Function getFileRange

src/lib/libwasmfs_fetch.js:23–108

(file, offset, len)

    // Get a promise that fetches the data and stores it in JS memory (if it has
    // not already been fetched).
    async function getFileRange(file, offset, len) {
      var url = '';
      var fileUrl_p = __wasmfs_fetch_get_file_url(file);
      var fileUrl = UTF8ToString(fileUrl_p);
      var isAbs = fileUrl.indexOf('://') !== -1;
      if (isAbs) {
        url = fileUrl;
      } else {
        try {
          var u = new URL(fileUrl, self.location.origin);
          url = u.toString();
        } catch (_e) {
          throw {status: 404};
        }
      }
      var chunkSize = __wasmfs_fetch_get_chunk_size(file);
      offset ??= 0;
      len ??= chunkSize;
      // In which chunk does the seeked range start?  E.g., 5-14 with chunksize 8 will start in chunk 0.
      if (!(file in wasmFS$JSMemoryRanges)) {
        var fileInfo = await fetch(url, {method:'HEAD', headers:{'Range': 'bytes=0-'}});
        if (fileInfo.ok &&
            fileInfo.headers.has('Content-Length') &&
            fileInfo.headers.get('Accept-Ranges') == 'bytes' &&
            (parseInt(fileInfo.headers.get('Content-Length'), 10) > chunkSize*2)) {
          var size = parseInt(fileInfo.headers.get('Content-Length'), 10);
          wasmFS$JSMemoryRanges[file] = {
            size,
            chunks: [],
            chunkSize: chunkSize
          };
          len = Math.min(len, size-offset);
        } else {
          // may as well/forced to download the whole file
          var wholeFileReq = await fetch(url);
          if (!wholeFileReq.ok) {
            throw wholeFileReq;
          }
          var wholeFileData = new Uint8Array(await wholeFileReq.arrayBuffer());
          wasmFS$JSMemoryRanges[file] = {
            size: wholeFileData.byteLength,
            chunks: [wholeFileData],
            chunkSize: wholeFileData.byteLength
          };
          return;
        }
      }
      var firstChunk = (offset / chunkSize) | 0;
      // In which chunk does the seeked range end?  E.g., 5-14 with chunksize 8 will end in chunk 1, as will 5-16 (since byte 16 isn't requested).
      // This will always give us a chunk >= firstChunk since len > 0.
      var lastChunk = ((offset+len-1) / chunkSize) | 0;
      var allPresent = true;
      var i;
      // Do we have all the chunks already?  If so, we don't need to do any fetches.
      for (i = firstChunk; i <= lastChunk; i++) {
        if (!wasmFS$JSMemoryRanges[file].chunks[i]) {
          allPresent = false;
          break;