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

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop tries to produce a complexity score that's a measure of the complexity the reader experiences when looking at a method. For that reason it considers when nodes as something that doesn't add as much complexity as an if or a &&. Except if it's one of those special case/when constructs where there's no expression after case. Then the cop treats it as an if/elsif/elsif... and lets all the when nodes count. In contrast to the CyclomaticComplexity cop, this cop considers else nodes as adding complexity.

Example:

def my_method                   # 1
  if cond                       # 1
    case var                    # 2 (0.8 + 4 * 0.2, rounded)
    when 1 then func_one
    when 2 then func_two
    when 3 then func_three
    when 4..10 then func_other
    end
  else                          # 1
    do_something until a && b   # 2
  end                           # ===
end                             # 7 complexity points

This cop checks if the length of a method exceeds some maximum value. Comment lines can optionally be ignored. The maximum allowed length is configurable.

This cop checks that the cyclomatic complexity of methods is not higher than the configured maximum. The cyclomatic complexity is the number of linearly independent paths through a method. The algorithm counts decision points and adds one.

An if statement (or unless or ?:) increases the complexity by one. An else branch does not, since it doesn't add a decision point. The && operator (or keyword and) can be converted to a nested if statement, and ||/or is shorthand for a sequence of ifs, so they also add one. Loops can be said to have an exit condition, so they add one.

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 checks that include, extend and prepend exists at the top level. Using these at the top level affects the behavior of Object. There will not be using include, extend and prepend at the top level. Let's use it inside class or module.

Example:

# bad
include M

class C
end

# bad
extend M

class C
end

# bad
prepend M

class C
end

# good
class C
  include M
end

# good
class C
  extend M
end

# good
class C
  prepend M
end

This cop checks that include, extend and prepend exists at the top level. Using these at the top level affects the behavior of Object. There will not be using include, extend and prepend at the top level. Let's use it inside class or module.

Example:

# bad
include M

class C
end

# bad
extend M

class C
end

# bad
prepend M

class C
end

# good
class C
  include M
end

# good
class C
  extend M
end

# good
class C
  prepend M
end

This cop checks that include, extend and prepend exists at the top level. Using these at the top level affects the behavior of Object. There will not be using include, extend and prepend at the top level. Let's use it inside class or module.

Example:

# bad
include M

class C
end

# bad
extend M

class C
end

# bad
prepend M

class C
end

# good
class C
  include M
end

# good
class C
  extend M
end

# good
class C
  prepend M
end

If the else branch of a conditional consists solely of an if node, it can be combined with the else to become an elsif. This helps to keep the nesting level from getting too deep.

Example:

# bad
if condition_a
  action_a
else
  if condition_b
    action_b
  else
    action_c
  end
end

# good
if condition_a
  action_a
elsif condition_b
  action_b
else
  action_c
end