144 lines
5.6 KiB
JavaScript
144 lines
5.6 KiB
JavaScript
"use strict";
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Object.defineProperty(exports, "__esModule", { value: true });
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const { pow, floor } = Math;
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const TWO_POW_7 = pow(2, 7);
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const TWO_POW_14 = pow(2, 14);
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const TWO_POW_21 = pow(2, 21);
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const TWO_POW_28 = pow(2, 28);
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const TWO_POW_35 = pow(2, 35);
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const TWO_POW_42 = pow(2, 42);
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const TWO_POW_49 = pow(2, 49);
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const TWO_POW_56 = pow(2, 56);
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/**
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* This class provides encoding and decoding methods for writing and reading
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* ZigZag-encoded LEB128-64b9B-variant (Little Endian Base 128) values to/from a
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* {@link ByteBuffer}. LEB128's variable length encoding provides for using a
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* smaller nuber of bytes for smaller values, and the use of ZigZag encoding
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* allows small (closer to zero) negative values to use fewer bytes. Details
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* on both LEB128 and ZigZag can be readily found elsewhere.
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*
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* The LEB128-64b9B-variant encoding used here diverges from the "original"
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* LEB128 as it extends to 64 bit values: In the original LEB128, a 64 bit
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* value can take up to 10 bytes in the stream, where this variant's encoding
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* of a 64 bit values will max out at 9 bytes.
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*
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* As such, this encoder/decoder should NOT be used for encoding or decoding
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* "standard" LEB128 formats (e.g. Google Protocol Buffers).
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*/
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class ZigZagEncoding {
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/**
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* Writes a long value to the given buffer in LEB128 ZigZag encoded format
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* (negative numbers not supported)
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* @param buffer the buffer to write to
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* @param value the value to write to the buffer
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*/
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static encode(buffer, value) {
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if (value >= 0) {
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value = value * 2;
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}
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else {
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value = -value * 2 - 1;
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}
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if (value < TWO_POW_7) {
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buffer.put(value);
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}
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else {
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buffer.put(value | 0x80);
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if (value < TWO_POW_14) {
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buffer.put(floor(value / TWO_POW_7));
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}
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else {
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buffer.put(floor(value / TWO_POW_7) | 0x80);
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if (value < TWO_POW_21) {
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buffer.put(floor(value / TWO_POW_14));
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}
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else {
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buffer.put(floor(value / TWO_POW_14) | 0x80);
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if (value < TWO_POW_28) {
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buffer.put(floor(value / TWO_POW_21));
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}
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else {
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buffer.put(floor(value / TWO_POW_21) | 0x80);
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if (value < TWO_POW_35) {
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buffer.put(floor(value / TWO_POW_28));
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}
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else {
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buffer.put(floor(value / TWO_POW_28) | 0x80);
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if (value < TWO_POW_42) {
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buffer.put(floor(value / TWO_POW_35));
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}
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else {
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buffer.put(floor(value / TWO_POW_35) | 0x80);
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if (value < TWO_POW_49) {
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buffer.put(floor(value / TWO_POW_42));
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}
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else {
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buffer.put(floor(value / TWO_POW_42) | 0x80);
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if (value < TWO_POW_56) {
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buffer.put(floor(value / TWO_POW_49));
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}
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else {
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// should not happen
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buffer.put(floor(value / TWO_POW_49) + 0x80);
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buffer.put(floor(value / TWO_POW_56));
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}
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}
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}
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}
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}
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}
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}
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}
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}
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/**
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* Read an LEB128-64b9B ZigZag encoded long value from the given buffer
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* (negative numbers not supported)
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* @param buffer the buffer to read from
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* @return the value read from the buffer
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*/
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static decode(buffer) {
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let v = buffer.get();
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let value = v & 0x7f;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_7;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_14;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_21;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_28;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_35;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_42;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_49;
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if ((v & 0x80) != 0) {
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v = buffer.get();
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value += (v & 0x7f) * TWO_POW_56;
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}
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}
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}
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}
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}
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}
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}
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}
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if (value % 2 === 0) {
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value = value / 2;
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}
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else {
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value = -(value + 1) / 2;
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}
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return value;
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}
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}
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exports.default = ZigZagEncoding;
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//# sourceMappingURL=ZigZagEncoding.js.map
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