Performance

# bad
A.ancestors.include?(B)

# good
A <= B

# bad
array[..2]
array[...2]
array[2..]
array[2...]
array.slice(..2)

# good
array.take(3)
array.take(2)
array.drop(2)
array.drop(2)
array.take(3)

# bad
BigDecimal(1, 2)
BigDecimal(1.2, 3, exception: true)

# good
BigDecimal('1', 2)
BigDecimal('1.2', 3, exception: true)

# bad
umethod.bind(obj).call(foo, bar)
umethod.bind(obj).(foo, bar)

# good
umethod.bind_call(obj, foo, bar)

# bad
def method(&block)
  do_something if block_given?
end

# good
def method(&block)
  do_something if block
end

# good - block is reassigned
def method(&block)
  block ||= -> { do_something }
  warn "Using default ..." unless block_given?
  # ...
end

# bad
caller[1]
caller.first
caller_locations[1]
caller_locations.first

# good
caller(2..2).first
caller(1..1).first
caller_locations(2..2).first
caller_locations(1..1).first

# bad
case foo
when *condition
  bar
when baz
  foobar
end

case foo
when *[1, 2, 3, 4]
  bar
when 5
  baz
end

# good
case foo
when baz
  foobar
when *condition
  bar
end

case foo
when 1, 2, 3, 4
  bar
when 5
  baz
end

# bad
str.downcase == 'abc'
str.upcase.eql? 'ABC'
'abc' == str.downcase
'ABC'.eql? str.upcase
str.downcase == str.downcase

# good
str.casecmp('ABC').zero?
'abc'.casecmp(str).zero?

# bad
array = ["a", "b", "c"]
array.compact.flatten.map { |x| x.downcase }

# good.
array = ["a", "b", "c"]
array.compact!
array.flatten!
array.map! { |x| x.downcase }
array

# bad
users.select do |user|
  %i[superadmin admin].include?(user.role)
end

# good
admin_roles = %i[superadmin admin]
users.select do |user|
  admin_roles.include?(user.role)
end

# good
ADMIN_ROLES = %i[superadmin admin]
...
users.select do |user|
  ADMIN_ROLES.include?(user.role)
end

# bad
array.sort { |a, b| a.foo <=> b.foo }
array.max { |a, b| a.foo <=> b.foo }
array.min { |a, b| a.foo <=> b.foo }
array.sort { |a, b| a[:foo] <=> b[:foo] }

# good
array.sort_by(&:foo)
array.sort_by { |v| v.foo }
array.sort_by do |var|
  var.foo
end
array.max_by(&:foo)
array.min_by(&:foo)
array.sort_by { |a| a[:foo] }

# bad
Concurrent.monotonic_time

# good
Process.clock_gettime(Process::CLOCK_MONOTONIC)

# bad
def tokens(pattern)
  pattern.scan(TOKEN).reject { |token| token.match?(/\A#{SEPARATORS}\Z/) }
end

# good
ALL_SEPARATORS = /\A#{SEPARATORS}\Z/
def tokens(pattern)
  pattern.scan(TOKEN).reject { |token| token.match?(ALL_SEPARATORS) }
end

# good
def tokens(pattern)
  pattern.scan(TOKEN).reject { |token| token.match?(/\A#{SEPARATORS}\Z/o) }
end

# good
def separators
  @separators ||= /\A#{SEPARATORS}\Z/
end

`Model.where(id: [1, 2, 3]).select { |m| m.method == true }.size`

`Model.where(id: [1, 2, 3]).to_a.count { |m| m.method == true }`

# bad
[1, 2, 3].select { |e| e > 2 }.size
[1, 2, 3].reject { |e| e > 2 }.size
[1, 2, 3].select { |e| e > 2 }.length
[1, 2, 3].reject { |e| e > 2 }.length
[1, 2, 3].select { |e| e > 2 }.count { |e| e.odd? }
[1, 2, 3].reject { |e| e > 2 }.count { |e| e.even? }
array.select(&:value).count

# good
[1, 2, 3].count { |e| e > 2 }
[1, 2, 3].count { |e| e < 2 }
[1, 2, 3].count { |e| e > 2 && e.odd? }
[1, 2, 3].count { |e| e < 2 && e.even? }
Model.select('field AS field_one').count
Model.select(:value).count

# bad
str.gsub(/\Aprefix/, '')
str.gsub!(/\Aprefix/, '')

str.sub(/\Aprefix/, '')
str.sub!(/\Aprefix/, '')

# good
str.delete_prefix('prefix')
str.delete_prefix!('prefix')

# bad
str.gsub(/suffix\z/, '')
str.gsub!(/suffix\z/, '')

str.sub(/suffix\z/, '')
str.sub!(/suffix\z/, '')

