class ActionView::Template::Handlers::ERB

def find_lineno_offset(compiled, source_lines, highlight, error_lineno)

determine the earliest line that could contain the highlight.
Use the difference between the compiled and source sizes to
The compiled template is likely to be longer than the source.

chance of finding the correct line
Searches in reverse from the backtrace lineno so we have a better
Return the offset between the error lineno and the source lineno. 
def find_lineno_offset(compiled, source_lines, highlight, error_lineno)
  first_index = error_lineno - 1 - compiled.size + source_lines.size
  first_index = 0 if first_index < 0
  last_index = error_lineno - 1
  last_index = source_lines.size - 1 if last_index >= source_lines.size
  last_index.downto(first_index) do |line_index|
    next unless source_lines[line_index].include?(highlight)
    return error_lineno - 1 - line_index
  end
  raise LocationParsingError, "Couldn't find code snippet"
end