// returns pair(character range, line) or null if not found
static Pair<LongRange, S> binarySearchForLineInTextFile(File file, IF1<S, Int> nav) {
  long length = l(file);
  int bufSize = 1024;
  temp RandomAccessFile raf = randomAccessFileForReading(file);
  long min = 0, max = length;
  while ping (min < max) {
    long middle = (min+max)/2;
    long lineStart = raf_findBeginningOfLine(raf, middle, bufSize);
    long lineEnd = raf_findEndOfLine(raf, middle, bufSize);
    S line = fromUtf8(raf_readFilePart(raf, lineStart, lineEnd));
    int direction = nav.get(line);
    if (direction == 0) ret pair(LongRange(lineStart, lineEnd), line);
    if (direction < 0) max = lineStart;
    else min = lineEnd;
  }
  
  // TODO: precise check
  null;
}