# good
str.delete_suffix('suffix')
str.delete_suffix!('suffix')

# bad
[].select { |item| true }.first
[].select { |item| true }.last
[].find_all { |item| true }.first
[].find_all { |item| true }.last
[].filter { |item| true }.first
[].filter { |item| true }.last
[].filter { |item| true }[0]
[].filter { |item| true }[-1]

# good
[].detect { |item| true }
[].reverse.detect { |item| true }

# bad
str.start_with?("a") || str.start_with?(Some::CONST)
str.start_with?("a", "b") || str.start_with?("c")
str.end_with?(var1) || str.end_with?(var2)

# good
str.start_with?("a", Some::CONST)
str.start_with?("a", "b", "c")
str.end_with?(var1, var2)

# bad
'abc'.match?(/bc\Z/)
/bc\Z/.match?('abc')
'abc' =~ /bc\Z/
/bc\Z/ =~ 'abc'
'abc'.match(/bc\Z/)
/bc\Z/.match('abc')

# good
'abc'.end_with?('bc')

# String methods
# bad
'foo'.size
%q[bar].count
%(qux).length

# Symbol methods
# bad
:fred.size
:'baz'.length

# Array methods
# bad
[1, 2, thud].count
%W(1, 2, bar).size

# Hash methods
# bad
{ a: corge, b: grault }.length

# good
foo.size
bar.count
qux.length

# good
:"#{fred}".size
CONST = :baz.length

# good
[1, 2, *thud].count
garply = [1, 2, 3]
garply.size

# good
{ a: corge, **grault }.length
waldo = { a: corge, b: grault }
waldo.size

# bad
[1, 2, 3, 4].map { |e| [e, e] }.flatten(1)
[1, 2, 3, 4].collect { |e| [e, e] }.flatten(1)

# good
[1, 2, 3, 4].flat_map { |e| [e, e] }
[1, 2, 3, 4].map { |e| [e, e] }.flatten
[1, 2, 3, 4].collect { |e| [e, e] }.flatten

# bad
{ a: 1, b: 2 }.keys.include?(:a)
{ a: 1, b: 2 }.keys.include?(:z)
h = { a: 1, b: 2 }; h.keys.include?(100)

# good
{ a: 1, b: 2 }.key?(:a)
{ a: 1, b: 2 }.has_key?(:z)
h = { a: 1, b: 2 }; h.key?(100)

# bad
{ a: 1, b: 2 }.values.include?(2)
{ a: 1, b: 2 }.values.include?('garbage')
h = { a: 1, b: 2 }; h.values.include?(nil)

# good
{ a: 1, b: 2 }.value?(2)
{ a: 1, b: 2 }.has_value?('garbage')
h = { a: 1, b: 2 }; h.value?(nil)

# bad
File.readlines('testfile').each { |l| puts l }
IO.readlines('testfile', chomp: true).each { |l| puts l }

conn.readlines(10).map { |l| l.size }
file.readlines.find { |l| l.start_with?('#') }
file.readlines.each { |l| puts l }

# good
File.open('testfile', 'r').each_line { |l| puts l }
IO.open('testfile').each_line(chomp: true) { |l| puts l }

conn.each_line(10).map { |l| l.size }
file.each_line.find { |l| l.start_with?('#') }
file.each_line { |l| puts l }

# bad
ary.map(&:foo).compact
ary.collect(&:foo).compact

# good
ary.filter_map(&:foo)
ary.map(&:foo).compact!
ary.compact.map(&:foo)

# bad
array.map(&method(:do_something))
[1, 2, 3].each(&out.method(:puts))

# good
array.map { |x| do_something(x) }
[1, 2, 3].each { |x| out.puts(x) }

# bad
class MyClass
  def my_method
    OpenStruct.new(my_key1: 'my_value1', my_key2: 'my_value2')
  end
end

# good
class MyClass
  MyStruct = Struct.new(:my_key1, :my_key2)
  def my_method
    MyStruct.new('my_value1', 'my_value2')
  end
end

# bad
('a'..'z').include?('b') # => true
('a'..'z').member?('b')  # => true

# good
('a'..'z').cover?('b') # => true

# Example of a case where `Range#cover?` may not provide
# the desired result:

('a'..'z').cover?('yellow') # => true

# bad
def method(&block)
  block.call
end
def another(&func)
  func.call 1, 2, 3
end

# good
def method
  yield
end
def another
  yield 1, 2, 3
end

# bad
items.all? { |item| pattern === item }
items.all? { |item| item == other }
items.all? { |item| item.is_a?(Klass) }
items.all? { |item| item.kind_of?(Klass) }

