From: duerst@...
Date: 2018-10-18T06:17:58+00:00
Subject: [ruby-core:89456] [Ruby trunk Bug#15233] Speeding Up Matrix Powers

Issue #15233 has been updated by duerst (Martin D��rst).


The code doesn't look very Ruby-ish. What about rewriting it as follows:

~~~ ruby
def pow(a, num)
  if num==1) 
    a
  elsif num.odd?
    a*pow(a, num-1)
  else
    ret = pow(a, num/2)
    ret*ret
  end
end
~~~



----------------------------------------
Bug #15233: Speeding Up Matrix Powers
https://bugs.ruby-lang.org/issues/15233#change-74496

* Author: octonion (Christopher Long)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
I've been looking at SymPy's slow matrix power speed, and noticed that replacing Ruby's matrix power code with the equivalent code from SymPy makes it significantly faster. As this is a recursively defined function, presumably it can be made even faster with a proper iterative version.

Base:

~~~ ruby
require 'matrix'

Q = Matrix[[0,0,1,0,2,0],
           [0,0,0,1,0,0],
           [1,0,0,1,0,2],
           [0,2,1,0,0,0],
           [1,0,0,0,0,0],
           [0,0,1,0,0,0]]

r = Vector[1,0,0,0,0,0]

p = (Q**100000000*r).sum

puts p.to_s.size
~~~

Output:

~~~
time ruby main.rb
35950264

real    1m42.250s
user    1m41.050s
sys     0m1.184s
~~~

Modified:

~~~ ruby
require 'matrix'

def pow(a,num)
  if (num==1)
    return a
  end
  if (num%2)==1
    return a*pow(a,num-1)
  end
  ret = pow(a,(num/2))
  ret*ret
end

Q = Matrix[[0,0,1,0,2,0],
           [0,0,0,1,0,0],
           [1,0,0,1,0,2],
           [0,2,1,0,0,0],
           [1,0,0,0,0,0],
           [0,0,1,0,0,0]]

r = Vector[1,0,0,0,0,0]

p = (pow(Q,100000000)*r).sum

puts p.to_s.size
~~~

Output:

~~~
time ruby main.rb
35950264

real    1m9.489s
user    1m8.661s
sys     0m0.828s
~~~






-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>