Questo script non dovrebbe essere installato direttamente. È una libreria per altri script da includere con la chiave // @require https://update.greasyfork.org/scripts/428485/1028790/lzw-encoder.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;
};
