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// (c) Dean McNamee <dean@gmail.com>, 2013.
//
// https://github.com/deanm/omggif
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to
// deal in the Software without restriction, including without limitation the
// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
// sell copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
// IN THE SOFTWARE.
//
// omggif is a JavaScript implementation of a GIF 89a encoder and decoder,
// including animation and compression. It does not rely on any specific
// underlying system, so should run in the browser, Node, or Plask.

"use strict";

function GifWriter(buf, width, height, gopts) {
var p = 0;

var gopts = gopts === undefined ? { } : gopts;
var loop_count = gopts.loop === undefined ? null : gopts.loop;
var global_palette = gopts.palette === undefined ? null : gopts.palette;

if (width <= 0 || height <= 0 || width > 65535 || height > 65535)
throw new Error("Width/Height invalid.");

function check_palette_and_num_colors(palette) {
var num_colors = palette.length;
if (num_colors < 2 || num_colors > 256 || num_colors & (num_colors-1)) {
throw new Error(
"Invalid code/color length, must be power of 2 and 2 .. 256.");
}
return num_colors;
}

// - Header.
buf[p++] = 0x47; buf[p++] = 0x49; buf[p++] = 0x46; // GIF
buf[p++] = 0x38; buf[p++] = 0x39; buf[p++] = 0x61; // 89a

// Handling of Global Color Table (palette) and background index.
var gp_num_colors_pow2 = 0;
var background = 0;
if (global_palette !== null) {
var gp_num_colors = check_palette_and_num_colors(global_palette);
while (gp_num_colors >>= 1) ++gp_num_colors_pow2;
gp_num_colors = 1 << gp_num_colors_pow2;
--gp_num_colors_pow2;
if (gopts.background !== undefined) {
background = gopts.background;
if (background >= gp_num_colors)
throw new Error("Background index out of range.");
// The GIF spec states that a background index of 0 should be ignored, so
// this is probably a mistake and you really want to set it to another
// slot in the palette. But actually in the end most browsers, etc end
// up ignoring this almost completely (including for dispose background).
if (background === 0)
throw new Error("Background index explicitly passed as 0.");
}
}

// - Logical Screen Descriptor.
// NOTE(deanm): w/h apparently ignored by implementations, but set anyway.
buf[p++] = width & 0xff; buf[p++] = width >> 8 & 0xff;
buf[p++] = height & 0xff; buf[p++] = height >> 8 & 0xff;
// NOTE: Indicates 0-bpp original color resolution (unused?).
buf[p++] = (global_palette !== null ? 0x80 : 0) | // Global Color Table Flag.
gp_num_colors_pow2; // NOTE: No sort flag (unused?).
buf[p++] = background; // Background Color Index.
buf[p++] = 0; // Pixel aspect ratio (unused?).

// - Global Color Table
if (global_palette !== null) {
for (var i = 0, il = global_palette.length; i < il; ++i) {
var rgb = global_palette[i];
buf[p++] = rgb >> 16 & 0xff;
buf[p++] = rgb >> 8 & 0xff;
buf[p++] = rgb & 0xff;
}
}

if (loop_count !== null) { // Netscape block for looping.
if (loop_count < 0 || loop_count > 65535)
throw new Error("Loop count invalid.");
// Extension code, label, and length.
buf[p++] = 0x21; buf[p++] = 0xff; buf[p++] = 0x0b;
// NETSCAPE2.0
buf[p++] = 0x4e; buf[p++] = 0x45; buf[p++] = 0x54; buf[p++] = 0x53;
buf[p++] = 0x43; buf[p++] = 0x41; buf[p++] = 0x50; buf[p++] = 0x45;
buf[p++] = 0x32; buf[p++] = 0x2e; buf[p++] = 0x30;
// Sub-block
buf[p++] = 0x03; buf[p++] = 0x01;
buf[p++] = loop_count & 0xff; buf[p++] = loop_count >> 8 & 0xff;
buf[p++] = 0x00; // Terminator.
}

var ended = false;

this.addFrame = function(x, y, w, h, indexed_pixels, opts) {
if (ended === true) { --p; ended = false; } // Un-end.

opts = opts === undefined ? { } : opts;

