class RuboCop::Cop::Performance::Casecmp

‘abc’.casecmp(str).zero?
str.casecmp(‘ABC’).zero?
# good
str.downcase == str.downcase
’ABC’.eql? str.upcase
’abc’ == str.downcase
str.upcase.eql? ‘ABC’
str.downcase == ‘abc’
# bad
@example
can better be implemented using ‘casecmp`.
This cop identifies places where a case-insensitive string comparison

def autocorrect(node) 

def autocorrect(node)
  return unless (parts = take_method_apart(node))
  receiver, method, arg, variable = parts
  correction(node, receiver, method, arg, variable)
end

def build_good_method(arg, variable) 

def build_good_method(arg, variable)
  # We want resulting call to be parenthesized
  # if arg already includes one or more sets of parens, don't add more
  # or if method call already used parens, again, don't add more
  if arg.send_type? || !parentheses?(arg)
    "#{variable.source}.casecmp(#{arg.source}).zero?"
  else
    "#{variable.source}.casecmp#{arg.source}.zero?"
  end
end

def correction(node, _receiver, method, arg, variable) 

def correction(node, _receiver, method, arg, variable)
  lambda do |corrector|
    corrector.insert_before(node.loc.expression, '!') if method == :!=
    replacement = build_good_method(arg, variable)
    corrector.replace(node.loc.expression, replacement)
  end
end

def on_send(node) 

def on_send(node)
  return unless downcase_eq(node) || eq_downcase(node)
  return unless (parts = take_method_apart(node))
  _, _, arg, variable = parts
  good_method = build_good_method(arg, variable)
  add_offense(
    node,
    message: format(MSG, good: good_method, bad: node.source)
  )
end

def take_method_apart(node) 

def take_method_apart(node)
  if downcase_downcase(node)
    receiver, method, rhs = *node
    arg, = *rhs
  elsif downcase_eq(node)
    receiver, method, arg = *node
  elsif eq_downcase(node)
    arg, method, receiver = *node
  else
    return
  end
  variable, = *receiver
  [receiver, method, arg, variable]
end