From: Ryan Davis <ryand-ruby@...>
Date: 2011-01-20T08:13:33+09:00
Subject: Re: Improving performance of hash math


On Jan 18, 2011, at 21:23 , dblock wrote:

>> Switch to using #each with an explicit accumulator variable instead of #inject.
> 
> Looks about twice as slow. Either way, about 36% of the time is spent
> in Hash.each.

I don't agree with Gary's suggestion, because in this case it is a proper application of inject, but if you think it is 2x slower, then you're benchmarking is probably at fault.

My measurements show each to be roughly equivalent on 1.9 and quite a bit faster on 1.8. That could be explained by my modification to your treatment of rhs or it could also be explained by my fake data.

% ./q.rb 10000
# of iterations = 10000
                          user     system      total        real
null_time             0.000000   0.000000   0.000000 (  0.001118)
euclidian_distance1  18.960000   0.010000  18.970000 ( 19.011762)
euclidian_distance2  13.050000   0.010000  13.060000 ( 13.069581)
euclidian_distance3  13.090000   0.010000  13.100000 ( 13.109778)

against:

def euclidian_distance1(lhs, rhs)
   @s = lhs.inject(0.0) do |s, (k,v)|
     rhs.has_key?(k) ? (s + (v - rhs[k])**2) : (s + v**2)
   end
   @s = rhs.inject(@s) do |s, (k,v)|
     lhs.has_key?(k) ? s : (s + v**2)
   end
   @s
end

def euclidian_distance2(lhs, rhs)
  s = 0.0
  lhs.each do |k,v|
    s += rhs.has_key?(k) ? ((v - rhs[k])**2) : (v**2)
  end
  rhs.each do |k,v|
    s += v**2 if lhs.has_key?(k)
  end
  s
end

def euclidian_distance3(lhs, rhs)
  s = lhs.inject(0.0) do |n, (k,v)|
    n + (rhs.has_key?(k) ? v - rhs[k] : v)**2
  end
  rhs.each do |k,v|
    s += v**2 if lhs.has_key? k
  end
  s
end