// TODO(deanm): Bounds check x, y. Do they need to be within the virtual
// canvas width/height, I imagine?
if (x < 0 || y < 0 || x > 65535 || y > 65535)
throw new Error("x/y invalid.");

if (w <= 0 || h <= 0 || w > 65535 || h > 65535)
throw new Error("Width/Height invalid.");

if (indexed_pixels.length < w * h)
throw new Error("Not enough pixels for the frame size.");

var using_local_palette = true;
var palette = opts.palette;
if (palette === undefined || palette === null) {
using_local_palette = false;
palette = global_palette;
}

if (palette === undefined || palette === null)
throw new Error("Must supply either a local or global palette.");

var num_colors = check_palette_and_num_colors(palette);

// Compute the min_code_size (power of 2), destroying num_colors.
var min_code_size = 0;
while (num_colors >>= 1) ++min_code_size;
num_colors = 1 << min_code_size; // Now we can easily get it back.

var delay = opts.delay === undefined ? 0 : opts.delay;

// From the spec:
// 0 - No disposal specified. The decoder is
// not required to take any action.
// 1 - Do not dispose. The graphic is to be left
// in place.
// 2 - Restore to background color. The area used by the
// graphic must be restored to the background color.
// 3 - Restore to previous. The decoder is required to
// restore the area overwritten by the graphic with
// what was there prior to rendering the graphic.
// 4-7 - To be defined.
// NOTE(deanm): Dispose background doesn't really work, apparently most
// browsers ignore the background palette index and clear to transparency.
var disposal = opts.disposal === undefined ? 0 : opts.disposal;
if (disposal < 0 || disposal > 3) // 4-7 is reserved.
throw new Error("Disposal out of range.");

var use_transparency = false;
var transparent_index = 0;
if (opts.transparent !== undefined && opts.transparent !== null) {
use_transparency = true;
transparent_index = opts.transparent;
if (transparent_index < 0 || transparent_index >= num_colors)
throw new Error("Transparent color index.");
}

if (disposal !== 0 || use_transparency || delay !== 0) {
// - Graphics Control Extension
buf[p++] = 0x21; buf[p++] = 0xf9; // Extension / Label.
buf[p++] = 4; // Byte size.

buf[p++] = disposal << 2 | (use_transparency === true ? 1 : 0);
buf[p++] = delay & 0xff; buf[p++] = delay >> 8 & 0xff;
buf[p++] = transparent_index; // Transparent color index.
buf[p++] = 0; // Block Terminator.
}

// - Image Descriptor
buf[p++] = 0x2c; // Image Seperator.
buf[p++] = x & 0xff; buf[p++] = x >> 8 & 0xff; // Left.
buf[p++] = y & 0xff; buf[p++] = y >> 8 & 0xff; // Top.
buf[p++] = w & 0xff; buf[p++] = w >> 8 & 0xff;
buf[p++] = h & 0xff; buf[p++] = h >> 8 & 0xff;
// NOTE: No sort flag (unused?).
// TODO(deanm): Support interlace.
buf[p++] = using_local_palette === true ? (0x80 | (min_code_size-1)) : 0;

// - Local Color Table
if (using_local_palette === true) {
for (var i = 0, il = palette.length; i < il; ++i) {
var rgb = palette[i];
buf[p++] = rgb >> 16 & 0xff;
buf[p++] = rgb >> 8 & 0xff;
buf[p++] = rgb & 0xff;
}
}

p = GifWriterOutputLZWCodeStream(
buf, p, min_code_size < 2 ? 2 : min_code_size, indexed_pixels);

return p;
};

this.end = function() {
if (ended === false) {
buf[p++] = 0x3b; // Trailer.
ended = true;
}
return p;
};

this.getOutputBuffer = function() { return buf; };
this.setOutputBuffer = function(v) { buf = v; };
this.getOutputBufferPosition = function() { return p; };
this.setOutputBufferPosition = function(v) { p = v; };
}

// Main compression routine, palette indexes -> LZW code stream.
// |index_stream| must have at least one entry.
function GifWriterOutputLZWCodeStream(buf, p, min_code_size, index_stream) {
buf[p++] = min_code_size;
var cur_subblock = p++; // Pointing at the length field.

