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125 lines
4.0 KiB
JavaScript
125 lines
4.0 KiB
JavaScript
/* Copyright 2013 Google Inc. All Rights Reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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Bit reading helpers
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*/
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var BROTLI_READ_SIZE = 4096;
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var BROTLI_IBUF_SIZE = (2 * BROTLI_READ_SIZE + 32);
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var BROTLI_IBUF_MASK = (2 * BROTLI_READ_SIZE - 1);
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var kBitMask = new Uint32Array([
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0, 1, 3, 7, 15, 31, 63, 127, 255, 511, 1023, 2047, 4095, 8191, 16383, 32767,
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65535, 131071, 262143, 524287, 1048575, 2097151, 4194303, 8388607, 16777215
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]);
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/* Input byte buffer, consist of a ringbuffer and a "slack" region where */
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/* bytes from the start of the ringbuffer are copied. */
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function BrotliBitReader(input) {
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this.buf_ = new Uint8Array(BROTLI_IBUF_SIZE);
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this.input_ = input; /* input callback */
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this.reset();
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}
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BrotliBitReader.READ_SIZE = BROTLI_READ_SIZE;
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BrotliBitReader.IBUF_MASK = BROTLI_IBUF_MASK;
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BrotliBitReader.prototype.reset = function() {
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this.buf_ptr_ = 0; /* next input will write here */
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this.val_ = 0; /* pre-fetched bits */
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this.pos_ = 0; /* byte position in stream */
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this.bit_pos_ = 0; /* current bit-reading position in val_ */
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this.bit_end_pos_ = 0; /* bit-reading end position from LSB of val_ */
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this.eos_ = 0; /* input stream is finished */
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this.readMoreInput();
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for (var i = 0; i < 4; i++) {
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this.val_ |= this.buf_[this.pos_] << (8 * i);
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++this.pos_;
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}
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return this.bit_end_pos_ > 0;
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};
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/* Fills up the input ringbuffer by calling the input callback.
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Does nothing if there are at least 32 bytes present after current position.
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Returns 0 if either:
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- the input callback returned an error, or
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- there is no more input and the position is past the end of the stream.
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After encountering the end of the input stream, 32 additional zero bytes are
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copied to the ringbuffer, therefore it is safe to call this function after
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every 32 bytes of input is read.
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*/
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BrotliBitReader.prototype.readMoreInput = function() {
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if (this.bit_end_pos_ > 256) {
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return;
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} else if (this.eos_) {
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if (this.bit_pos_ > this.bit_end_pos_)
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throw new Error('Unexpected end of input ' + this.bit_pos_ + ' ' + this.bit_end_pos_);
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} else {
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var dst = this.buf_ptr_;
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var bytes_read = this.input_.read(this.buf_, dst, BROTLI_READ_SIZE);
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if (bytes_read < 0) {
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throw new Error('Unexpected end of input');
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}
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if (bytes_read < BROTLI_READ_SIZE) {
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this.eos_ = 1;
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/* Store 32 bytes of zero after the stream end. */
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for (var p = 0; p < 32; p++)
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this.buf_[dst + bytes_read + p] = 0;
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}
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if (dst === 0) {
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/* Copy the head of the ringbuffer to the slack region. */
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for (var p = 0; p < 32; p++)
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this.buf_[(BROTLI_READ_SIZE << 1) + p] = this.buf_[p];
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this.buf_ptr_ = BROTLI_READ_SIZE;
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} else {
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this.buf_ptr_ = 0;
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}
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this.bit_end_pos_ += bytes_read << 3;
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}
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};
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/* Guarantees that there are at least 24 bits in the buffer. */
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BrotliBitReader.prototype.fillBitWindow = function() {
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while (this.bit_pos_ >= 8) {
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this.val_ >>>= 8;
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this.val_ |= this.buf_[this.pos_ & BROTLI_IBUF_MASK] << 24;
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++this.pos_;
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this.bit_pos_ = this.bit_pos_ - 8 >>> 0;
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this.bit_end_pos_ = this.bit_end_pos_ - 8 >>> 0;
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}
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};
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/* Reads the specified number of bits from Read Buffer. */
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BrotliBitReader.prototype.readBits = function(n_bits) {
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if (32 - this.bit_pos_ < n_bits) {
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this.fillBitWindow();
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}
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var val = ((this.val_ >>> this.bit_pos_) & kBitMask[n_bits]);
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this.bit_pos_ += n_bits;
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return val;
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};
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module.exports = BrotliBitReader;
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