src/main/data/bai.js
function computeIndexChunks(buffer) {
  var BLOCK_SIZE = 65536;
  var view = new jDataView(buffer, 0, buffer.byteLength, true /* little endian */);

  var minBlockIndex = Infinity;
  var contigStartOffsets = [];
  view.getInt32();  // magic
  var n_ref = view.getInt32();
  for (var j = 0; j < n_ref; j++) {
    contigStartOffsets.push(view.tell());
    var n_bin = view.getInt32();
    for (var i = 0; i < n_bin; i++) {
      view.getUint32();  // bin ID
      var n_chunk = view.getInt32();
      view.skip(n_chunk * 16);
    }
    var n_intv = view.getInt32();
    if (n_intv) {
      var offset = VirtualOffset.fromBlob(view.getBytes(8), 0),
          coffset = offset.coffset + (offset.uoffset ? BLOCK_SIZE : 0);
      minBlockIndex = coffset ? Math.min(coffset, minBlockIndex) : BLOCK_SIZE;
      view.skip((n_intv - 1) * 8);
    }
  }
  contigStartOffsets.push(view.tell());

  // At this point, `minBlockIndex` should be non-Infinity (see #405 & #406)
  return {
    chunks: _.zip(_.initial(contigStartOffsets), _.rest(contigStartOffsets)),
    minBlockIndex
  };
}
function computeIndexChunks(buffer) {
  var BLOCK_SIZE = 65536;
  var view = new jDataView(buffer, 0, buffer.byteLength, true /* little endian */);

  var minBlockIndex = Infinity;
  var contigStartOffsets = [];
  view.getInt32();  // magic
  var n_ref = view.getInt32();
  for (var j = 0; j < n_ref; j++) {
    contigStartOffsets.push(view.tell());
    var n_bin = view.getInt32();
    for (var i = 0; i < n_bin; i++) {
      view.getUint32();  // bin ID
      var n_chunk = view.getInt32();
      view.skip(n_chunk * 16);
    }
    var n_intv = view.getInt32();
    if (n_intv) {
      var offset = VirtualOffset.fromBlob(view.getBytes(8), 0),
          coffset = offset.coffset + (offset.uoffset ? BLOCK_SIZE : 0);
      minBlockIndex = coffset ? Math.min(coffset, minBlockIndex) : BLOCK_SIZE;
      view.skip((n_intv - 1) * 8);
    }
  }
  contigStartOffsets.push(view.tell());

  // At this point, `minBlockIndex` should be non-Infinity (see #405 & #406)
  return {
    chunks: _.zip(_.initial(contigStartOffsets), _.rest(contigStartOffsets)),
    minBlockIndex
  };
}
In the event that index chunks aren't available from an external source, it winds up saving time to do a fast pass over the data to compute them. This allows us to parse a single contig at a time using jBinary.