // user settable max # bits/code
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/*private*/ var maxcode/*int*/ // maximum code, given n_bits
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/*private*/ var maxmaxcode/*int*/ = 1 << BITS; // should NEVER generate this code
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/*private*/ var htab/*Array*/ = new Array;
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/*private*/ var codetab/*Array*/ = new Array;
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/*private*/ var hsize/*int*/ = HSIZE; // for dynamic table sizing
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/*private*/ var free_ent/*int*/ = 0; // first unused entry
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// block compression parameters -- after all codes are used up,
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// and compression rate changes, start over.
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/*private*/ var clear_flg/*Boolean*/ = false;
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// Algorithm: use open addressing double hashing (no chaining) on the
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// prefix code / next character combination. We do a variant of Knuth's
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// algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
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// secondary probe. Here, the modular division first probe is gives way
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// to a faster exclusive-or manipulation. Also do block compression with
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// an adaptive reset, whereby the code table is cleared when the compression
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// ratio decreases, but after the table fills. The variable-length output
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// codes are re-sized at this point, and a special CLEAR code is generated
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// for the decompressor. Late addition: construct the table according to
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// file size for noticeable speed improvement on small files. Please direct
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// questions about this implementation to ames!jaw.
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/*private*/ var g_init_bits/*int*/;
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/*private*/ var ClearCode/*int*/;
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/*private*/ var EOFCode/*int*/;
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// output
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// Output the given code.
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// Inputs:
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// code: A n_bits-bit integer. If == -1, then EOF. This assumes
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// that n_bits =< wordsize - 1.
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// Outputs:
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// Outputs code to the file.
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// Assumptions:
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// Chars are 8 bits long.
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// Algorithm:
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// Maintain a BITS character long buffer (so that 8 codes will
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// fit in it exactly). Use the VAX insv instruction to insert each
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// code in turn. When the buffer fills up empty it and start over.
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/*private*/ var cur_accum/*int*/ = 0;
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/*private*/ var cur_bits/*int*/ = 0;
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/*private*/ var masks/*Array*/ = [ 0x0000, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF, 0x01FF, 0x03FF, 0x07FF, 0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF ];
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// Number of characters so far in this 'packet'
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/*private*/ var a_count/*int*/;
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// Define the storage for the packet accumulator
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/*private*/ var accum/*ByteArray*/ = [];
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var LZWEncoder = exports.LZWEncoder = function LZWEncoder (width/*int*/, height/*int*/, pixels/*ByteArray*/, color_depth/*int*/)
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{
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imgW = width;
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imgH = height;
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pixAry = pixels;
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initCodeSize = Math.max(2, color_depth);
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}
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// Add a character to the end of the current packet, and if it is 254
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// characters, flush the packet to disk.