var clear_code = 1 << min_code_size;
var code_mask = clear_code - 1;
var eoi_code = clear_code + 1;
var next_code = eoi_code + 1;

var cur_code_size = min_code_size + 1; // Number of bits per code.
var cur_shift = 0;
// We have at most 12-bit codes, so we should have to hold a max of 19
// bits here (and then we would write out).
var cur = 0;

function emit_bytes_to_buffer(bit_block_size) {
while (cur_shift >= bit_block_size) {
buf[p++] = cur & 0xff;
cur >>= 8; cur_shift -= 8;
if (p === cur_subblock + 256) { // Finished a subblock.
buf[cur_subblock] = 255;
cur_subblock = p++;
}
}
}

function emit_code(c) {
cur |= c << cur_shift;
cur_shift += cur_code_size;
emit_bytes_to_buffer(8);
}

// I am not an expert on the topic, and I don't want to write a thesis.
// However, it is good to outline here the basic algorithm and the few data
// structures and optimizations here that make this implementation fast.
// The basic idea behind LZW is to build a table of previously seen runs
// addressed by a short id (herein called output code). All data is
// referenced by a code, which represents one or more values from the
// original input stream. All input bytes can be referenced as the same
// value as an output code. So if you didn't want any compression, you
// could more or less just output the original bytes as codes (there are
// some details to this, but it is the idea). In order to achieve
// compression, values greater then the input range (codes can be up to
// 12-bit while input only 8-bit) represent a sequence of previously seen
// inputs. The decompressor is able to build the same mapping while
// decoding, so there is always a shared common knowledge between the
// encoding and decoder, which is also important for "timing" aspects like
// how to handle variable bit width code encoding.
//
// One obvious but very important consequence of the table system is there
// is always a unique id (at most 12-bits) to map the runs. 'A' might be
// 4, then 'AA' might be 10, 'AAA' 11, 'AAAA' 12, etc. This relationship
// can be used for an effecient lookup strategy for the code mapping. We
// need to know if a run has been seen before, and be able to map that run
// to the output code. Since we start with known unique ids (input bytes),
// and then from those build more unique ids (table entries), we can
// continue this chain (almost like a linked list) to always have small
// integer values that represent the current byte chains in the encoder.
// This means instead of tracking the input bytes (AAAABCD) to know our
// current state, we can track the table entry for AAAABC (it is guaranteed
// to exist by the nature of the algorithm) and the next character D.
// Therefor the tuple of (table_entry, byte) is guaranteed to also be
// unique. This allows us to create a simple lookup key for mapping input
// sequences to codes (table indices) without having to store or search
// any of the code sequences. So if 'AAAA' has a table entry of 12, the
// tuple of ('AAAA', K) for any input byte K will be unique, and can be our
// key. This leads to a integer value at most 20-bits, which can always
// fit in an SMI value and be used as a fast sparse array / object key.

// Output code for the current contents of the index buffer.
var ib_code = index_stream[0] & code_mask; // Load first input index.
var code_table = { }; // Key'd on our 20-bit "tuple".

emit_code(clear_code); // Spec says first code should be a clear code.

// First index already loaded, process the rest of the stream.
for (var i = 1, il = index_stream.length; i < il; ++i) {
var k = index_stream[i] & code_mask;
var cur_key = ib_code << 8 | k; // (prev, k) unique tuple.
var cur_code = code_table[cur_key]; // buffer + k.

// Check if we have to create a new code table entry.
if (cur_code === undefined) { // We don't have buffer + k.
// Emit index buffer (without k).
// This is an inline version of emit_code, because this is the core
// writing routine of the compressor (and V8 cannot inline emit_code
// because it is a closure here in a different context). Additionally
// we can call emit_byte_to_buffer less often, because we can have
// 30-bits (from our 31-bit signed SMI), and we know our codes will only
// be 12-bits, so can safely have 18-bits there without overflow.
// emit_code(ib_code);
cur |= ib_code << cur_shift;
cur_shift += cur_code_size;
while (cur_shift >= 8) {
buf[p++] = cur & 0xff;
cur >>= 8; cur_shift -= 8;
if (p === cur_subblock + 256) { // Finished a subblock.
buf[cur_subblock] = 255;
cur_subblock = p++;
}
}

