From: Brian Buckley <briankbuckley@...>
Date: 2005-10-17T23:12:25+09:00
Subject: declaratively caching results of a method

Hello

I have a class Foo which has a number of heavy, CPU intensive, number
crunching methods that take parameters.

class Foo
  attr_reader :x, :y, ...

  def calc1(a,b,c)
    ...# complicated, time-consuming calculation...
  end
  def calc2(a,b)
    ...# complicated, time-consuming calculation...
  end
  ...
end

Foo is immutable.  If a client calls a method more than once using the
same set of parameters, the same result will be calculated and
returned.

To avoid the work of recalculating a result if a client calls a method
more than once with the same set of parameters, I revised Foo to cache
results in a hash for each method for each set of parameters.  The
keys in the hashes are arrays of parameters.

class Foo
  def calc1(a,b,c)
    param_array = [a,b,c]
    @cache_calc1 ||= {}
    return @cache_calc1[param_array] if @cache_calc1[param_array]

    ans = ... #same complicated, time-consuming calculation...
    @cache_calc1[param_array] = ans
  end

  def calc2(a,b)
    param_array = [a,b]
    @cache_calc2 ||= {}
    return @cache_calc2[param_array] if @cache_calc2[param_array]

    ans = ... #same complicated, time-consuming calculation...
    @cache_calc2[param_array] = ans
  end
end

This works great  -- calculations for a given set of parameters on
each method are now done only once. However, the code seems rote,
repetitive and intrusive to the original methods.

Is there a way to do this in Ruby in a more declarative, more DRY way?
I am envisioning it should be possible to add a simple, single line to
the original Foo, a la

class Foo
  cache_calculation :calc1, :calc2

  #rest of Foo unchanged
end

My newbie ruby skills are not up to task writing such a
cache_calculation method on Class, to wrap and call the original
method and provide the caching, but it seems it should be simple to
do.

class Class
  def cache_calculation(syms)
    syms.each do |sym|
      ???
    end
  end
end

Can anyone provide the necessary snippet or otherwise advise or point
me to a solution to this?

Thanks!

Brian