From: L7 <jesse.r.brown@...>
Date: 2006-08-19T00:15:05+09:00
Subject: Re: Question on speed


Paul Battley wrote:
> On 18/08/06, L7 <jesse.r.brown@gmail.com> wrote:
> > In the first when, (/not found/, /^[gBbTi]/) is there any benefit in
> > setting up the statement in either of the following ways?
>
> Ruby doesn't compile the separate expressions: it sends ===(subject)
> to each argument in turn. They don't have to be regular expressions,
> after all. Putting the more frequent match first is indeed quicker, as
> expected - but that's an optimisation that can only be made with
> foreknowledge of the data set.
>
> Separating each operation into its own argument is significantly
> slower due to the Ruby method call overhead. Knowing this, however, we
> can derive a further optimisation by combining the two regular
> expressions together: /^[gBbTi]|not found/
>
> Paul.
>
> PS: I did a bit of quick unscientific profiling to check. Here's the code:
>
> data = [
>   "goat",
>   "Badger",
>   "bear",
>   "Tiger",
>   "ibis",
>   "not found",
>   "Start",
>   "Data goes here"
> ] * 10000
>
> t0 = Time.now
> data.each do |line|
>   case line
>   when /not found/, /^[gBbTi]/
>     next
>   end
> end
> p Time.now - t0 # 0.236658
>
> t0 = Time.now
> data.each do |line|
>   case line
>   when /^[gBbTi]/, /not found/
>     next
>   end
> end
> p Time.now - t0 # 0.176375
>
> t0 = Time.now
> data.each do |line|
>   case line
>   when /^[gBbTi]|not found/
>     next
>   end
> end
> p Time.now - t0 # 0.145403
>
> t0 = Time.now
> data.each do |line|
>   case line
>   when /^g/, /^B/, /^b/, /^T/, /^i/, /not found/
>     next
>   end
> end
> p Time.now - t0 # 0.299182


Thank you for the explanation and example (it's plenty 'un'scientific
for my needs).