if (next_code === 4096) { // Table full, need a clear.
emit_code(clear_code);
next_code = eoi_code + 1;
cur_code_size = min_code_size + 1;
code_table = { };
} else { // Table not full, insert a new entry.
// Increase our variable bit code sizes if necessary. This is a bit
// tricky as it is based on "timing" between the encoding and
// decoder. From the encoders perspective this should happen after
// we've already emitted the index buffer and are about to create the
// first table entry that would overflow our current code bit size.
if (next_code >= (1 << cur_code_size)) ++cur_code_size;
code_table[cur_key] = next_code++; // Insert into code table.
}

ib_code = k; // Index buffer to single input k.
} else {
ib_code = cur_code; // Index buffer to sequence in code table.
}
}

emit_code(ib_code); // There will still be something in the index buffer.
emit_code(eoi_code); // End Of Information.

// Flush / finalize the sub-blocks stream to the buffer.
emit_bytes_to_buffer(1);

// Finish the sub-blocks, writing out any unfinished lengths and
// terminating with a sub-block of length 0. If we have already started
// but not yet used a sub-block it can just become the terminator.
if (cur_subblock + 1 === p) { // Started but unused.
buf[cur_subblock] = 0;
} else { // Started and used, write length and additional terminator block.
buf[cur_subblock] = p - cur_subblock - 1;
buf[p++] = 0;
}
return p;
}

function GifReader(buf) {
var p = 0;

// - Header (GIF87a or GIF89a).
if (buf[p++] !== 0x47 || buf[p++] !== 0x49 || buf[p++] !== 0x46 ||
buf[p++] !== 0x38 || (buf[p++]+1 & 0xfd) !== 0x38 || buf[p++] !== 0x61) {
throw new Error("Invalid GIF 87a/89a header.");
}

// - Logical Screen Descriptor.
var width = buf[p++] | buf[p++] << 8;
var height = buf[p++] | buf[p++] << 8;
var pf0 = buf[p++]; // <Packed Fields>.
var global_palette_flag = pf0 >> 7;
var num_global_colors_pow2 = pf0 & 0x7;
var num_global_colors = 1 << (num_global_colors_pow2 + 1);
var background = buf[p++];
buf[p++]; // Pixel aspect ratio (unused?).

var global_palette_offset = null;
var global_palette_size = null;

if (global_palette_flag) {
global_palette_offset = p;
global_palette_size = num_global_colors;
p += num_global_colors * 3; // Seek past palette.
}

var no_eof = true;

var frames = [ ];

var delay = 0;
var transparent_index = null;
var disposal = 0; // 0 - No disposal specified.
var loop_count = null;

this.width = width;
this.height = height;

while (no_eof && p < buf.length) {
switch (buf[p++]) {
case 0x21: // Graphics Control Extension Block
switch (buf[p++]) {
case 0xff: // Application specific block
// Try if it's a Netscape block (with animation loop counter).
if (buf[p ] !== 0x0b || // 21 FF already read, check block size.
// NETSCAPE2.0
buf[p+1 ] == 0x4e && buf[p+2 ] == 0x45 && buf[p+3 ] == 0x54 &&
buf[p+4 ] == 0x53 && buf[p+5 ] == 0x43 && buf[p+6 ] == 0x41 &&
buf[p+7 ] == 0x50 && buf[p+8 ] == 0x45 && buf[p+9 ] == 0x32 &&
buf[p+10] == 0x2e && buf[p+11] == 0x30 &&
// Sub-block
buf[p+12] == 0x03 && buf[p+13] == 0x01 && buf[p+16] == 0) {
p += 14;
loop_count = buf[p++] | buf[p++] << 8;
p++; // Skip terminator.
} else { // We don't know what it is, just try to get past it.
p += 12;
while (true) { // Seek through subblocks.
var block_size = buf[p++];
// Bad block size (ex: undefined from an out of bounds read).
if (!(block_size >= 0)) throw Error("Invalid block size");
if (block_size === 0) break; // 0 size is terminator
p += block_size;
}
}
break;

case 0xf9: // Graphics Control Extension
if (buf[p++] !== 0x4 || buf[p+4] !== 0)
throw new Error("Invalid graphics extension block.");
var pf1 = buf[p++];
delay = buf[p++] | buf[p++] << 8;
transparent_index = buf[p++];
if ((pf1 & 1) === 0) transparent_index = null;
disposal = pf1 >> 2 & 0x7;
p++; // Skip terminator.
break;

