formats.exposed/public/index/routeFile.js

// @ts-check

import { SUPPORTED_FILE_TYPES } from "./constants.js";

/**
 * Figure out the route for a given file.
 *
 * For example, PNG files should get routed to `/png`.
 *
 * @param {File} file
 * @returns {Promise<null | string>} The route for the file, or null if the file is not supported.
 */
export default async (file) => {
  const fileType = findFileTypeUsingMimeType(file.type) ??
    fineFileTypeUsingFileName(file.name) ??
    (await findFileTypeWithMimeSniffing(file));
  return fileType ? fileType.route : null;
};

/**
 * @param {string} mimeType
 */
const findFileTypeUsingMimeType = (mimeType) =>
  SUPPORTED_FILE_TYPES.find((t) => t.mimeType === mimeType);

/**
 * @param {string} name
 */
const fineFileTypeUsingFileName = (name) => {
  if (!name.includes(".")) return;
  return SUPPORTED_FILE_TYPES.find((fileType) =>
    fileType.extensions.some((extension) => name.endsWith(extension))
  );
};

/**
 * @returns {number}
 */
const getLongestMimeSniffPattern = () => {
  let result = 0;
  for (const fileType of SUPPORTED_FILE_TYPES) {
    for (const pattern of fileType.mimeSniffPatterns) {
      result = Math.max(result, pattern.bytes.length);
    }
  }
  return result;
};

/**
 * See the ["MIME Sniffing" spec][0].
 * [0]: https://mimesniff.spec.whatwg.org
 * @param {Blob} file
 */
const findFileTypeWithMimeSniffing = async (file) => {
  // We only need to sample the first few bytes.
  const sampleLength = getLongestMimeSniffPattern();
  const sampleAsBlob = file.slice(0, sampleLength);
  const sampleAsBuffer = await sampleAsBlob.arrayBuffer();
  const sample = new Uint8Array(sampleAsBuffer);

  return SUPPORTED_FILE_TYPES.find(({ mimeSniffPatterns }) => (
    mimeSniffPatterns.some(({ bytes, mask }) => {
      // Roughly follows the [pattern matching algorithm from the spec][1].
      // [1]: https://mimesniff.spec.whatwg.org/#pattern-matching-algorithm
      if (sample.length < bytes.length) return false;

      for (let p = 0; p < bytes.length; p++) {
        const maskedData = sample[p] & mask[p];
        if (maskedData !== bytes[p]) return false;
      }

      return true;
    })
  ));
};