On Thu, Dec 29, 2005 at 06:51:29AM +0900, ara.t.howard / noaa.gov wrote:
> On Thu, 29 Dec 2005, Johannes Friestad wrote:
> >That's where the 'can be' part comes in :)
> >The point is that symbols support quicker lookup by their nature.
> >Whether they are quicker in practice will depend on the
> >implementation. From the timings you give, it looks like symbol lookup
> >is implemented by converting the symbol to a string and doing string
> >lookup. Which is obviously not quicker :)
> 
> i never consider that as an impl - i bet your right though... time for me to
> read the source.

I think :sym.hash uses the method definition from Kernel (inherited through
Object) and :sym.eql?(:foo) would also reuse the existing definition.

[see C code at the bottom]

So there's seemingly no reason for symbol hashing/comparison not to be
faster than for strings. The benchmarks you linked to, as well as Jim's,
use relatively short strings, but one can exaggerate the effect:


# >> Strings Filling
# >>   5.800000   0.000000   5.800000 (  6.302649)
# >> Strings Fetching
# >>   3.120000   0.010000   3.130000 (  3.404679)
# >> 
# >> Symbols Filling
# >>   2.120000   0.000000   2.120000 (  2.326393)
# >> Symbols Fetching
# >>   0.640000   0.000000   0.640000 (  0.700178)


#!/usr/bin/env ruby

require 'benchmark'

SIZE = 100
N = 10000

RUBY_VERSION                                       # => "1.8.4"

def make_str_keys
  (1..SIZE).collect { |i| "long key" * i}
end

def make_sym_keys(strs)
  strs.collect { |s| s.intern }
end

def populate(keys)
  result = {}
  keys.each_with_index do |k, i|
    result[k] = i
  end
  result
end

def fetch(keys, hash)
  keys.each do |key| hash[key] end
end

strs = make_str_keys
syms = make_sym_keys(strs)

str_hash = populate(strs)
sym_hash = populate(syms)

puts "Strings Filling"
puts Benchmark.measure {
  N.times do
    populate(strs)
  end
}

puts "Strings Fetching"
puts Benchmark.measure {
  N.times do
    fetch(strs, str_hash)
  end
}

puts
puts "Symbols Filling"
puts Benchmark.measure {
  N.times do
    populate(syms)
  end
}

puts "Symbols Fetching"
puts Benchmark.measure {
  N.times do
    fetch(syms, sym_hash)
  end
}


    rb_define_method(rb_mKernel, "hash", rb_obj_id, 0);
...

VALUE
rb_obj_id(VALUE obj)
{
    if (SPECIAL_CONST_P(obj)) {
	return LONG2NUM((long)obj);
    }
    return (VALUE)((long)obj|FIXNUM_FLAG);
}


    rb_define_method(rb_mKernel, "eql?", rb_obj_equal, 1);
...

static VALUE
rb_obj_equal(VALUE obj1, VALUE obj2)
{
    if (obj1 == obj2) return Qtrue;
    return Qfalse;
}

-- 
Mauricio Fernandez