// Plain Text Extension could be present and we just want to be able
// to parse past it. It follows the block structure of the comment
// extension enough to reuse the path to skip through the blocks.
case 0x01: // Plain Text Extension (fallthrough to Comment Extension)
case 0xfe: // Comment Extension.
while (true) { // Seek through subblocks.
var block_size = buf[p++];
// Bad block size (ex: undefined from an out of bounds read).
if (!(block_size >= 0)) throw Error("Invalid block size");
if (block_size === 0) break; // 0 size is terminator
// console.log(buf.slice(p, p+block_size).toString('ascii'));
p += block_size;
}
break;

default:
throw new Error(
"Unknown graphic control label: 0x" + buf[p-1].toString(16));
}
break;

case 0x2c: // Image Descriptor.
var x = buf[p++] | buf[p++] << 8;
var y = buf[p++] | buf[p++] << 8;
var w = buf[p++] | buf[p++] << 8;
var h = buf[p++] | buf[p++] << 8;
var pf2 = buf[p++];
var local_palette_flag = pf2 >> 7;
var interlace_flag = pf2 >> 6 & 1;
var num_local_colors_pow2 = pf2 & 0x7;
var num_local_colors = 1 << (num_local_colors_pow2 + 1);
var palette_offset = global_palette_offset;
var palette_size = global_palette_size;
var has_local_palette = false;
if (local_palette_flag) {
var has_local_palette = true;
palette_offset = p; // Override with local palette.
palette_size = num_local_colors;
p += num_local_colors * 3; // Seek past palette.
}

var data_offset = p;

p++; // codesize
while (true) {
var block_size = buf[p++];
// Bad block size (ex: undefined from an out of bounds read).
if (!(block_size >= 0)) throw Error("Invalid block size");
if (block_size === 0) break; // 0 size is terminator
p += block_size;
}

frames.push({x: x, y: y, width: w, height: h,
has_local_palette: has_local_palette,
palette_offset: palette_offset,
palette_size: palette_size,
data_offset: data_offset,
data_length: p - data_offset,
transparent_index: transparent_index,
interlaced: !!interlace_flag,
delay: delay,
disposal: disposal});
break;

case 0x3b: // Trailer Marker (end of file).
no_eof = false;
break;

default:
throw new Error("Unknown gif block: 0x" + buf[p-1].toString(16));
break;
}
}

this.numFrames = function() {
return frames.length;
};

this.loopCount = function() {
return loop_count;
};

this.frameInfo = function(frame_num) {
if (frame_num < 0 || frame_num >= frames.length)
throw new Error("Frame index out of range.");
return frames[frame_num];
};

this.decodeAndBlitFrameBGRA = function(frame_num, pixels) {
var frame = this.frameInfo(frame_num);
var num_pixels = frame.width * frame.height;
var index_stream = new Uint8Array(num_pixels); // At most 8-bit indices.
GifReaderLZWOutputIndexStream(
buf, frame.data_offset, index_stream, num_pixels);
var palette_offset = frame.palette_offset;

// NOTE(deanm): It seems to be much faster to compare index to 256 than
// to === null. Not sure why, but CompareStub_EQ_STRICT shows up high in
// the profile, not sure if it's related to using a Uint8Array.
var trans = frame.transparent_index;
if (trans === null) trans = 256;

// We are possibly just blitting to a portion of the entire frame.
// That is a subrect within the framerect, so the additional pixels
// must be skipped over after we finished a scanline.
var framewidth = frame.width;
var framestride = width - framewidth;
var xleft = framewidth; // Number of subrect pixels left in scanline.

