class Kramdown::Utils::StringScanner

start_line_number override for nested StringScanners.
This patched StringScanner adds line number information for current scan position and a

def current_line_number

However we add the one indirectly by using a one-based start_line_number.
NOTE: Normally we'd have to add one to the count of newlines to get the correct line number.

NOTE: Requires that all line endings are normalized to '\n'

Returns the line number for current charpos. 
def current_line_number
  # Not using string[@previous_pos..best_pos].count('\n') because it is slower
  strscan = ::StringScanner.new(string)
  strscan.pos = @previous_pos
  old_pos = pos + 1
  @previous_line_number += 1 while strscan.skip_until(/\n/) && strscan.pos <= old_pos
  @previous_pos = (eos? ? pos : pos + 1)
  @previous_line_number
end

def initialize(string, start_line_number = 1)

Note: The original second argument is no longer used so this should be safe.

Takes the start line number as optional second argument. 
def initialize(string, start_line_number = 1)
  super(string)
  @start_line_number = start_line_number || 1
  @previous_pos = 0
  @previous_line_number = @start_line_number
end

def pos=(pos)

and don't use any other method for that!
Note: This also resets some internal variables, so always use pos= when setting the position

Sets the byte position of the scan pointer. 
def pos=(pos)
  if self.pos > pos
    @previous_line_number = @start_line_number
    @previous_pos = 0
  end
  super
end

def revert_pos(data)

Revert the position to one saved by #save_pos. 
def revert_pos(data)
  self.pos = data[0]
  @previous_pos, @previous_line_number = data[1], data[2]
end

def save_pos

Note: Just saving #pos won't be enough.

The returned data can be fed to #revert_pos to revert the position to the saved one.

way.
Return information needed to revert the byte position of the string scanner in a performant 
def save_pos
  [pos, @previous_pos, @previous_line_number]
end