From: Ilmari Heikkinen <kig@...>
Date: 2004-12-28T23:21:18+09:00
Subject: Re: help on making ruby code faster

Hello, here's some precalc to speed up the lots-of-conversions case
It makes the only-few-conversions case slower and by uses some more 
(25*36 numbers) memory. So if you have a shell script that converts a 
single guid, this'll just slow it down.

> ===================================================================
> class Guid
>   @@d36 = ('a'..'z').to_a + ('0'..'9').to_a
>   @@rd36 = {}
>   @@d36.each_with_index {|d, i| @@rd36[d[0]] = i}

   # add these
   @@exp_a = (0..24).to_a.reverse.map{|i| 36**i}
   @@exps = {}
   @@rd36.map{|char,index|
     @@exps[char] = @@exp_a.map{|e| index*e}
   }

>   def Guid.from_base36(val)
>     val = val.downcase
>     raise ArgumentError unless val =~ /\A[a-z][a-z0-9]{24}\z/
>     n = 0

     mult = 0
     val.each_byte {|c|
       n += @@exps[c][mult]
       mult += 1
     }

>     Guid.from_i(n)
>   end

Got around 35% speedup in the 1000 conversions benchmark.

-Ilmari