// 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);

}