This cop checks that the ABC size of methods is not higher than the configured maximum. The ABC size is based on assignments, branches (method calls), and conditions. See http://c2.com/cgi/wiki?AbcMetric

Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

This cop identifies places where gsub can be replaced by tr or delete.

Example:

# bad
'abc'.gsub('b', 'd')
'abc'.gsub('a', '')
'abc'.gsub(/a/, 'd')
'abc'.gsub!('a', 'd')

# good
'abc'.gsub(/.*/, 'a')
'abc'.gsub(/a+/, 'd')
'abc'.tr('b', 'd')
'a b c'.delete(' ')

This cop checks for redundant access modifiers, including those with no code, those which are repeated, and leading public modifiers in a class or module body. Conditionally-defined methods are considered as always being defined, and thus access modifiers guarding such methods are not redundant.

Example:

class Foo
  public # this is redundant (default access is public)

  def method
  end

  private # this is not redundant (a method is defined)
  def method2
  end

  private # this is redundant (no following methods are defined)
end

Example:

class Foo
  # The following is not redundant (conditionally defined methods are
  # considered as always defining a method)
  private

  if condition?
    def method
    end
  end

  protected # this is not redundant (method is defined)

  define_method(:method2) do
  end

  protected # this is redundant (repeated from previous modifier)

  [1,2,3].each do |i|
    define_method("foo#{i}") do
    end
  end

  # The following is redundant (methods defined on the class'
  # singleton class are not affected by the public modifier)
  public

  def self.method3
  end
end

Example:

# Lint/UselessAccessModifier:
#   ContextCreatingMethods:
#     - concerning
require 'active_support/concern'
class Foo
  concerning :Bar do
    def some_public_method
    end

    private

    def some_private_method
    end
  end

  # this is not redundant because `concerning` created its own context
  private

  def some_other_private_method
  end
end

Example:

# Lint/UselessAccessModifier:
#   MethodCreatingMethods:
#     - delegate
require 'active_support/core_ext/module/delegation'
class Foo
  # this is not redundant because `delegate` creates methods
  private

  delegate :method_a, to: :method_b
end