fast-png/src/helpers/text.ts

PNG image decoder and encoder written entirely in JavaScript

import type { IOBuffer } from 'iobuffer';

import { writeCrc } from './crc';

// https://www.w3.org/TR/png/#11tEXt

export const textChunkName = 'tEXt';

const NULL = 0;

const latin1Decoder = new TextDecoder('latin1');

function validateKeyword(keyword: string) {
  validateLatin1(keyword);
  if (keyword.length === 0 || keyword.length > 79) {
    throw new Error('keyword length must be between 1 and 79');
  }
}

// eslint-disable-next-line no-control-regex
const latin1Regex = /^[\u0000-\u00FF]*$/;
function validateLatin1(text: string) {
  if (!latin1Regex.test(text)) {
    throw new Error('invalid latin1 text');
  }
}

export function decodetEXt(
  text: Record<string, string>,
  buffer: IOBuffer,
  length: number,
) {
  const keyword = readKeyword(buffer);
  text[keyword] = readLatin1(buffer, length - keyword.length - 1);
}

export function encodetEXt(buffer: IOBuffer, keyword: string, text: string) {
  validateKeyword(keyword);
  validateLatin1(text);
  const length = keyword.length + 1 /* NULL */ + text.length;

  buffer.writeUint32(length);
  buffer.writeChars(textChunkName);
  buffer.writeChars(keyword);
  buffer.writeByte(NULL);
  buffer.writeChars(text);
  writeCrc(buffer, length + 4);
}

// https://www.w3.org/TR/png/#11keywords
export function readKeyword(buffer: IOBuffer): string {
  buffer.mark();
  while (buffer.readByte() !== NULL) {
    /* advance */
  }
  const end = buffer.offset;
  buffer.reset();
  const keyword = latin1Decoder.decode(
    buffer.readBytes(end - buffer.offset - 1),
  );
  // NULL
  buffer.skip(1);

  validateKeyword(keyword);

  return keyword;
}

export function readLatin1(buffer: IOBuffer, length: number): string {
  return latin1Decoder.decode(buffer.readBytes(length));
}