81 lines
2.2 KiB
JavaScript
81 lines
2.2 KiB
JavaScript
import { PLAINTEXT, LANGUAGES } from "./languages.js";
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/**
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* Data options are stored in a binary header before the payload.
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*
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* Header format:
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*
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* Byte 1
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* |xxx----y|
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* x = amount of extra bytes (other than this byte) in the header (uint)
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* y = highest bit of language ID (the rest in byte 2)
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*
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* Byte 2 (+ lowest bit of byte 1) is language ID (uint), i.e. index of the
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* language in the LANGUAGE_NAMES list.
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*
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* ---
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*
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* NOTE: If options are set to their default values, the header is minimised
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* to not include those bytes if possible.
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*/
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export class DataOptions {
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language = PLAINTEXT;
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/**
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* Parse options from uncompressed bytes.
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* @param {Uint8Array} data
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* @returns {Uint8Array} The data without the header.
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*/
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parseFrom(data) {
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const byte1 = data[0];
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const totalBytes = (byte1 & 0b11100000) >>> 5;
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if (totalBytes >= 1) {
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const languageIDLowByte = data[1];
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const languageIDHighBit = (byte1 & 0b00000001);
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const languageID = (languageIDHighBit << 8) | languageIDLowByte;
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if (LANGUAGES.has(languageID)) {
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this.language = LANGUAGES.get(languageID);
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} else {
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this.language = PLAINTEXT;
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}
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} else {
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this.language = PLAINTEXT;
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}
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return data.subarray(totalBytes + 1);
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}
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/**
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* Serialize options to uncompressed bytes.
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* @param {Uint8Array} data
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* @returns {Uint8Array} Data with the options in a header.
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*/
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serializeTo(data) {
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let byte1LowBit = null;
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const extra_bytes = [];
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if (this.language !== PLAINTEXT) {
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const languageID = LANGUAGES.get(this.language);
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const languageIDLowByte = languageID & 0b011111111;
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const languageIDHighBit = languageID & 0b100000000;
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byte1LowBit = languageIDHighBit >>> 8;
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extra_bytes.unshift(languageIDLowByte);
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}
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let byte1 = (extra_bytes.length & 0b00000111) << 5;
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if (byte1LowBit !== null) {
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byte1 |= byte1LowBit;
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}
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const headerBytes = new Uint8Array([byte1, ...extra_bytes]);
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const combined = new Uint8Array(1 + extra_bytes.length + data.length);
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combined.set(headerBytes, 0);
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combined.set(data, headerBytes.length);
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return combined;
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}
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}
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