mirror of
https://github.com/a-sync/game-server-watcher.git
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56 lines
2.6 KiB
JavaScript
56 lines
2.6 KiB
JavaScript
/** Distance model used by gzip/deflate.
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* Encodes distances starting at 0 (for deflate compatibility, subtract
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* one from distance to encode).
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* Uses ~32-entry model to predict ln2(distance) (more-or-less) and then
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* encodes a few more bits for the actual distance. */
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if (typeof define !== 'function') { var define = require('amdefine')(module); }
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define(['./Util'],function(Util){
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// lengthBitsModelFactory will be called with arguments 2, 4, 8, 16, etc
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// and must return an appropriate model or coder.
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var DeflateDistanceModel = function(size, extraStates,
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lgDistanceModelFactory,
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lengthBitsModelFactory) {
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var i;
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var bits = Util.fls(size-1);
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this.extraStates = +extraStates || 0;
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this.lgDistanceModel = lgDistanceModelFactory(2*bits + extraStates);
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// this.distanceModel[n] used for distances which are n-bits long,
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// but only n-2 bits are encoded: the top bit is known to be one,
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// and the next bit is encoded by the lgDistanceModel.
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this.distanceModel = [];
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for (i=3 ; i <= bits; i++) {
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var numBits = i - 2;
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this.distanceModel[i] = lengthBitsModelFactory(1<<numBits);
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}
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};
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/* you can give this model arguments between 0 and (size-1), or else
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a negative argument which is one of the 'extra states'. */
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DeflateDistanceModel.prototype.encode = function(distance) {
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if (distance < 4) { // small distance or an 'extra state'
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this.lgDistanceModel.encode(distance + this.extraStates);
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return;
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}
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var lgDistance = Util.fls(distance);
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console.assert(distance & (1<<(lgDistance-1))); // top bit is set
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console.assert(lgDistance >= 3);
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var nextBit = (distance & (1 << (lgDistance-2))) ? 1 : 0;
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var l = 4 + ((lgDistance-3)*2) + nextBit;
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this.lgDistanceModel.encode(l + this.extraStates);
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// now encode the rest of the bits.
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var rest = distance & ((1 << (lgDistance-2)) - 1);
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this.distanceModel[lgDistance].encode(rest);
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};
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DeflateDistanceModel.prototype.decode = function() {
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var l = this.lgDistanceModel.decode() - this.extraStates;
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if (l < 4) {
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return l; // this is a small distance or an 'extra state'
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}
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var nextBit = (l&1);
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var lgDistance = ((l-4) >>> 1) + 3;
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var rest = this.distanceModel[lgDistance].decode();
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return ((2+nextBit) << (lgDistance-2)) + rest;
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};
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return DeflateDistanceModel;
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});
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