class SourceMap::Map

def build_vlq_string
  source_id        = 0
  source_line      = 1
  source_column    = 0
  name_id          = 0
  by_lines = @mappings.group_by { |m| m.generated.line }
  sources_index = Hash[sources.each_with_index.to_a]
  names_index   = Hash[names.each_with_index.to_a]
  ary = (1..(by_lines.keys.max || 1)).map do |line|
    generated_column = 0
    (by_lines[line] || []).map do |mapping|
      group = []
      group << mapping.generated.column - generated_column
      group << sources_index[mapping.source] - source_id
      group << mapping.original.line - source_line
      group << mapping.original.column - source_column
      group << names_index[mapping.name] - name_id if mapping.name
      generated_column = mapping.generated.column
      source_id        = sources_index[mapping.source]
      source_line      = mapping.original.line
      source_column    = mapping.original.column
      name_id          = names_index[mapping.name] if mapping.name
      group
    end
  end
  VLQ.encode_mappings(ary)
end