From: Jason Creighton <androflux@...>
Date: 2004-07-13T13:07:22+09:00
Subject: Re: A little algorithmic help requested...

On Mon, 12 Jul 2004 15:47:41 +0200, 
"Robert Klemme" <bob.news@gmx.net> wrote:

> 
> "Jason Creighton" <androflux@softhome.net.remove.to.reply> schrieb im
> Newsbeitrag
> news:20040711153514.4b895d41.androflux@softhome.net.remove.to.reply...
> > On Sun, 11 Jul 2004 16:10:34 +0900,
> > Hal Fulton <hal9000@hypermetrics.com> wrote:
> >
> > > Here's a problem my tired brain is having trouble with.
> > >
> > > Given a sorted array of integers, convert them into as many
> > > ranges as possible (ranges of three or more).
> > >
> > > Example:
> > > [1,2,3,4,6,7,8,11,12,15,16,17] ==> [1..4,6..8,11,12,15..17]
> > >
> > > How would *you* do this?
> >
> > Here's one that doesn't follow that "need at least three of more"
> > limitation, but it's how *I* would do it, because it returns an array of
> > *only* ranges, which seems like it would be more fun to deal with that a
> > mix of ranges and numbers.
> >
> > module Enumerable
> >   def to_ranges
> >     ranges = Array.new
> >     self.sort.each do |e|
> >       if ranges[-1] == nil or ranges[-1].end.succ != e
> >         ranges << Range.new(e,e)
> >         next
> >       end
> >       ranges[-1] = Range.new(ranges[-1].begin, e)
> >     end
> >     return ranges
> >   end
> > end
> 
> Nice and short, although I'd use "else" instead of "next"! :-)

Oh, right, that would be easier. :)

> However, there is a performance drawback: you recreate ranges all over
> again.  In the worst case of an array that contains all numbers from 1 to
> 1000 you create 999 Range instances and keep only one of them.  That's not
> efficient.

I realized I was doing this, but I didn't think the performance hit
would be that much. But doing it a nicer was is much faster:

~/prog/rb$ ruby range-problem.rb 
                user     system      total        real
to_ranges   2.340000   0.000000   2.340000 (  2.338682)
to_ranges2  0.410000   0.000000   0.410000 (  0.400007)

range-problem.rb:
#! /usr/bin/ruby -w

# NOTE: to_ranges assumes sorted object.

module Enumerable
  def to_ranges
    ranges = Array.new
    self.each do |e|
      if ranges[-1] == nil or ranges[-1].end.succ != e
        ranges << Range.new(e,e)
      else
        ranges[-1] = Range.new(ranges[-1].begin, e)
      end
    end
    return ranges
  end

  def to_ranges2
    ranges = Array.new
    low, high = nil, nil
    self.each do |e|
      if low == nil
        # First item in collection
        low, high = e,e
      elsif high.succ != e
        # We hit something out of sequence
        ranges << Range.new(low,high)
        low, high = e, e
      else
        high = e
      end
    end
    ranges << Range.new(low,high)
    return ranges
  end
end

if __FILE__ == $0
  require 'benchmark'
  Benchmark.bm(10) do |b|
    b.report("to_ranges") { (1..2**16).to_ranges }
    b.report("to_ranges2") { (1..2**16).to_ranges2 }
  end
end

Jason Creighton