Performance-CaseWhenSplat.md

Pattern: Possibly slow when condition with splat

Issue: -

Description

Place when conditions that use splat at the end of the list of when branches.

Ruby has to allocate memory for the splat expansion every time that the case when statement is run. Since Ruby does not support fall through inside of case when, like some other languages do, the order of the when branches does not matter. By placing any splat expansions at the end of the list of when branches you will reduce the number of times that memory has to be allocated for the expansion.

This is not a guaranteed performance improvement. If the data being processed by the case condition is normalized in a manner that favors hitting a condition in the splat expansion, it is possible that moving the splat condition to the end will use more memory, and run slightly slower.

Examples

# 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

Further Reading

• RuboCop - Performance/CaseWhenSplat