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decoder.js
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Sat, Dec 14, 19:33

decoder.js
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var hpack = require('../hpack');
var utils = hpack.utils;
var huffman = hpack.huffman.decode;
var assert = utils.assert;
var OffsetBuffer = require('obuf');
function Decoder() {
this.buffer = new OffsetBuffer();
this.bitOffset = 0;
// Used internally in decodeStr
this._huffmanNode = null;
}
module.exports = Decoder;
Decoder.create = function create() {
return new Decoder();
};
Decoder.prototype.isEmpty = function isEmpty() {
return this.buffer.isEmpty();
};
Decoder.prototype.push = function push(chunk) {
this.buffer.push(chunk);
};
Decoder.prototype.decodeBit = function decodeBit() {
// Need at least one octet
assert(this.buffer.has(1), 'Buffer too small for an int');
var octet;
var offset = this.bitOffset;
if (++this.bitOffset === 8) {
octet = this.buffer.readUInt8();
this.bitOffset = 0;
} else {
octet = this.buffer.peekUInt8();
}
return (octet >>> (7 - offset)) & 1;
};
// Just for testing
Decoder.prototype.skipBits = function skipBits(n) {
this.bitOffset += n;
this.buffer.skip(this.bitOffset >> 3);
this.bitOffset &= 0x7;
};
Decoder.prototype.decodeInt = function decodeInt() {
// Need at least one octet
assert(this.buffer.has(1), 'Buffer too small for an int');
var prefix = 8 - this.bitOffset;
// We are going to end up octet-aligned
this.bitOffset = 0;
var max = (1 << prefix) - 1;
var octet = this.buffer.readUInt8() & max;
// Fast case - int fits into the prefix
if (octet !== max)
return octet;
// TODO(indutny): what about > 32bit numbers?
var res = 0;
var isLast = false;
var len = 0;
do {
octet = this.buffer.readUInt8();
isLast = (octet & 0x80) === 0;
res <<= 7;
res |= octet & 0x7f;
len++;
} while (!isLast);
assert(isLast, 'Incomplete data for multi-octet integer');
assert(len <= 4, 'Integer does not fit into 32 bits');
// Reverse bits
res = (res >>> 21) |
(((res >> 14) & 0x7f) << 7) |
(((res >> 7) & 0x7f) << 14) |
((res & 0x7f) << 21);
res >>= (4 - len) * 7;
// Append prefix max
res += max;
return res;
};
Decoder.prototype.decodeHuffmanWord = function decodeHuffmanWord(input,
inputBits,
out) {
var root = huffman;
var node = this._huffmanNode;
var word = input;
var bits = inputBits;
for (; bits > 0; word &= (1 << bits) - 1) {
// Nudge the word bit length to match it
for (var i = Math.max(0, bits - 8); i < bits; i++) {
var subnode = node[word >>> i];
if (typeof subnode !== 'number') {
node = subnode;
bits = i;
break;
}
if (subnode === 0)
continue;
// Word bit length should match
if ((subnode >>> 9) !== bits - i) {
subnode = 0;
continue;
}
var octet = subnode & 0x1ff;
assert(octet !== 256, 'EOS in encoding');
out.push(octet);
node = root;
bits = i;
break;
}
if (subnode === 0)
break;
}
this._huffmanNode = node;
return bits;
};
Decoder.prototype.decodeStr = function decodeStr() {
var isHuffman = this.decodeBit();
var len = this.decodeInt();
assert(this.buffer.has(len), 'Not enough octets for string');
if (!isHuffman)
return this.buffer.take(len);
this._huffmanNode = huffman;
var out = [];
var word = 0;
var bits = 0;
var lastKey = 0;
for (var i = 0; i < len; i++) {
word <<= 8;
word |= this.buffer.readUInt8();
bits += 8;
bits = this.decodeHuffmanWord(word, bits, out);
lastKey = word >> bits;
word &= (1 << bits) - 1;
}
assert(this._huffmanNode === huffman, '8-bit EOS');
assert(word + 1 === (1 << bits), 'Final sequence is not EOS');
this._huffmanNode = null;
return out;
};

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