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var char_out = function char_out(c/*Number*/, outs/*ByteArray*/)/*void*/
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{
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accum[a_count++] = c;
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if (a_count >= 254) flush_char(outs);
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}
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// Clear out the hash table
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// table clear for block compress
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var cl_block = function cl_block(outs/*ByteArray*/)/*void*/
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{
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cl_hash(hsize);
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free_ent = ClearCode + 2;
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clear_flg = true;
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output(ClearCode, outs);
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}
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// reset code table
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var cl_hash = function cl_hash(hsize/*int*/)/*void*/
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{
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for (var i/*int*/ = 0; i < hsize; ++i) htab[i] = -1;
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}
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var compress = exports.compress = function compress(init_bits/*int*/, outs/*ByteArray*/)/*void*/
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{
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var fcode/*int*/;
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var i/*int*/ /* = 0 */;
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var c/*int*/;
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var ent/*int*/;
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var disp/*int*/;
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var hsize_reg/*int*/;
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var hshift/*int*/;
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// Set up the globals: g_init_bits - initial number of bits
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g_init_bits = init_bits;
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// Set up the necessary values
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clear_flg = false;
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n_bits = g_init_bits;
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maxcode = MAXCODE(n_bits);
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ClearCode = 1 << (init_bits - 1);
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EOFCode = ClearCode + 1;
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free_ent = ClearCode + 2;
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a_count = 0; // clear packet
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ent = nextPixel();
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hshift = 0;
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for (fcode = hsize; fcode < 65536; fcode *= 2)
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++hshift;
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hshift = 8 - hshift; // set hash code range bound
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hsize_reg = hsize;
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cl_hash(hsize_reg); // clear hash table
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output(ClearCode, outs);
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outer_loop: while ((c = nextPixel()) != EOF)
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{
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fcode = (c << maxbits) + ent;
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i = (c << hshift) ^ ent; // xor hashing
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if (htab[i] == fcode)
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{
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ent = codetab[i];
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continue;
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} else if (htab[i] >= 0) // non-empty slot
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{
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disp = hsize_reg - i; // secondary hash (after G. Knott)
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if (i == 0)
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disp = 1;
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do
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{
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if ((i -= disp) < 0) i += hsize_reg;
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if (htab[i] == fcode)
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{
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ent = codetab[i];
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continue outer_loop;
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}
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} while (htab[i] >= 0);
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}
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output(ent, outs);
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ent = c;
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if (free_ent < maxmaxcode)
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{
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codetab[i] = free_ent++; // code -> hashtable
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htab[i] = fcode;
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} else cl_block(outs);
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}
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// Put out the final code.
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output(ent, outs);
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output(EOFCode, outs);
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}
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// ----------------------------------------------------------------------------
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var encode = exports.encode = function encode(os/*ByteArray*/)/*void*/
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{
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os.writeByte(initCodeSize); // write "initial code size" byte
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remaining = imgW * imgH; // reset navigation variables
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curPixel = 0;
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compress(initCodeSize + 1, os); // compress and write the pixel data
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os.writeByte(0); // write block terminator
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}
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// Flush the packet to disk, and reset the accumulator
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var flush_char = function flush_char(outs/*ByteArray*/)/*void*/
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{
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if (a_count > 0)
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{
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outs.writeByte(a_count);
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outs.writeBytes(accum, 0, a_count);
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a_count = 0;
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}
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}
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var MAXCODE = function MAXCODE(n_bits/*int*/)/*int*/
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{
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return (1 << n_bits) - 1;
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}
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// ----------------------------------------------------------------------------
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// Return the next pixel from the image
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// ----------------------------------------------------------------------------
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var nextPixel = function nextPixel()/*int*/
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{
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if (remaining == 0) return EOF;
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--remaining;
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var pix/*Number*/ = pixAry[curPixel++];
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return pix & 0xff;
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}
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var output = function output(code/*int*/, outs/*ByteArray*/)/*void*/
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{
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cur_accum &= masks[cur_bits];
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if (cur_bits > 0) cur_accum |= (code << cur_bits);
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else cur_accum = code;
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cur_bits += n_bits;
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while (cur_bits >= 8)
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{
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char_out((cur_accum & 0xff), outs);
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cur_accum >>= 8;
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cur_bits -= 8;
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}
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// If the next entry is going to be too big for the code size,
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// then increase it, if possible.
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if (free_ent > maxcode || clear_flg)
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{
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if (clear_flg)
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{
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maxcode = MAXCODE(n_bits = g_init_bits);
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clear_flg = false;
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} else
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{
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++n_bits;
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if (n_bits == maxbits) maxcode = maxmaxcode;
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else maxcode = MAXCODE(n_bits);
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}
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}
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if (code == EOFCode)
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{
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// At EOF, write the rest of the buffer.
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while (cur_bits > 0)
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{
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char_out((cur_accum & 0xff), outs);
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cur_accum >>= 8;
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cur_bits -= 8;
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}
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flush_char(outs);
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}
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}
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LZWEncoder.apply(this, arguments);
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return exports;
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}
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