class Sprockets::Preprocessors::DefaultSourceMap

available.
Because other generators run after might depend on having a valid source map
result. Otherwise it maps 1 for 1 lines original to generated. This is needed
If the input file already has a source map, it effectively returns the original
Private: Adds a default map to assets when one is not present

def call(input) 

def call(input)
  result = { data: input[:data] }
  map    = input[:metadata][:map]
  if map.nil? || map.empty?
    result[:map] ||= []
    input[:data].each_line.with_index do |_, index|
      line = index + 1
      result[:map] << { source: input[:source_path], generated: [line , 0], original: [line, 0] }
    end
  end
  return result
end