128 lines
3.5 KiB
JavaScript
128 lines
3.5 KiB
JavaScript
import { PLAINTEXT, LANGUAGES } from "./languages.js";
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const UTF8_ENCODER = new TextEncoder();
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const UTF8_DECODER = new TextDecoder();
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class DataOptionsError extends Error {}
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class DecodeError extends DataOptionsError {}
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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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* The header consists of a fixed section, followed by a dynamic section.
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* The fixed section consists of bytes with a fixed meaning and positioning.
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* The dynamic section is based on the contents of the fixed section and
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* contains data related to the fixed section.
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*
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* Fixed section:
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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's fixed
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* section (uid)
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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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* Byte 3 is number of bytes reserved for the page title.
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*
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* Dynamic section parts:
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*
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* 1. Bytes of page title (UTF-8).
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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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title = "";
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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 fixedHeaderBytes = ((byte1 & 0b11100000) >>> 5) + 1;
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if (fixedHeaderBytes >= 2) {
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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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let titleLen = 0;
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if (fixedHeaderBytes >= 3) {
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titleLen = data[2];
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if (titleLen > data) {
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throw new DecodeError(
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`Title length ${titleLen} was bigger than chunk size ${data.length}.`
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);
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}
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this.title = UTF8_DECODER.decode(
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data.subarray(fixedHeaderBytes, fixedHeaderBytes + titleLen)
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);
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} else {
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this.title = "";
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}
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return data.subarray(fixedHeaderBytes + titleLen);
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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 fixedBytes = [];
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const dynamicBytes = [];
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const hasTitleBytes = this.title !== "";
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const hasLanguageBytes = this.language !== PLAINTEXT || hasTitleBytes;
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if (hasLanguageBytes) {
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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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fixedBytes.push(languageIDLowByte);
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}
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if (hasTitleBytes) {
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const languageBytes = UTF8_ENCODER.encode(this.title);
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fixedBytes.push(languageBytes.length);
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dynamicBytes.push(...languageBytes);
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}
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let byte1 = (fixedBytes.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, ...fixedBytes, ...dynamicBytes]);
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const combined = new Uint8Array(headerBytes.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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