From: Florian Frank <flori@...>
Date: 2004-05-04T04:15:10+09:00
Subject: Re: ruby 1.8 differences?

On 2004-05-03 20:23:41 +0900, Jean-Hugues ROBERT wrote:
> I started refactoring my code to use it. But I use a lot of sort!()
> and sort_by! does not exists :-(
> 
> Question : Why no sort_by!() ?

sort_by is a more efficent way of Schwartzian Transform (because it's
implemented in C). So instead of 

ary.map {|x| [f(x), x] }.sort {|(ka,),(kb,)| ka <=> kb }.map { |k,v| v }

you can just write:

ary.sort_by { |x| f(x) }

Temporary arrays are created anyway. That's why it is not useful to have
sort_by!. In general Schwartzian Transform is a stupid idea, especially
for large arrays, but sometimes it's possible to find a function f with
an image that can be used for comparing the values more efficiently than
the values themselves.

Here's a perl-centered paper about a lot of sorting tricks, but most of
it is also true for Ruby: "A Fresh Look at Efficient Perl Sorting",
http://www.sysarch.com/perl/sort_paper.html

-- 
c, s, x = gets, c[0, 2].to_i, "dsfd;kfoA,.iyewrkldJKDHSUB"
puts (1...(c.size / 2)).inject("") do |p, i|
	p << (c[2 * i, 2].to_i(16) ^ x[(s += 1) && s - 1])
end