t/dataoptions.js

import { PLAINTEXT, LANGUAGES } from "./languages.js";

const UTF8_ENCODER = new TextEncoder();
const UTF8_DECODER = new TextDecoder();

class DataOptionsError extends Error {}
class DecodeError extends DataOptionsError {}

/**
 * Data options are stored in a binary header before the payload.
 *
 * The header consists of a fixed section, followed by a dynamic section.
 * The fixed section consists of bytes with a fixed meaning and positioning.
 * The dynamic section is based on the contents of the fixed section and
 * contains data related to the fixed section.
 *
 * Fixed section:
 *
 * Byte 1
 * |xxx----y|
 * x = amount of extra bytes (other than this byte) in the header's fixed
 *     section (uid)
 * y = highest bit of language ID (the rest in byte 2)
 *
 * Byte 2 (+ lowest bit of byte 1) is language ID (uint), i.e. index of the
 * language in the LANGUAGE_NAMES list.
 *
 * Byte 3 is number of bytes reserved for the page title.
 *
 * Dynamic section parts:
 *
 * 1. Bytes of page title (UTF-8).
 *
 * ---
 *
 * NOTE: If options are set to their default values, the header is minimised
 * to not include those bytes if possible.
 */
export class DataOptions {
  language = PLAINTEXT;
  title = "";

  /**
   * Parse options from uncompressed bytes.
   * @param {Uint8Array} data
   * @returns {Uint8Array} The data without the header.
   */
  parseFrom(data) {
    const byte1 = data[0];
    const fixedHeaderBytes = ((byte1 & 0b11100000) >>> 5) + 1;

    if (fixedHeaderBytes >= 2) {
      const languageIDLowByte = data[1];
      const languageIDHighBit = byte1 & 0b00000001;
      const languageID = (languageIDHighBit << 8) | languageIDLowByte;

      if (LANGUAGES.has(languageID)) {
        this.language = LANGUAGES.get(languageID);
      } else {
        this.language = PLAINTEXT;
      }
    } else {
      this.language = PLAINTEXT;
    }

    let titleLen = 0;

    if (fixedHeaderBytes >= 3) {
      titleLen = data[2];

      if (titleLen > data) {
        throw new DecodeError(
          `Title length ${titleLen} was bigger than chunk size ${data.length}.`
        );
      }

      this.title = UTF8_DECODER.decode(
        data.subarray(fixedHeaderBytes, fixedHeaderBytes + titleLen)
      );
    } else {
      this.title = "";
    }

    return data.subarray(fixedHeaderBytes + titleLen);
  }

  /**
   * Serialize options to uncompressed bytes.
   * @param {Uint8Array} data
   * @returns {Uint8Array} Data with the options in a header.
   */
  serializeTo(data) {
    let byte1LowBit = null;
    const fixedBytes = [];
    const dynamicBytes = [];

    const hasTitleBytes = this.title !== "";
    const hasLanguageBytes = this.language !== PLAINTEXT || hasTitleBytes;

    if (hasLanguageBytes) {
      const languageID = LANGUAGES.get(this.language);
      const languageIDLowByte = languageID & 0b011111111;
      const languageIDHighBit = languageID & 0b100000000;
      byte1LowBit = languageIDHighBit >>> 8;

      fixedBytes.push(languageIDLowByte);
    }

    if (hasTitleBytes) {
      const languageBytes = UTF8_ENCODER.encode(this.title);
      fixedBytes.push(languageBytes.length);
      dynamicBytes.push(...languageBytes);
    }

    let byte1 = (fixedBytes.length & 0b00000111) << 5;
    if (byte1LowBit !== null) {
      byte1 |= byte1LowBit;
    }

    const headerBytes = new Uint8Array([byte1, ...fixedBytes, ...dynamicBytes]);

    const combined = new Uint8Array(headerBytes.length + data.length);
    combined.set(headerBytes, 0);
    combined.set(data, headerBytes.length);

    return combined;
  }
}