class RuboCop::Cop::Style::TrailingUnderscoreVariable

def allow_named_underscore_variables
  @allow_named_underscore_variables ||=
    cop_config['AllowNamedUnderscoreVariables']
end

def autocorrect(node)
  ranges = unneeded_ranges(node)
  lambda do |corrector|
    ranges.each { |range| corrector.remove(range) if range }
  end
end

def children_offenses(variables)
  variables.select(&:mlhs_type?).flat_map { |v| unneeded_ranges(v) }
end

def find_first_offense(variables)
  first_offense = find_first_possible_offense(variables.reverse)
  return unless first_offense
  return if splat_variable_before?(first_offense, variables)
  first_offense
end

def find_first_possible_offense(variables)
  variables.reduce(nil) do |offense, variable|
    break offense unless %i[lvasgn splat].include?(variable.type)
    var, = *variable
    var, = *var
    if allow_named_underscore_variables
      break offense unless var == :_
    else
      break offense unless var.to_s.start_with?(UNDERSCORE)
    end
    variable
  end
end

def main_node_offense(node)
  node.masgn_type? ? (mlhs_node, right = *node) : mlhs_node = node
  variables = *mlhs_node
  first_offense = find_first_offense(variables)
  return unless first_offense
  if unused_variables_only?(first_offense, variables)
    return unused_range(node.type, mlhs_node, right)
  end
  if Util.parentheses?(mlhs_node)
    return range_for_parentheses(first_offense, mlhs_node)
  end
  range_between(first_offense.source_range.begin_pos,
                node.loc.operator.begin_pos)
end

def on_masgn(node)
  ranges = unneeded_ranges(node)
  ranges.each do |range|
    good_code = node.source
    offset = range.begin_pos - node.source_range.begin_pos
    good_code[offset, range.size] = ''
    add_offense(node,
                location: range,
                message: format(MSG, code: good_code))
  end
end

def range_for_parentheses(offense, left)
  range_between(
    offense.source_range.begin_pos - 1,
    left.loc.expression.end_pos - 1
  )
end

def reverse_index(collection, item)
  collection.size - 1 - collection.reverse.index(item)
end

def splat_variable_before?(first_offense, variables)
  # Account for cases like `_, *rest, _`, where we would otherwise get
  # the index of the first underscore.
  first_offense_index = reverse_index(variables, first_offense)
  variables[0...first_offense_index].any?(&:splat_type?)
end

def unneeded_ranges(node)
  node.masgn_type? ? (mlhs_node, = *node) : mlhs_node = node
  variables = *mlhs_node
  main_offense = main_node_offense(node)
  if main_offense.nil?
    children_offenses(variables)
  else
    children_offenses(variables) << main_offense
  end
end

def unused_range(node_type, mlhs_node, right)
  start_range = mlhs_node.source_range.begin_pos
  end_range = case node_type
              when :masgn
                right.source_range.begin_pos
              when :mlhs
                mlhs_node.source_range.end_pos
              end
  range_between(start_range, end_range)
end

def unused_variables_only?(offense, variables)
  offense.source_range == variables.first.source_range
end