// Output index of the top left corner of the subrect.
var opbeg = ((frame.y * width) + frame.x) * 4;
// Output index of what would be the left edge of the subrect, one row
// below it, i.e. the index at which an interlace pass should wrap.
var opend = ((frame.y + frame.height) * width + frame.x) * 4;
var op = opbeg;

var scanstride = framestride * 4;

// Use scanstride to skip past the rows when interlacing. This is skipping
// 7 rows for the first two passes, then 3 then 1.
if (frame.interlaced === true) {
scanstride += width * 4 * 7; // Pass 1.
}

var interlaceskip = 8; // Tracking the row interval in the current pass.

for (var i = 0, il = index_stream.length; i < il; ++i) {
var index = index_stream[i];

if (xleft === 0) { // Beginning of new scan line
op += scanstride;
xleft = framewidth;
if (op >= opend) { // Catch the wrap to switch passes when interlacing.
scanstride = framestride * 4 + width * 4 * (interlaceskip-1);
// interlaceskip / 2 * 4 is interlaceskip << 1.
op = opbeg + (framewidth + framestride) * (interlaceskip << 1);
interlaceskip >>= 1;
}
}

if (index === trans) {
op += 4;
} else {
var r = buf[palette_offset + index * 3];
var g = buf[palette_offset + index * 3 + 1];
var b = buf[palette_offset + index * 3 + 2];
pixels[op++] = b;
pixels[op++] = g;
pixels[op++] = r;
pixels[op++] = 255;
}
--xleft;
}
};

// I will go to copy and paste hell one day...
this.decodeAndBlitFrameRGBA = function(frame_num, pixels) {
var frame = this.frameInfo(frame_num);
var num_pixels = frame.width * frame.height;
var index_stream = new Uint8Array(num_pixels); // At most 8-bit indices.
GifReaderLZWOutputIndexStream(
buf, frame.data_offset, index_stream, num_pixels);
var palette_offset = frame.palette_offset;

// NOTE(deanm): It seems to be much faster to compare index to 256 than
// to === null. Not sure why, but CompareStub_EQ_STRICT shows up high in
// the profile, not sure if it's related to using a Uint8Array.
var trans = frame.transparent_index;
if (trans === null) trans = 256;

// We are possibly just blitting to a portion of the entire frame.
// That is a subrect within the framerect, so the additional pixels
// must be skipped over after we finished a scanline.
var framewidth = frame.width;
var framestride = width - framewidth;
var xleft = framewidth; // Number of subrect pixels left in scanline.

// Output index of the top left corner of the subrect.
var opbeg = ((frame.y * width) + frame.x) * 4;
// Output index of what would be the left edge of the subrect, one row
// below it, i.e. the index at which an interlace pass should wrap.
var opend = ((frame.y + frame.height) * width + frame.x) * 4;
var op = opbeg;

var scanstride = framestride * 4;

// Use scanstride to skip past the rows when interlacing. This is skipping
// 7 rows for the first two passes, then 3 then 1.
if (frame.interlaced === true) {
scanstride += width * 4 * 7; // Pass 1.
}

var interlaceskip = 8; // Tracking the row interval in the current pass.

for (var i = 0, il = index_stream.length; i < il; ++i) {
var index = index_stream[i];

if (xleft === 0) { // Beginning of new scan line
op += scanstride;
xleft = framewidth;
if (op >= opend) { // Catch the wrap to switch passes when interlacing.
scanstride = framestride * 4 + width * 4 * (interlaceskip-1);
// interlaceskip / 2 * 4 is interlaceskip << 1.
op = opbeg + (framewidth + framestride) * (interlaceskip << 1);
interlaceskip >>= 1;
}
}

if (index === trans) {
op += 4;
} else {
var r = buf[palette_offset + index * 3];
var g = buf[palette_offset + index * 3 + 1];
var b = buf[palette_offset + index * 3 + 2];
pixels[op++] = r;
pixels[op++] = g;
pixels[op++] = b;
pixels[op++] = 255;
}
--xleft;
}
};
}

function GifReaderLZWOutputIndexStream(code_stream, p, output, output_length) {
var min_code_size = code_stream[p++];

var clear_code = 1 << min_code_size;
var eoi_code = clear_code + 1;
var next_code = eoi_code + 1;

var cur_code_size = min_code_size + 1; // Number of bits per code.
// NOTE: This shares the same name as the encoder, but has a different
// meaning here. Here this masks each code coming from the code stream.
var code_mask = (1 << cur_code_size) - 1;
var cur_shift = 0;
var cur = 0;

var op = 0; // Output pointer.

var subblock_size = code_stream[p++];

