// returns pair(character range, line) or null if not found
// if no exact match found, return line above
// nav takes a line and returns -1 (move up), 0 (found) or 1 (move down)
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;
  int direction = 0;
  Pair<LongRange, S> possibleResult = null;
  while ping (min < max) {
    long middle = (min+max)/2;
    long lineStart = raf_findBeginningOfLine(raf, middle, bufSize);
    long lineEnd = raf_findEndOfLine(raf, middle, bufSize);
    ifdef binarySearchForLineInTextFile_debug
      printVars_str("binsearch", +middle, +lineStart, +lineEnd);
    endifdef
    S line = fromUtf8(raf_readFilePart(raf, lineStart, (int) (lineEnd-1-lineStart)));
    direction = nav.get(line);
    possibleResult = pair(LongRange(lineStart, lineEnd), line);
    ifdef binarySearchForLineInTextFile_debug
      S _line = dropTrailingBackslashR(line);
      printVars_str("binsearch", +min, +max, +middle, +lineStart, +lineEnd, line := _line, +direction);
    endifdef
    if (direction == 0) ret possibleResult;
    // asserts are to assure that loop terminates
    if (direction < 0) max = assertLessThan(max, lineStart);
    else min = assertBiggerThan(min, lineEnd);
    ifdef binarySearchForLineInTextFile_debug
      printVars_str("binsearch", +min, +max);
    endifdef
  }
  
  ifdef binarySearchForLineInTextFile_debug
    printVars_str("binsearch ending", +direction);
  endifdef
  if (direction >= 0) ret possibleResult;
  
  long lineStart = raf_findBeginningOfLine(raf, min-1, bufSize);
  S line = fromUtf8(raf_readFilePart(raf, lineStart, (int) (min-1-lineStart)));
  ifdef binarySearchForLineInTextFile_debug
    printVars_str("binsearch end",+lineStart, +line);
  endifdef
  ret pair(LongRange(lineStart, min), line);
}