tools/eu4_han_convert/src/codec.js

import { Buffer } from 'node:buffer';

const UTF8_BOM = Buffer.from([0xef, 0xbb, 0xbf]);
const LOW_BYTE_OFFSET = 14;
const HIGH_BYTE_OFFSET = -9;

const CURRENT_INTERNAL_BYTES = new Set([
  0x00, 0x0a, 0x0d, 0x20, 0x22, 0x24, 0x40, 0x5b, 0x5c, 0x7b,
  0x7d, 0x7e, 0x80, 0xa3, 0xa4, 0xa7, 0xbd, 0x3b, 0x5d, 0x5f,
  0x3d, 0x21,
]);

const LEGACY_INTERNAL_BYTES = new Set([
  0x00, 0x0a, 0x0d, 0x20, 0x22, 0x23, 0x24, 0x2f, 0x3a, 0x3b,
  0x3d, 0x40, 0x5b, 0x5c, 0x5d, 0x5f, 0x7b, 0x7d, 0x7e, 0x80,
  0xa3, 0xa4, 0xa7, 0xbd,
]);

const COMPATIBLE_INTERNAL_BYTES = new Set([
  ...CURRENT_INTERNAL_BYTES,
  ...LEGACY_INTERNAL_BYTES,
]);

const PROFILES = new Map([
  ['compatible', COMPATIBLE_INTERNAL_BYTES],
  ['current', CURRENT_INTERNAL_BYTES],
  ['legacy', LEGACY_INTERNAL_BYTES],
]);

const CP1252_TO_UNICODE = new Map([
  [0x80, 0x20ac], [0x82, 0x201a], [0x83, 0x0192], [0x84, 0x201e],
  [0x85, 0x2026], [0x86, 0x2020], [0x87, 0x2021], [0x88, 0x02c6],
  [0x89, 0x2030], [0x8a, 0x0160], [0x8b, 0x2039], [0x8c, 0x0152],
  [0x8e, 0x017d], [0x91, 0x2018], [0x92, 0x2019], [0x93, 0x201c],
  [0x94, 0x201d], [0x95, 0x2022], [0x96, 0x2013], [0x97, 0x2014],
  [0x98, 0x02dc], [0x99, 0x2122], [0x9a, 0x0161], [0x9b, 0x203a],
  [0x9c, 0x0153], [0x9e, 0x017e], [0x9f, 0x0178],
]);

const UNICODE_TO_CP1252 = new Map(
  [...CP1252_TO_UNICODE].map(([byte, codePoint]) => [codePoint, byte]),
);

function byteToUtf8CodePoint(byte) {
  return CP1252_TO_UNICODE.get(byte) ?? byte;
}

function utf8CodePointToByte(codePoint) {
  return UNICODE_TO_CP1252.get(codePoint) ?? codePoint;
}

function assertByte(value, position) {
  if (!Number.isInteger(value) || value < 0 || value > 0xff) {
    throw new SyntaxError(`Invalid EU4 byte at encoded character ${position}`);
  }
}

export function encodeText(text, options = {}) {
  const profile = options.profile ?? 'compatible';
  const internalBytes = PROFILES.get(profile);
  if (!internalBytes) {
    throw new TypeError(`Unknown encoding profile: ${profile}`);
  }
  let result = '';

  for (const character of text) {
    const codePoint = character.codePointAt(0);
    if (codePoint < 0x100) {
      result += character;
      continue;
    }
    if (codePoint > 0xffff) {
      throw new RangeError(
        `EU4 double-byte encoding only supports BMP characters: U+${codePoint.toString(16).toUpperCase()}`,
      );
    }

    let low = codePoint & 0xff;
    let high = codePoint >>> 8;
    let marker = 0x10;

    if (internalBytes.has(high)) marker += 2;
    if (internalBytes.has(low)) marker += 1;

    if (marker === 0x11 || marker === 0x13) low += LOW_BYTE_OFFSET;
    if (marker === 0x12 || marker === 0x13) high += HIGH_BYTE_OFFSET;

    result += String.fromCodePoint(
      marker,
      byteToUtf8CodePoint(low),
      byteToUtf8CodePoint(high),
    );
  }

  return result;
}

export function decodeText(text, options = {}) {
  const characters = [...text];
  let result = '';

  for (let index = 0; index < characters.length; index += 1) {
    const marker = characters[index].codePointAt(0);
    if (marker < 0x10 || marker > 0x13) {
      result += characters[index];
      continue;
    }
    if (index + 2 >= characters.length) {
      throw new SyntaxError(`Truncated EU4 escape at encoded character ${index}`);
    }

    let low = utf8CodePointToByte(characters[index + 1].codePointAt(0));
    let high = utf8CodePointToByte(characters[index + 2].codePointAt(0));

    assertByte(low, index + 1);
    assertByte(high, index + 2);

    if (marker === 0x11 || marker === 0x13) low -= LOW_BYTE_OFFSET;
    if (marker === 0x12 || marker === 0x13) high -= HIGH_BYTE_OFFSET;

    assertByte(low, index + 1);
    assertByte(high, index + 2);

    const codePoint = (high << 8) | low;
    if (codePoint < 0x100) {
      throw new SyntaxError(`Invalid EU4 escape at encoded character ${index}`);
    }

    result += String.fromCodePoint(codePoint);
    index += 2;
  }

  return result;
}

export function encodeBuffer(text, options = {}) {
  const encoded = Buffer.from(encodeText(text, options), 'utf8');
  return options.bom === false ? encoded : Buffer.concat([UTF8_BOM, encoded]);
}

export function decodeBuffer(buffer, options = {}) {
  const input = Buffer.isBuffer(buffer) ? buffer : Buffer.from(buffer);
  const hasBom = input.subarray(0, UTF8_BOM.length).equals(UTF8_BOM);

  if (options.requireBom !== false && !hasBom) {
    throw new SyntaxError('EU4 UTF-8 localisation file is missing the EF BB BF BOM');
  }

  const content = hasBom ? input.subarray(UTF8_BOM.length) : input;
  const encodedText = new TextDecoder('utf-8', { fatal: true }).decode(content);
  return decodeText(encodedText, options);
}

export function hasUtf8Bom(buffer) {
  const input = Buffer.isBuffer(buffer) ? buffer : Buffer.from(buffer);
  return input.subarray(0, UTF8_BOM.length).equals(UTF8_BOM);
}

export { UTF8_BOM };