// TODO(deanm): Would using a TypedArray be any faster? At least it would
// solve the fast mode / backing store uncertainty.
// var code_table = Array(4096);
var code_table = new Int32Array(4096); // Can be signed, we only use 20 bits.

var prev_code = null; // Track code-1.

while (true) {
// Read up to two bytes, making sure we always 12-bits for max sized code.
while (cur_shift < 16) {
if (subblock_size === 0) break; // No more data to be read.

cur |= code_stream[p++] << cur_shift;
cur_shift += 8;

if (subblock_size === 1) { // Never let it get to 0 to hold logic above.
subblock_size = code_stream[p++]; // Next subblock.
} else {
--subblock_size;
}
}

// TODO(deanm): We should never really get here, we should have received
// and EOI.
if (cur_shift < cur_code_size)
break;

var code = cur & code_mask;
cur >>= cur_code_size;
cur_shift -= cur_code_size;

// TODO(deanm): Maybe should check that the first code was a clear code,
// at least this is what you're supposed to do. But actually our encoder
// now doesn't emit a clear code first anyway.
if (code === clear_code) {
// We don't actually have to clear the table. This could be a good idea
// for greater error checking, but we don't really do any anyway. We
// will just track it with next_code and overwrite old entries.

next_code = eoi_code + 1;
cur_code_size = min_code_size + 1;
code_mask = (1 << cur_code_size) - 1;

// Don't update prev_code ?
prev_code = null;
continue;
} else if (code === eoi_code) {
break;
}

// We have a similar situation as the decoder, where we want to store
// variable length entries (code table entries), but we want to do in a
// faster manner than an array of arrays. The code below stores sort of a
// linked list within the code table, and then "chases" through it to
// construct the dictionary entries. When a new entry is created, just the
// last byte is stored, and the rest (prefix) of the entry is only
// referenced by its table entry. Then the code chases through the
// prefixes until it reaches a single byte code. We have to chase twice,
// first to compute the length, and then to actually copy the data to the
// output (backwards, since we know the length). The alternative would be
// storing something in an intermediate stack, but that doesn't make any
// more sense. I implemented an approach where it also stored the length
// in the code table, although it's a bit tricky because you run out of
// bits (12 + 12 + 8), but I didn't measure much improvements (the table
// entries are generally not the long). Even when I created benchmarks for
// very long table entries the complexity did not seem worth it.
// The code table stores the prefix entry in 12 bits and then the suffix
// byte in 8 bits, so each entry is 20 bits.

var chase_code = code < next_code ? code : prev_code;

// Chase what we will output, either {CODE} or {CODE-1}.
var chase_length = 0;
var chase = chase_code;
while (chase > clear_code) {
chase = code_table[chase] >> 8;
++chase_length;
}

var k = chase;

var op_end = op + chase_length + (chase_code !== code ? 1 : 0);
if (op_end > output_length) {
console.log("Warning, gif stream longer than expected.");
return;
}

// Already have the first byte from the chase, might as well write it fast.
output[op++] = k;

op += chase_length;
var b = op; // Track pointer, writing backwards.

if (chase_code !== code) // The case of emitting {CODE-1} + k.
output[op++] = k;

chase = chase_code;
while (chase_length--) {
chase = code_table[chase];
output[--b] = chase & 0xff; // Write backwards.
chase >>= 8; // Pull down to the prefix code.
}

if (prev_code !== null && next_code < 4096) {
code_table[next_code++] = prev_code << 8 | k;
// TODO(deanm): Figure out this clearing vs code growth logic better. I
// have an feeling that it should just happen somewhere else, for now it
// is awkward between when we grow past the max and then hit a clear code.
// For now just check if we hit the max 12-bits (then a clear code should
// follow, also of course encoded in 12-bits).
if (next_code >= code_mask+1 && cur_code_size < 12) {
++cur_code_size;
code_mask = code_mask << 1 | 1;
}
}

prev_code = code;
}

if (op !== output_length) {
console.log("Warning, gif stream shorter than expected.");
}

return output;
}

// CommonJS.
try { exports.GifWriter = GifWriter; exports.GifReader = GifReader } catch(e) {}

MediaWiki:Omggif.js - Histórico de revisão

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