case..when can be modified for performance RB-PR1002

Reordering when conditions with a splat to the end of the when branches can improve performance.

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 should not matter. By placing any splat expansions at the end of the list of when branches we will reduce the number of times that memory has to be allocated for the expansion. The exception to this is if multiple when conditions can be true for any given condition. A likely scenario for this is defining a higher level when condition to override a condition that is inside of the splat 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.

Bad practice

case foo
when *condition
  bar
when baz
  foobar
end

Recommended

case foo
when baz
  foobar
when *condition
  bar
end

Similarly,

Bad practice

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

Recommended

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