LZWEncoder.js

/**
 * This class handles LZW encoding
 * Adapted from Jef Poskanzer's Java port by way of J. M. G. Elliott.
 * @author Kevin Weiner (original Java version - kweiner@fmsware.com)
 * @author Thibault Imbert (AS3 version - bytearray.org)
 * @author Kevin Kwok (JavaScript version - https://github.com/antimatter15/jsgif)
 * @version 0.1 AS3 implementation
 */

LZWEncoder = function() {

	var exports = {};
	var EOF = -1;
	var imgW;
	var imgH;
	var pixAry;
	var initCodeSize;
	var remaining;
	var curPixel;

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

	var BITS = 12;
	var 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)

	var n_bits; // number of bits/code
	var maxbits = BITS; // user settable max # bits/code
	var maxcode; // maximum code, given n_bits
	var maxmaxcode = 1 << BITS; // should NEVER generate this code
	var htab = [];
	var codetab = [];
	var hsize = HSIZE; // for dynamic table sizing
	var free_ent = 0; // first unused entry

	// block compression parameters -- after all codes are used up,
	// and compression rate changes, start over.

	var 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.

	var g_init_bits;
	var ClearCode;
	var 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.

	var cur_accum = 0;
	var cur_bits = 0;
	var 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'
	var a_count;

	// Define the storage for the packet accumulator
	var accum = [];

	var LZWEncoder = exports.LZWEncoder = function LZWEncoder(width, height, pixels, 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.
	var char_out = function char_out(c, outs) {
		accum[a_count++] = c;
		if (a_count >= 254) flush_char(outs);
	};

	// Clear out the hash table
	// table clear for block compress

	var cl_block = function cl_block(outs) {
		cl_hash(hsize);
		free_ent = ClearCode + 2;
		clear_flg = true;
		output(ClearCode, outs);
	};

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

	var compress = exports.compress = function compress(init_bits, outs) {

		var fcode;
		var i; /* = 0 */
		var c;
		var ent;
		var disp;
		var hsize_reg;
		var 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);
	};

	// ----------------------------------------------------------------------------
	var encode = exports.encode = function encode(os) {
		os.writeByte(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.writeByte(0); // write block terminator
	};

	// Flush the packet to disk, and reset the accumulator
	var flush_char = function flush_char(outs) {
		if (a_count > 0) {
			outs.writeByte(a_count);
			outs.writeBytes(accum, 0, a_count);
			a_count = 0;
		}
	};

	var MAXCODE = function MAXCODE(n_bits) {
		return (1 << n_bits) - 1;
	};

	// ----------------------------------------------------------------------------
	// Return the next pixel from the image
	// ----------------------------------------------------------------------------

	var nextPixel = function nextPixel() {
		if (remaining === 0) return EOF;
		--remaining;
		var pix = pixAry[curPixel++];
		return pix & 0xff;
	};

	var output = function output(code, outs) {

		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((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((cur_accum & 0xff), outs);
				cur_accum >>= 8;
				cur_bits -= 8;
			}

			flush_char(outs);
		}
	};

	LZWEncoder.apply(this, arguments);
	return exports;
};