From: "Shot (Piotr Szotkowski)" <shot@...>
Date: 2009-09-19T19:04:52+09:00
Subject: Re: How can I cache param-based Enumerator call results?

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Shot (Piotr Szotkowski):

> I=E2=80=99m trying to cache a lazy-evaluated, param-based Enumerator call=
s.

> Considering the following code:


> def arith a, b
>   Enumerator.new do |yielder|
>     ['+', '*'].each do |oper|
>       sleep 1 # simulates a param-based, long-running process
>       yielder.yield [oper, eval("#{a} #{oper} #{b}")]
>     end
>   end
> end

> [[1,2], [3,4], [5,6], [1,2], [3,4], [5,6], [7,8]].each do |a, b|
>   arith(a,b).each do |oper, result|=20
>     puts "#{a} #{oper} #{b} =3D #{result}"
>   end
> end


> should take about 14 seconds to run, with arith() lazily returning the
> sum and product of its params. Still, if you look closely, arith(1,2),
> arith(3,4) and arith(5,6) are called twice; if there was caching of
> the yielded values there would be no need to recompute these sums
> and products (and the whole process would take around eight seconds
> instead).

I ended up doing the below, which nicely caches the stuff outside the
Enumerator. Any improvements (I have a feeling that the each=E2=80=A6puts=
=E2=80=A6end
blocks can be DRY-ed up by factoring them out) most welcome, of course!


def arith a, b
  Enumerator.new do |yielder|
    ['+', '*'].each do |oper|
      sleep 1 # simulates a param-based, long-running process
      yielder.yield [oper, eval("#{a} #{oper} #{b}")]
    end
  end
end

cache =3D {}
[[1,2], [3,4], [5,6], [1,2], [3,4], [5,6], [7,8]].each do |a, b|
  if cache[[a,b]]
    cache[[a,b]].each do |oper, result|
      puts "#{a} #{oper} #{b} =3D #{result}"
    end
  else
    cache[[a,b]] =3D []
    arith(a,b).each do |oper, result|
      puts "#{a} #{oper} #{b} =3D #{result}"
      cache[[a,b]] << [oper, result]
    end
  end
end


=E2=80=94 Shot
--=20
This sentence contradicts itself =E2=80=93 no actually
it doesn=E2=80=99t.               [Douglas Hofstadter]

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