# good
items.all?(pattern)

# bad
do_something if str.match(/regex/)
while regex.match('str')
  do_something
end

# good
method(str =~ /regex/)
return value unless regex =~ 'str'

# bad
hash.merge!(a: 1)
hash.merge!({'key' => 'value'})

# good
hash[:a] = 1
hash['key'] = 'value'

# bad
array.sort { |a, b| a <=> b }

# good
array.sort

# bad
'a,b,c'.split(/,/)

# good
'a,b,c'.split(',')

# bad
str.chars[0..2]
str.chars.slice(0..2)

# good
str[0..2].chars

# bad
str.chars.first
str.chars.first(2)

# good
str[0]
str[0...2].chars

# bad
str.chars.take(2)
str.chars.length
str.chars.size
str.chars.empty?

# good
str[0...2].chars
str.length
str.size
str.empty?

# For example, if the receiver is a blank string, it will be incompatible.
# If a negative value is specified for the receiver, `nil` is returned.
str.chars.last    # Incompatible with `str[-1]`.
str.chars.last(2) # Incompatible with `str[-2..-1].chars`.
str.chars.drop(2) # Incompatible with `str[2..-1].chars`.

# bad
def foo
  if x =~ /re/
    do_something
  end
end

# bad
def foo
  if x !~ /re/
    do_something
  end
end

# bad
def foo
  if x.match(/re/)
    do_something
  end
end

# bad
def foo
  if /re/ === x
    do_something
  end
end

# good
def foo
  if x.match?(/re/)
    do_something
  end
end

# good
def foo
  if !x.match?(/re/)
    do_something
  end
end

# good
def foo
  if x =~ /re/
    do_something(Regexp.last_match)
  end
end

# good
def foo
  if x.match(/re/)
    do_something($~)
  end
end

# good
def foo
  if /re/ === x
    do_something($~)
  end
end

# bad
items.reverse.each

# good
items.reverse_each

# bad
array.reverse.first(5)
array.reverse.first

# good
array.last(5).reverse
array.last

# bad
ary.select(&:foo).map(&:bar)
ary.filter(&:foo).map(&:bar)

# good
ary.filter_map { |o| o.bar if o.foo }

# bad
[1, 2, 3].count
(1..3).to_a.count
Array[*1..3].count
Array(1..3).count

# bad
{a: 1, b: 2, c: 3}.count
[[:foo, :bar], [1, 2]].to_h.count
Hash[*('a'..'z')].count
Hash(key: :value).count

# good
[1, 2, 3].size
(1..3).to_a.size
Array[*1..3].size
Array(1..3).size

# good
{a: 1, b: 2, c: 3}.size
[[:foo, :bar], [1, 2]].to_h.size
Hash[*('a'..'z')].size
Hash(key: :value).size

# good
[1, 2, 3].count { |e| e > 2 }

# bad
array.sort { |a, b| b <=> a }

# good
array.sort.reverse

# bad
str.gsub(/a+/, 'a')
str.gsub!(/a+/, 'a')

# good
str.squeeze('a')
str.squeeze!('a')

# bad
'abc'.match?(/\Aab/)
/\Aab/.match?('abc')
'abc' =~ /\Aab/
/\Aab/ =~ 'abc'
'abc'.match(/\Aab/)
/\Aab/.match('abc')

# good
'abc'.start_with?('ab')

# bad
'abc'.match?(/ab/)
/ab/.match?('abc')
'abc' =~ /ab/
/ab/ =~ 'abc'
'abc'.match(/ab/)
/ab/.match('abc')

# good
'abc'.include?('ab')

# 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(' ')

# bad
[1, 2, 3].inject(:+)                        # These bad cases with no initial value are unsafe and
[1, 2, 3].inject(&:+)                       # will not be auto-correced by default. If you want to
[1, 2, 3].reduce { |acc, elem| acc + elem } # auto-corrected, you can set `SafeAutoCorrect: false`.
[1, 2, 3].reduce(10, :+)
[1, 2, 3].map { |elem| elem ** 2 }.sum
[1, 2, 3].collect(&:count).sum(10)

# good
[1, 2, 3].sum
[1, 2, 3].sum(10)
[1, 2, 3].sum { |elem| elem ** 2 }
[1, 2, 3].sum(10, &:count)

-1.times{}    # does nothing
Array.new(-1) # ArgumentError: negative array size

# bad
9.times.map do |i|
  i.to_s
end

# good
Array.new(9) do |i|
  i.to_s
end

# bad
''.dup
"something".dup
String.new
String.new('')
String.new('something')

# good
+'something'
+''

# bad
URI::Parser.new

# good
URI::DEFAULT_PARSER